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  • 1995-1999  (4.033)
  • 1997  (1.612)
  • 1995  (2.421)
  • Life and Medical Sciences  (3.042)
  • Theoretical, Physical and Computational Chemistry  (699)
  • crystal structure
  • Nuclear reactions
Materialart
Erscheinungszeitraum
  • 1995-1999  (4.033)
Jahr
  • 1
    ISSN: 1572-8862
    Schlagwort(e): Rhenium ; dirhenium complexes ; rhenium–rhenium multiple bonds ; isocyanide ligands ; carbonyl ligand ; structural isomers ; crystal structure
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The reaction of the unsymmetrical, coordinatively unsaturated dirhenium(II) complex [(XylNC)(OC)CIRe(μ-dppm)2ReCl2]O3SCF3 (dppm = Ph2PCH2PPh2) with one equivalent of XylNC in CH2Cl2 affords a fifth structural isomer of the [Re2Cl3(μ-dppm)2(CO)(CNXyl)2] + cation; this is believed to have a CO-bridged structure of the type [(XylNC)ClRe(μ-Cl)(μ-CO)(μ-dppm)2ReCl(CNXyl)]+. The latter complex reacts with a further equivalent of XylNC in the presence of Tl+ to form the [Re2Cl2(μ-dppm)2(CO)(CNXyl)3]2+ cation, which has been shown by IR spectroscopy, and by the X-ray crystallographic characterization of its neutral congener Re2Cl2(μ-dppm)2(CO)(CNXyl)3, to contain a very weak and unsymmetrical CO bridge.
    Materialart: Digitale Medien
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  • 2
    Digitale Medien
    Digitale Medien
    Springer
    Journal of thermal analysis and calorimetry 50 (1997), S. 569-586 
    ISSN: 1572-8943
    Schlagwort(e): crystal structure ; metal(II) picolinate and quinaldinate ; thermal degradation of imidazole and pyrazole complexes
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Complexes of the type M(Pa)2(HAz)2 and M(QA)2(HAz)2 (M=cobalt(II) and nickel(II); HPa=picolinic acid, HQa=quinaldic acid; HAz=azoles like imidazole (Him), pyrazole (HPz), benzimidazole (HBzIm) etc.) show a similar thermal behaviour. In the first step of decomposition the corresponding azolinium picolinates or quinaldinates (H2AzPa, H2AzQa) are split off with formation of polymeric mixed ligand complexes M(Pa)(Az) or M(Qa)(Az). X-ray analysis of Co(Qa)2(HBzIm)2 XIIIa illustrates a proton transfer and a subsequent thermal removal of benzimidazolinium quinaldinate (H2BzImQa): Hydrogen bridges from pyrrole nitrogen of the benzimidazole to the non-coordinated oxygen of the quinaldinate predetermine the thermal initiated proton transfer. The high volatility of the heterocyclic acids and the nitrogen coordination are responsible for the formation of the mixed ligand complex Co(Qa)(BzIm) XIVa. Exceptions are the complexes M(Pa)2(HPz)2 XIa-b and M(Qa)2(HIm)2 XVIIa-b. Pyrazole is eliminated from the complexes XIa-b with formation of the solvent-free inner complex M(Pa)2 XIIa-b. From compounds XVIIIa-b quinaldic acid or their decomposition products are split off and a high temperature modification of M(Im)2 XVIIIa-b is formed at elevated temperature. XVIIIa-b are decomposed to the cyanides M(CN)2 similarly to the thermal behaviour of Cu(Im). In the first step the thermal degradation of imidazole and pyrazole adducts of copper(II) picolinates and quinaldinates is characterized by the elimination of azoles. The reason for this thermal behaviour is the weaker coordination of the azole heterocycles in copper chelate compounds.
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    Springer
    Journal of cluster science 8 (1997), S. 521-531 
    ISSN: 1572-8862
    Schlagwort(e): Zirconium clusters ; isocyanide ; synthesis ; crystal structure
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The first isocyanide ligated hexanuclear zirconium halide cluster is reported. The unoxidized [(Zr6Be)Cl12(CNXyl)6] (CNXyl = 2,6-dimethylphenyl isocyanide) was obtained from the solid state precursor K3Zr6Cl15Be by dissolution in CH3CN in the presence of CNXyl. The CNXyl ligands occupy all the axial positions on the cluster. The compound was recrystallized from CH2Cl2 and Et2O. [(Zr6Be)Cl12(CNXyl)6].2CH2Cl2 crystallizes in the space group $${\text{P}}\overline {\text{1}} $$ (#2) with a = 12.092(5) Å, b=12.728(5) Å, c = 14.102(8) Å, α = 104.98(4)°, β =107.11°, γ = 100.94°, V = 1919(2) Å3, Z = l, R = 11.3% and R W = 27.0%. For the bound isocyanide ligands, v CN increases to 2140 cm−1.
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    Springer
    Structural chemistry 8 (1997), S. 141-147 
    ISSN: 1572-9001
    Schlagwort(e): Dibenzanthracene ; trinitrobenzene complex ; trinitrobenzene complex ; π-complex ; crystal structure
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The crystal structure of the complex between the polycyclic aromatic hydrocarbon di-benz[a,c]anthracene and 1,3,5-trinitrobenzene is reported. The crystals are triclinic, space group P¯1 with unit cell dimensionsa=7.277(2) å,b=11.237(6) å, andc=13.902(5) å,α= 104.13(4)
    Materialart: Digitale Medien
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  • 5
    ISSN: 1572-9001
    Schlagwort(e): Tautomerism ; anthraquinone ; crystal structure ; semiempirical computations
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Substituted 1-hydroxy-9,10-anthraquinone-9-imines have been found to exhibit tautomeric interconversions between the 9,10- and 1,10-quinonoid forms in the solid state as well as in solution. Single-crystal X-ray crystallography was used to study the structures of 4-(N-acetyl-p-tolylamino)-9-amino-1,10-anthracenedione and 4-hydroxy-1-phenylamino-10-mesitylimino-9(10H)-anthracenone at ambient and low temperatures. The former compound gave crystals belonging to the monoclinic space group P2l/c and, at 295 K,a=9.684(2),b=16.371(3),c=12.097(2) å,Β=110.41(1)
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    Springer
    Structural chemistry 8 (1997), S. 453-457 
    ISSN: 1572-9001
    Schlagwort(e): Benzopsoralen ; photochemotherapeutic agent ; crystal structure ; molecular mechanics ; AM1 theoretical calculations
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract 5-Methoxy-4-methyl-2H-benzofuro[3,2-g]benzo-1-pyran-2-one was synthesized and its crystal structure was determined and compared with the optimal conformation arrived at by MM and AM1 theoretical calculations. The latter indicated that the tetracyclic skeleton is planar with total length (C2–C8) 9.23 å, and that the line joining the conters of the terminal-benzene and furan rings makes an angle of 30.5
    Materialart: Digitale Medien
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  • 7
    ISSN: 1573-1111
    Schlagwort(e): β-Dimorph ; crystal structure ; dianilinegossypol ethylacetate 1 : 1 clathrate ; packing motifs
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Depending on crystallization conditions, dianilinegossypol and ethylacetate form low (ambient temperature, α-phase) and high temperature (t = 35°C, β-phase) clathrate modifications. The structure of the α-phase has been discussed earlier [1]. Crystals of the 1 : 1 β-phase complex, C42H40O6N2·C4O2H8, are monoclinic, space group P21/c, a = 11.362(6), b = 19.479(9), c = 19.085(9) Å, β = 103.21(4)°, V = 4112(3)Å3, Z = 4, R = 0.084 for 3210 observed reflections. In these complexes centrosymmetric dimers of dianilinegossypol molecules formed via O(5)—H···O(3) hydrogen bonds are associated into columns by a weak O(8)—H···O(7) H-bond. A difference in the structure of these two phases is in the packing mode of the columns. The angle formed by intersecting host columns is about 126° for the α-phase and 104° for the β-modification. Guest molecules are hydrogen bonded to the host molecules via an O(1)—H···O(10) bond and are accommodated in channels in α-phase complex and in cavities in β-phase complex.
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    Springer
    Journal of inclusion phenomena and macrocyclic chemistry 28 (1997), S. 17-32 
    ISSN: 1573-1111
    Schlagwort(e): Azocrown ether ; azoxycrown ether ; sodium complexes ; crystal structure ; X-ray analysis ; conformation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Sodium iodide complexes of 13-membered azo-(I)and azoxycrown ethers (II) have been synthesized. Compound I[Na(L1 trans)2]⋅I⋅H2O is triclinic witha = 11.53(2), b = 15.74(2), c = 19.17(2) Å,α = 98.93(9), β = 105.51(9),γ = 89.20(9) deg.; Z = 4, space groupP1. Compound II [Na(L2)2]⋅I is orthorhombic witha = 12.451(2), b = 13.796(3), c = 18.667(4)Å; Z = 4, space group P212121. In bothcomplexes the cation is coordinated tochain oxygen atoms and to one nitrogen atom of the azoor azoxy unit. The ability of bothligands to form complexes in relation to thegeometry of the azo or azoxy subunit of themacrocycle is discussed.
    Materialart: Digitale Medien
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  • 9
    Digitale Medien
    Digitale Medien
    Springer
    Journal of inclusion phenomena and macrocyclic chemistry 27 (1997), S. 13-20 
    ISSN: 1573-1111
    Schlagwort(e): Uranyl complexes ; calixarenes ; crystal structure
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The synthesis and crystal structure of the inclusion complex between uranyl and p-tert-butylcalix[5]arene are reported. [UO2 (p-tert-butylcalix[5]arene-4H]2- · $${\text{2HNE}}_{{\text{t}}_{\text{3}} }^{\text{ + }} $$ &·2MeOH(1) crystallizes in the monoclinic space group C2/c, a = 30.06(2), b = 18.20(3), c = 31.35(2) Å, β = 128.51(6)°, V = 13423(40) Å3, Z = 8. Refinement led to a final conventional R value of 0.043 for 4155 reflections. The uranyl ion is bonded, in its equatorial plane, to the five oxygen atoms of the calixarene, four of which are deprotonated. A protonated triethylamine molecule is located inside the calixarene cavity and hydrogen bonded to a uranyl oxygen atom, and another one outside and hydrogen bonded to a calixarene oxygen atom. The calixarene conformation is the usual cone one.
    Materialart: Digitale Medien
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  • 10
    ISSN: 1573-1111
    Schlagwort(e): Calix[4]arene ; polyether ; crystal structure ; alkali metal ion binding
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The ligand 5,11,17,23-tetra-t-butyl-25,27-di(phenylmethoxy)-26,28-di(2-methoxy-ethoxy)calix[4]arene,designed as an analogue of some calixcrown speciesin order to evaluate possible origins of their selectivity in alkali metal ion binding, has been synthesised and structurally characterised by X-ray crystallography. The crystals are monoclinic, P21/n, a = 15.940(6), b = 19.388(5), c = 20.020(5) Å,β = 109.10(2) deg., Z = 4, conventional R on |F| being 0.073 for 3454 independent, ’observed‘ (I 〉 3σ(I)) reflections. 1H-NMR studies in 1:1 CD3CN/CDCl3solvent have shown that the ligand exerts a strong preference for the lighteralkali metal ions (Li+ and Na+) contrary to the binding behaviour of knowncalixcrowns. This may reflect interactions restricted to the lower rim donor atoms without concomitant interaction with the calixarene π-electrons, perhaps because the latter interactions are substituted by those with the benzyl group π-electrons.
    Materialart: Digitale Medien
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  • 11
    ISSN: 1573-1111
    Schlagwort(e): Dianilinegossypol ; crystal structure ; host–guest complexes ; H-bond ; α- and β-dimorphs
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Dianilinegossypol forms a 1 : 2 host-guest complex with DMSO:monoclinic, space group P21/n, a = 8.522(3), b = 18.034(4), c= 28.462(6) Å, β = 94.14(2)°, V = 4362Å3, Z = 4, D x = 1.26 g cm-3, T = 295 K.Final R value is 0.102 for 1793 observed reflections. A 1 : 1 : 1 adduct ofdianilinegossypol with 1,4-dioxane and 1,2-dichloroethane is found to beisostructural with the dianilinegossypol complex with DMSO: monoclinic,space group P21/n, a = 8.281(2), b = 19.245(3), c = 27.970(7)Å, β = 95.18°, V = 4439 Å3, Z = 4, D x =1.28 g cm-3, T = 295 K. Final R value is 0.114 for 2458observed reflections.The host molecules are associated by O(4)—H ...O(3) H-bonds toinfinite chains running in the direction of the c-axis The chains areincorporated into layers through 1,4-dioxane or DMSO molecules havingH-bonds with dianilinegossypol molecules. Another DMSO or 1,2-dichloroethanemolecule is included as a guest in the channels formed between the layers.At 60 °C a cryptate-type inclusion complex of dianilinegossypol isformed with DMSO or 1,4-dioxane. It is isostructural with the acetonecomplex reported in Part IV of the present series.
    Materialart: Digitale Medien
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  • 12
    Digitale Medien
    Digitale Medien
    Springer
    Journal of inclusion phenomena and macrocyclic chemistry 29 (1997), S. 175-185 
    ISSN: 1573-1111
    Schlagwort(e): p-tert-Butylcalix[5]arene ; synthesis ; complexation ; crystal structure
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The formation of p-tert-butylcalix[5]arene by the opening ofp-tert-butyldihomooxacalix [4]arene and the addition of a monomer has beenstudied. Various facets, including the effects of bases and the nature ofthe monomer added to the p-tert-butyldihomooxacalix[4]arene, have beeninvestigated. p-tert-Butylcalix[5]arene can be prepared in yields up to30%. The structure of its 1 : 2 complex with DMF has been determinedby X-ray crystallography. Crystals are triclinic, space group P¯1, a =1428.2(3) pm, b = 1837.3(3) pm, c = 1276.1(2) pm, α = 108.98(1)°,β = 105.02(2)°, γ = 95.21(1)°, Z = 2, D c = 1.059 kg m-3,final R value = 0.087. The macrocycle adopts a cone conformation, one guestenclosed inside the cavity, the other one outside.
    Materialart: Digitale Medien
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  • 13
    Digitale Medien
    Digitale Medien
    Springer
    Journal of inclusion phenomena and macrocyclic chemistry 27 (1997), S. 169-178 
    ISSN: 1573-1111
    Schlagwort(e): Bridged calix[4]arene ; ditopic receptor ; crystal structure
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The crystal structures of a new solvate of the ditopic receptor 1,3-calix[4]-bis-crown-6, Bis-C6, and of 1,3-calix[4]-bis-(benzo-crown-6), Bis-benzoC6, are reported. Bis-C6.3 CH3CN (1) crystallizes in the monoclinic space group P21/n, a = 14.388(3), b = 26.947(8), c = 14.707(4) Å, β = 113.19(3)°, V = 5241(5) Å3, Z = 4. Refinement led to a final conventional R value of 0.092 for 2723 reflections. The structure of (1) differs from the previously reported structure of Bis-C6.4 CH3CN by the conformation of one crown either chain. Two acetonitrile molecules are in the close neighbourhood of the crown ether cavities. Bis-benzoC6.3 CH3CN (2) crystallizes in the monoclinic space group P21/c, a = 10.391(4), b = 17.264(11), c = 30.426(9) Å, β = 94.62(3)°, V = 5440(7) Å3, Z = 4. Refinement led to a final conventional R value of 0.106 for 2965 reflections. Two acetonitrile molecules are located near the crown ether cavities, as in (1). One of the crown ether conformations is the same as in the binuclear caesium complex of Bis-benzoC6, supporting the hypothesis of a preorganization of this ligand towards the complexation of this ion; the second crown ether chain is partially disordered.
    Materialart: Digitale Medien
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  • 14
    Digitale Medien
    Digitale Medien
    Springer
    Journal of inclusion phenomena and macrocyclic chemistry 28 (1997), S. 125-140 
    ISSN: 1573-1111
    Schlagwort(e): X-ray ; crystal structure ; Li-complex ; triamides ; 1H-NMR
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract The structure of the lithium complex with1,3,5-tris[oxymethylene(N,N-dicyclohexyl)carboxyamido]cyclohexanehas been determined by the X-ray method.The compound is triclinic, space group P¯1,a = 15.623(3), b = 19.279(4),c = 19.295(4)Å α = 102.32(3), β = 92.45(3),γ = 105.67(3)0, V = 5436(2)Å3, Z = 4. Itscomposition is represented by the formulaC48H82N3O6LiI 0.5H2O. The lithium cationis encapsulated in a polar pseudo-cavity of six oxygen atoms of the ligandmolecule and displays a distorted trigonal prism coordination. The conformationof the ligand in the solid state complex has been compared with the conformationof the complex in solution determined by 1H-NMR measurements. Supplementary data relevant to this publication have been deposited with the British Library, No. SUP 82224 (21 pages).
    Materialart: Digitale Medien
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  • 15
    Digitale Medien
    Digitale Medien
    Springer
    Journal of inclusion phenomena and macrocyclic chemistry 29 (1997), S. 335-346 
    ISSN: 1573-1111
    Schlagwort(e): Glycophane ; macrocycle ; carbohydrate ; crystal structure
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie
    Notizen: Abstract Preparation of two new glycophanes is reported. These compounds arecomprised of two glucose molecules linked by hydrocarbon units at the 1,1′ and 3, 3′ or 3, 3′ and 6, 6′ positions. Thecrystal structure of one of the glycophanes is also described.
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  • 16
    ISSN: 0887-3585
    Schlagwort(e): small-angle scattering ; x-rays ; allosteric enzymes ; crystal structure ; rigid body modeling ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Medizin
    Notizen: Solution scattering curves evaluated from the crystal structures of the T and R states of the allosteric enzyme aspartate transcarbamylase from Escherichia coli were compared with the experimental x-ray scattering patterns. Whereas the scattering from the crystal structure of the T state agrees with the experiment, large deviations reflecting a significant difference between the quaternary structures in the crystal and in solution are observed for the R state. The experimental curve of the R state was fitted by rigid body movements of the subunits in the crystal R structure which displace the latter further away from the T structure along the reaction coordinates of the T→R transition observed in the crystals. Taking the crystal R structure as a reference, it was found that in solution the distance between the catalytic trimers along the threefold axis is 0.34 nm larger and the trimers are rotated by 11° in opposite directions around the same axis; each of the three regulatory dimers is rotated by 9° around the corresponding twofold axis and displaced by 0.14 nm away from the molecular center along this axis. Proteins 27:110-117 © 1997 Wiley-Liss, Inc.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 17
    ISSN: 0887-3585
    Schlagwort(e): Brownian dynamics ; molecular recognition ; site-directed mutagenesis ; facilitated diffusion ; crystal structure ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Medizin
    Notizen: Key charged residues in Cu,Zn superoxide dismutase (Cu,Zn SOD) promote electrostatic steering of the superoxide substrate to the active site Cu ion, resulting in dismutation of superoxide to oxygen and hydrogen peroxide. Lys-136, along with the adjacent residues Glu-132 and Glu-133, forms a proposed electrostatic triad contributing to substrate recognition. Human Cu,Zn SODs with single-site replacements of Lys-136 by Arg, Ala, Gln, or Glu or with a triple-site substitution (Glu-132 and Glu-133 to Gln and Lys-136 to Ala) were made to test hypotheses regarding contributions of these residues to Cu,Zn SOD activity. The structural effects of these mutations were modeled computationally and validated by the X-ray crystallographic structure determination of Cu,Zn SOD having the Lys-136-to-Glu replacement. Brownian dynamics simulations and multiple-site titration calculations predicted mutant reaction rates as well as ionic strength and pH effects measured by pulse-radiolytic experiments. Lys-136-to-Glu charge reversal decreased dismutation activity 50% from 2.2 × 109 to 1.2 × 109 M-1 s-1 due to repulsion of negatively charged superoxide, whereas charge-neutralizing substitutions (Lys-136 to Gln or Ala) had a less dramatic influence. In contrast, the triple-mutant Cu,Zn SOD (all three charges in the electrostatic triad neutralized) surprisingly doubled the reaction rate compared with wild-type enzyme but introduced phosphate inhibition. Computational and experimental reaction rates decreased with increasing ionic strength in all of the Lys-136 mutants, with charge reversal having a more pronounced effect than charge neutralization, implying that local electrostatic effects still govern the dismutation rates. Multiple-site titration analysis showed that deprotonation events throughout the enzyme are likely responsible for the gradual decrease in SOD activity above pH 9.5 and predicted a pKa value of 11.7 for Lys-136. Overall, Lys-136 and Glu-132 make comparable contributions to substrate recognition but are less critical to enzyme function than Arg-143, which is both mechanistically and electrostatically essential. Thus, the sequence-conserved residues of this electrostatic triad are evidently important solely for their electrostatic properties, which maintain the high catalytic rate and turnover of Cu,Zn SOD while simultaneously providing specificity by selecting against binding by other anions. Proteins 29:103-112, 1997. © 1997 Wiley-Liss, Inc.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
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  • 18
    ISSN: 0887-3585
    Schlagwort(e): bactericidal antibody ; crystal structure ; Neisseria meningitidis ; peptide-fluorescein conjugate ; PorA outer membrane protein ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Medizin
    Notizen: Class 1 outer membrane protein PorA of Neisseria meningitidis is a vaccine candidate against bacterial meningitis. Antibodies against PorA are able to induce complement-mediated bacterial killing and thereby play an important role in protection against meningococcal disease. Bactericidal antibodies are all directed against variable regions VR1 and VR2 of the PorA sequence, corresponding to loops 1 and 4 of a two-dimensional topology model of the porin with eight extracellular loops. We have determined the crystal structure to 2.6 Å resolution of the Fab fragment of bactericidal antibody MN12H2 against meningococcal PorA in complex with a linear fluorescein-conjugated peptide TKDTNNNL derived from the VR2 sequence of sero-subtype P1.7,16 (residues 180-187) from meningococcal strain H44/76. The peptide folds deeply into the binding cavity of the Fab molecule in a type I β-turn, with the minimal P1.16 epitope DTNNN virtually completely buried. The structure reveals H-bonds and van der Waals interactions with all minimal epitope residues and one essential salt bridge between Asp-182 of the peptide and His-31 of the MN12H2 light chain. The key components of the recognition of PorA epitope P1.16 by bactericidal antibody MN12H2 correspond well with available thermodynamic data from binding studies. Furthermore, they indicate the structural basis of an increased endemic incidence of infection by group B meningococci in England and Wales since 1981 associated with the occurrence of an Neisseria meningitidis escape mutant (strain MC58). The observed three-dimensional conformation of the peptide provides a rationale for the development of a synthetic peptide vaccine against meningococcal disease. Proteins 29:113-125, 1997. © 1997 Wiley-Liss, Inc.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
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  • 19
    Digitale Medien
    Digitale Medien
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 35 (1997), S. 1621-1625 
    ISSN: 0887-624X
    Schlagwort(e): 1,4-benzenedithiol ; 1,4-diethynylbenzene ; crystal structure ; solid-state polymerization ; layer structure ; Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The crystal structure of 1,4-benzenedithiol (BDT) was determined by the Rietveld method based on the calculation of the atomic coordinates of the BDT molecule using the Molecular Mechanics Program (MMP2). The refined crystal structure of BDT was monoclinic P21/c with dimensions, a = 7.795, b = 7.290, c = 5.955 Å, β = 92.16°, z = 2. The R factor of the refined structure was 0.038. Using above results, the mechanism of solid-state addition polymerization of BDT to 1,4-diethynylbenzene (DEB) was studied. Sublimed BDT piles up onto glass plate substrate and forms the layer structure along with the a axis. An inclination angle of the piled BDT column was 60° toward the substrate surface. DEB crystal structure was also monoclinic P21/c with a = 4.007, b = 6.018; c = 15.340 Å, β = 91.42°, z = 2. Sublimation of equimolar mixture of BDT and DEB gave a crystal having 1 : 1 composition, in which DEB column is situated between the columns of BDT. Relative arrangement of both monomers was suitable for the addition of —SH and —C=CH groups, since the distance between the two groups is 3.3 Å by CERIUS II calculation. Therefore, the addition polymerization of BDT to DEB easily proceeded by UV irradiation and the resulting polymer had a highly layer structure along with the a axis of BDT crystal. Tentatively estimated crystal structure of polymer obtained is monoclinic with a = 7.73, b = 7.30, c = 5.95 Å, β = 92.16°. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1621-1625, 1997
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  • 20
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 687-696 
    ISSN: 0894-3230
    Schlagwort(e): bsorption ; fluorescence ; solvatochromism ; hydrogen bonding ; dipole moment ; naphthalenones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Electronic absorption and steady-state fluorescence emission of seven 3,4-dihydro-1-(2-p-substituted benzylidene)naphthalenones (1-7) show sizable solvent dependence. The charge-transfer (CT) absorption maxima of these compounds in various solvents show a red shift for the electron-donating substituted compounds (1-5), whereas a blue shift is observed for compounds possessing electron-withdrawing substituents (6, 7). Excitation into the lowest energy absorption gives emission from the locally excited state, which relaxes to the emitting intramolecular charge-transfer state for compounds with strong electron-donating substituents (1-3) in hydroxylic solvents. However, in moderately polar solvents, dual emissions are observed for these compounds. No CT emission is observed for the compounds with moderate electron-donating substituents (3 and 4) or those with electron-withdrawing substituents (6 and 7). Compound 1 with an -N(CH3)2 substituent shows an excited-state dipole moment of 17 D. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 9 Ill.
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  • 21
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 717-724 
    ISSN: 0894-3230
    Schlagwort(e): SN1 solvolysis ; carbocation ; Grunwald-Winstein relationship ; solvent effect ; solvation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The rates of solvolysis in various solvents at 25 °C were determined for five tertiary alkyl chlorides: 2-chloro-2,4,4-trimethylpentane (4), 2-chloro-2,4-dimethylpentane, 2-chloro-2-methylpentane, 1-chloro-1,3,3-trimethylcyclopentane (7) and 1-chloro-1-methylcyclopentane. The rate data were analysed on the basis of the original and extended Grunwald-Winstein-type equation [log(k/k0)=mYCl+c and log(k/k0)=lNT+mYCl+c] and the results were compared with those reported for 2-chloro-2-methylpropane (1) and 2-chloro-2,3,3-trimethylbutane (3). The rate data for 4 in 18 solvents give an excellent correlation with l=0·00±0·02 and m=0·74±0·01. The neopentyl group in 4 more effectively shields the rear-side of the reaction center than the tert-butyl group in 3 that is correlated by l=0·10±0·04 and m=0·81±0·04. The rate ratio between 4 and 1 at 25 °C is 275 in TFE and predicted to increase to 950 in TFA. The previous 4/1 rate ratio of 21 in 80% ethanol evidently underestimates the B-strain effect on the solvolysis rate of 4 by a factor of at least 40. The remote methyl groups in 7 works to increase rear-side shielding without increasing B-strain. The marked difference in the effect of the remote methyl groups between 4 and 7 suggests that the leaving chloride ion in 4 takes a locus that is nearly antiperiplanar to the tert-butyl group. © 1997 John Wiley & Sons, Ltd.
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  • 22
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 725-730 
    ISSN: 0894-3230
    Schlagwort(e): p-nitrophenyl N-phenylcarbamates ; stepwise mechanism ; rate-limiting breakdown of T± ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Kinetic studies of the reactions of p-nitrophenyl N-phenylcarbamates with benzylamines were carried out in acetonitrile at 25·0 °C. Second-order (k2) and third-order (k3) rate constants were observed for all the Y-substituted carbamates except for Y=m-Cl. The relatively large magnitude of ρX (for X-substituted benzylamines) and ρY together with a positive cross-interaction constant ρXY supports a stepwise mechanism involving rate-limiting breakdown of the zwitterionic tetrahedral intermediate T±. Kinetic isotope effect studies with deuterated benzylamine (XC6H4CH2ND2) indicate that in the base-catalyzed path, k3, rate-limiting deprotonation occurs at the amino group of benzylamine within the T± intermediate. The low δH≠ and δS≠ values for the k3 process are in accord with the proposed mechanism © 1997 John Wiley & Sons, Ltd.
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  • 23
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 747-754 
    ISSN: 0894-3230
    Schlagwort(e): ureido sugars ; amino acids ; 1H NMR ; IR ; H-D exchange ; intramolecular hydrogen bonding ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Ureido 2-deoxy-β-D-glucopyranosides with seven different amino acid ester residues were studied by means of IR and 1H NMR spectroscopy. The H-D exchange rates increase in the order L-Val〈L-Leu〈L-Ala〈Gly for both NH protons; however, the exchange rate at N-1-H (linked to the glucopyranoside) is significantly faster than that at N-3-H (at the amino acid residue). The analysis of IR spectra in the region of the NH stretching vibrations shows, in agreement with other investigations, that the signals at 3454, 3423 and 3355 cm-1 are due to the free and the intramolecular associated NH groups forming a five- and seven-membered ring, respectively. It was found that the C7 associated ring is formed by hydrogen bonding between the N-1-H function and the C=O of the acetyl group at the C-3 position in the glucopyranoside. The N-3-H group is involved in a hydrogen bond with the C=O; function of the ester group protecting the amino acid residue. Furthermore, it was found that the ureido sugar with L-Val exhibits a stronger C7 hydrogen bond than the other amino acid residues. This result conforms with the lowest H-D exchange rate at the N-1 position of this compound. Steric effects resulting in the shielding of this hydrogen bond against OD attack are considered as a reason for this peculiarity. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 7 Ill.
    Materialart: Digitale Medien
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  • 24
    ISSN: 0894-3230
    Schlagwort(e): solvent effects ; aromatic nucleophilic substitutions ; empirical polarity parameter ; halonitrobenzenes ; aliphaticamines ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Empirical solvent polarity parameters ET(30) were determined by UV/VIS spectroscopy, using Dimroth-Reichardt's betaine dye, as a function of composition, for several binary solvent mixtures [i.e. polar hydrogen bond acceptor (PHBA) solvents+chloroform or dichloromethane]. Each solvent system was analyzed according to its deviations from additivity due to preferential solvation of the chemical probe and also from complicated intermolecular interactions of the mixed solvents. The ET(30) parameter of many of these mixtures has presented synergism. The synergetic effects were more significant for those binary solvent systems in which chloroform is the co-solvent. These results were related to the solvent effects on some aromatic nucleophilic substitution reactions. The kinetics of the reactions between 1-halo-2,4-dinitrobenzenes and primary or secondary aliphatic amines were studied in three solvent systems (PHBA+chloroform) where the synergism for the ET(30) polarity parameter is the rule. In all the aminodehalogenation reactions discussed the formation of the intermediate is the rate-determining step. The kinetic data show a tendency to decrease with decrease in the overall solvation capability of the binary mixture. In general, the reaction rates presented a gradual decrease in the PHBA solvent-rich zone and a large decrease at high co-solvent concentrations. The ET(30) values corresponding to binary dipolar hydrogen bond acceptor-hydrogen bond donor mixtures may be not generally valid for interpreting solvation effects on the reactions under consideration. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 11 Ill.
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  • 25
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 835-840 
    ISSN: 0894-3230
    Schlagwort(e): N-methylaminopyridines ; polar properties ; spectroscopic properties ; solvent effect ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Dipole moments of 2-N-methylamino-5-nitro-6-methylpyridine (I) and 2-N-methylamino-3-nitro-6-methylpyridine (II) were determined in solvents of different polarity and basicity. The solvent effect is discussed in terms of the two-parameter (π*-β) Kamlet-Taft expression. The solvent dependences of the energy of intramolecular charge-transfer transition and the frequency of the ν(NH) stretching vibration are in keeping with the dielectric results. It was found that the solvent-induced disturbance in the electronic structure of I is brought about by hydrogen bond interaction with the NH group. The strong intramolecular hydrogen bond in II is stable in weakly basic solvents and the electronic structure is insensitive to the environment. However, in more basic solvents the intramolecular hydrogen bond is partly broken and an N - H· · ·solvent hydrogen bond is formed, bringing about distinct changes in polarity and spectroscopic properties. © 1997 John Wiley & Sons, Ltd.
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  • 26
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 861-869 
    ISSN: 0894-3230
    Schlagwort(e): thiapyrylium salt ; photoinduced electron transfer ; laser flash photolysis ; trans-stilbene cation radical ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Photoinduced electron transfer reactions of trans-stilbene sensitized by 2,4,6-triphenylthiapyrylium tetrafluoroborate (STPP) were carried out by steady-state and laser flash photolysis techniques in the presence and absence of oxygen in dichloromethane. Rapid dimerization of the trans-stilbene cation radical with its neutral species was observed, as previously observed in 2,4,6-triphenylpyrylium tetrafluoroborate (TPP)-sensitized reactions in dichloromethane. Electrochemical and photophysical properties such as fluorescence quantum yield and T-T absorption spectrum of STPP were also studied, and the results were compared with those for TPP. The properties of STPP as an electron transfer sensitizer are discussed. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 8 Ill.
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  • 27
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 647-661 
    ISSN: 0894-3230
    Schlagwort(e): merocyanines ; solvatochromism ; solvent polarity ; VBHB model ; SA-SAB-SB model ; binary solvent mixtures ; solvation equilibria ; spectra simulation ; pyridinium betaines ; 7H-indolo[1,2-a]quinolinium betaines ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---A quantitative model of solvatochromism in a binary solvent system is presented. Although it is derived for merocyanine and betaine dyes, it explains a majority of known examples of solvatochromism in binary solvents. The model provides an estimation of equilibrium constants between particular types of solvates present therein. UV-VIS absorption spectra of solvated species can be simulated. They perfectly fit the experimental data. The model proposed describes the internal solvent picture from the solute point of view, which differs from other known models and may be useful for studying the structure of liquids © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 16 Ill.
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  • 28
    ISSN: 0894-3230
    Schlagwort(e): complex stabilities ; alkali metals ; alkaline earth metals ; dibenzo crown ethers ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---An experimental method was developed to determine the stability constants with nearly insoluble ligands in homogeneous solution. This method was tested using dibenzo crown ethers, which have a very low solubility in aqueous solution. The stability constants for the complexation of alkali and alkaline earth metal cations with different dibenzo crown ethers were determined in aqueous solution. Owing to the complex formation the total concentration of the ligand in solution increases. Dibenzo crown ethers absorb in the ultraviolet spectral range, hence the increase in the ligand concentration can be easily detected. Without the knowledge of the molar absorptivities of the ligands and of the corresponding complexes and of the solubilities, the stabilities of the complexes formed can be calculated under certain assumptions. To verify these assumptions, the solubilities of the dibenzo crown ethers and the molar absorptivities were determined. The kinetics of the solubilization process of the ligands was followed by spectrophotometric measurements. © 1997 John Wiley & Sons, Ltd.
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  • 29
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 777-780 
    ISSN: 0894-3230
    Schlagwort(e): tandem energy transfer-electron transfer ; photosensitized alkylation ; α,β-Unsaturated ketones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: -α,β-Unsaturated ketones are not conveniently alkylated by radicals generated from tetraalkylstannanes via photoinduced electron transfer (PET), either by direct irradiation or when a singlet sensitizer (an aromatic nitrile, a pyrilium salt) is used. However, the procedure is successful with tetramethyl pyromellitate (TMPM) as the sensitizer. TMPM is promoted to the triplet state by energy transfer from the unsaturated ketones and then sensitizes the cleavage of alkylstannanes by electron transfer. The alkyl radicals thus formed finally add to the unsaturated ketones, giving the β-alkyl derivatives. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Ill.
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  • 30
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 662-668 
    ISSN: 0894-3230
    Schlagwort(e): propan-1-ol oxidation ; alkaline hexacyanoferrate(III) ; ruthenium trichloride catalyst ; kinetic ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The oxidation kinetics of propan-1-ol by alkaline hexacyanoferrate(III) catalyzed by ruthenium trichloride were studied spectrophotometrically. The initial rate method was used for kinetic analysis. The reaction rate shows a fractional order in [oxidant] and [substrate] and a first-order dependence on [RuCl3]. The dependence on [OH-] is complicated. A reaction mechanism involving two active catalytic species is proposed. Each one of these species forms an intermediate complex with the substrate. The attack of these complexes by hexacyanoferrate(III) in rate-determining step produces a radical species which is further oxidized in the subsequent step. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 5 Ill.
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  • 31
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 731-736 
    ISSN: 0894-3230
    Schlagwort(e): solvatochromism ; preferential solvation ; aryliminomethylpyridinium iodides ; binary mixtures ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Studies on the solvatochromic behaviour of N-methyl-4- and N-methyl-2-[(4-dimethylaminophenyl)iminomethyl]pyridinium iodide dyes in a variety of solvents and the preferential solvation of the former dye in binary mixtures of protic and non-protic solvents are presented and interpreted in terms of solute-solvent and solvent-solvent interactions. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Ill.
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  • 32
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 787-796 
    ISSN: 0894-3230
    Schlagwort(e): carbonyl oxide ; ozone ; structure and reactivity ; oxygen atom transfer ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The structural and chemical properties of 1,3-dipolar peroxidic species (X - O - O) such as ozone (X=O) and carbonyl oxides (X=R2C) depend significantly on the nature of X. Although cyclic O3 is thermodynamically unstable, dioxiranes, cyclic isomers of carbonyl oxides, are isolable and have been fully characterized. In contrast to the well known electrophilic nature of ozone, carbonyl oxides usually act as a nucleophilic oxygen transfer agent, but their reactivity could be altered by substituents. It is expected that the chemistry of R2C - O2 species will provide a good starting point for a better understanding of dioxygen complexes (X - O2) with various Xs. Interesting aspects of carbonyl oxide chemistry are discussed, including methods of generation, cyclization to dioxiranes and the nature of their oxygen transfer activities. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 7 Ill.
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  • 33
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 825-834 
    ISSN: 0894-3230
    Schlagwort(e): N-tert-butoxycarbonyl-L-phenlylalanine-pyridine complex ; isotope effect ; x-ray structure ; vibrational spectroscopy ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The x-ray structure of the complex between N-tert-butoxycarbonyl-L-phenylalanine and pyridine shows that the crystals are held together by short hydrogen bonds between the OH group and the N atom of the pyridine ring [RO(H)· · ·N=2·574(3) Å]. Deuteration of the OH and NH groups results in a small expansion of the unit cell associated with an elongation of the O· · ·N distance [RO(D)· · ·N=2·610(3) Å]. The infrared spectra show very broad stretching protonic bands in the range 2750-600 cm-1. The isotopic ratio ν(OH· · ·H)/ν(OD· · ·N) of nearly unity suggests a double minimum potential with a low barrier for the hydrogen bridge. The Raman and near-infrared spectra are discussed. The perturbation of the fundamental modes and of the overtones of pyridine suggests that the complex is not of the proton transfer type. The broad absorptions of the hydrogen bridge disappear in the near-infrared region. The perturbation of the pyridine overtones is discussed. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 5 Ill.
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  • 34
    ISSN: 0894-3230
    Schlagwort(e): 13C and 1H magic angle spinning NMR ; x-ray diffraction ; 1,8-bis(dimethylamino)naphthalene ; picrolonic acid ; multicentre hydrogen bonding ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: An ionic complex of 1,8-bis(dimethylamino)naphthalene with picrolonic acid was studied by 13C and 1H magic angle spinning and x-ray diffraction. Structural and spectroscopic features of this compound were investigated. The detailed x-ray structure of this complex is described. A multicentre model of hydrogen bonding in proton sponges {[Me2N - H· · ·NMe2]+· · ·Xδ-} is proposed and the influence of weak intermolecular intractions with the nearest electronegative atom in crystal lattice of a proton sponge on the strong intramolecular [N - H· · ·N]+ hydrogen bonding is demonstrated. It appears that weak interactions of electronegative atoms with the nearest methyl hydrogen atoms can influence the localization of the proton in the intramolecular [N - H· · ·N]+ hydrogen bridge. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 8 Ill.
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  • 35
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 797-813 
    ISSN: 0894-3230
    Schlagwort(e): statistics ; regression ; correlation analysis ; incorrect data processing ; least-squares method ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Examples are given from older and more recent literature (kinetics, ionization equilibria, complex formation in solution, dipole moment determination, thermochemistry, resonance energies, NMR shifts, photoelectron spectroscopy) where experimental data were processed in an incorrect way from the point of view of statistics. The results were more or less biased, sometimes completely wrong. Corrected procedures, based entirely on the least-squares method, are reported; in several cases methods are proposed. Some hints are given as to how these mistakes can be avoided, how they can be revealed in the literature and how the literature data can be recalculated: the last task is the most difficult. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 13 Ill.
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  • 36
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 879-884 
    ISSN: 0894-3230
    Schlagwort(e): solvolysis ; correlation analysis ; azide salt effect ; β-deuterium kinetic isotope effect ; YBnCl ; 1-alkyl-1-chloro-1-(4-methyl)phenylmethanes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The solvolysis of 1-alkyl-1-chloro-1-(4-methyl)phenylmethanes (4a-d) in aqueous acetone, aqueous ethanol, aqueous methanol and ethanol-trifluoroethanol was studied. Grunwald-Winstein-type correlation analysis using the YBnCl scale suggests significant nucleophilic solvent intervention in the case of 1-chloro-1-(4-methyl)phenylethane (4a). Increasing bulkiness of the 1-alkyl substituent from methyl (4a) to ethyl (4b), to isopropyl (4c) and to tert-butyl (4d) resulted in a gradual change to limiting SN1 mechanisms. The observed excellent linear correlations with YBnCl and the good solubility in high-water-containing binary solvents made 4d a suitable reference standard for deriving more YBnCl values. A positive azide salt effect was realized in the solvolysis of 4a but not 4d. A small decrease in the β-deuterium kinetic isotope effect from 4a to 1-chloro-1-(4-methoxyphenyl)propane (5) suggested the presence of additional stabilization of the benzylic cationic transition state. Howver, no relationship between k(CH3)/k(CD3) and the solvent effect was found. The superiority of employing the YBnCl scale over the combination of YCl and I scales in the mehanistic study was observed. © 1997 John Wiley & Sons, Ltd.
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  • 37
    ISSN: 0894-3230
    Schlagwort(e): N-1-adamantyl-N-p-tolylcarbamoyl chloride ; solvolysis ; decompositions ; hydroxylic solvent ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---In hydroxylic solvents, N-1-adamantyl-N-p-tolylcarbamoyl chloride undergoes a facile rate-determing ionization with very little assistance from nucleophilic solvation (very low sensitivity to changes in solvent nucleophilicity). In the relatively nucleophilic aqueous ethanol or aqueous acetone, the cation formed reacts directly with the solvent. In fluoroalcohols, the low nucleophilicity of the solvent allows an effective competition from a pathway involving disproportionation to the 1-adamantyl cation and p-tolyl isocyanate, followed by reaction of the new cation either with solvent or with the chloride ion formed in the initial ionization. Grunwald-Winstein plots against YC1 values show for different binary solvent systems a marked dispersion, which can be considerably reduced by incorporation into the analyses of a term governed by the aromatic ring parameter (I). © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Ill.
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  • 38
    ISSN: 0894-3230
    Schlagwort(e): Electron transfer properties ; substituted flouranthene derivatives ; redox potentials ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The oxidation and reduction potentials of a series of related even non-alternant derivatives of 7,14-disubstituted acenaphth[1,2-k] fluoranthenes, and also fluoranthene, 7,10-diphenylfluoranthene and 8,9-dihydrodiindeno[1,2-j;2′,1′-] fluoranthene, were determined in organic solvents by cyclic voltammetry. The effects of steric hindrance on conjugation of the substituents with the central polycyclic aromatic hydrocarbon nucleus were evaluated. The semi-empirical molecular orbital calculation programs OMEGAMO, Extended Hückel, AM1 and PM3 were used to obtain optimal geometries and calculated HOMO and LUMO energies. As a further refinement, COSMO solvation was included in the AM1 calculations. The redox properties were correlated with data derived from the various semi-empirical calculations and the quality of these correlations is discussed. Inclusion of solvation energies in the computed molecular orbital energies results in a significant improvement in the correlation between observed and calculated oxidation potentials. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 7 Ill.
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  • 39
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 697-715 
    ISSN: 0894-3230
    Schlagwort(e): silanes ; polysilanes ; molecular mechanics calculations ; MM3 ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: -Molecular structures, conformational energies, heats of formation and vibrational spectra have all been fit using the MM3 force field for a group of 48 silanes and polysilanes for which experimental data are available. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 12 Tab.
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  • 40
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 680-686 
    ISSN: 0894-3230
    Schlagwort(e): 8-aminoquinoline ; x-ray structure ; vibrational spectra ; three-center hydrogen bond ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: -The x-ray structure of 8-aminoquinoline indicates that one of the NH bonds of the NH2 group forms a three-center hydrogen bond, the major component being the intramolecular hydrogen bond and the minor component the intermolecular hydrogen bond. The IR and Raman spectra of 8-aminoquinoline and of its N-deuterated counterpart reflect the non-equivalence of the two NH(D) bonds and the weakness of the hydrogen bridge. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 4 Ill.
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  • 41
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 675-679 
    ISSN: 0894-3230
    Schlagwort(e): chemical contrast ; scanning near-field optical microscopy ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Repeatable and stable scanning near-field optical microscopy (SNOM) images are obtained under shear-force control if uncoated, i.e. cold and sharp, tapered fiber tips are used. True chemical contrast is seen for the first time on organic crystals due to the different near-field reflectivities of different chemical species on partly oxidized anthracene. The topography spans a Z-range of 500 nm. The surface roughness is tolerable and the sites of reaction can be related to crystal structure data. A submicroscopic local resolution of 18 nm has been obtained for chemical contrast, discriminating different chemical species even with poorly reflecting organics on rough crystal surfaces. Thus, reflection-back-to-the-fiber SNOM will find wide applications for both transparent and opaque samples. All drawbacks with blunt and hot metal-coated tips at a 〈10 nm distance from the surface in previous SNOM setups are overcome. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
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  • 42
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 607-611 
    ISSN: 0894-3230
    Schlagwort(e): electron transfer photochemistry ; 7-(spirocyclopropane)quadricyclane ; cyclopropylcarbinyl rearrangements ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The electron transfer photo-sensitized reaction of 7-(spirocyclopropane)quadricyclane (1) with methanol produces two rearranged mono-methanol adducts, 2 and 3, and a bis-methanol adduct, 4. The products reveal that 1·+ reacts by stereo- and regiospecific attack of methanol on one trisubstituted cyclopropane ring. The resulting free radical rapidly undergoes one or two (consecutive) cyclopropylcarbinyl to butenyl rearrangements. The mono-adducts are formed by net hydrogen abstraction, the di-adduct via a (secondary) electron transfer reaction of 3. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 2 Ill.
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  • 43
    ISSN: 0894-3230
    Schlagwort(e): γ-lactones ; base-catalysed hydrolysis ; substituted isobenzofuranones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Rate coefficients were measured for the base-catalysed hydrolysis of a series of γ-lactones, i.e. 39 substituted 3-(aryl- and alkylmethylene)-(Z)-1(3H)-isobenzofuranones (3-aryl- and alkylmethylenephthalides) in 70% (v/v) aqueous dioxane at 30·0 °C. A Hammett reaction constant for the 3- or 4-substituted phenyl series is ca 1·5, whereas those for the 2-substituted phenyl and 4-substituted 1-naphthyl series, using para-σ values, are ca 1·0 and 1·9. These results are related to an electrostatic field effect model. A very successful correlation between the rates of alkaline hydrolysis of all 39 phthalides and the carbonyl stretching frequencies in chloroform was found. Substituent effects in widely different environments give linearly related effects on both reactivity and physical properties. Computational studies using the semi-empirical AM1 method correctly modelled both the details of the mechanistic pathway and the substituent effects. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 6 Ill.
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  • 44
    ISSN: 0894-3230
    Schlagwort(e): competitive reactivity ; reaction coordinates ; gas-phase ion-molecule chemistry ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Using the method of competitive reactivity of two functional groups in the same molecule, anionic elimination reactions show considerable kinetic selectivity for small differences in leaving group thermochemistry, in structures of the general type YCH2CH2CH2Z, where Y and Z are good anionic leaving groups. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 1 Ill.
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  • 45
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 612-622 
    ISSN: 0894-3230
    Schlagwort(e): neural networks ; structure-odour relationships ; sandalwood ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Neural networks have proved to be particularly successful in their ability to identify non-linear relationships. This paper shows that a three-layer back-propagation neural network is able to learn the relationship between the sandalwood odour and molecular structures of 85 organic compounds belonging to acyclic, cyclohexyl, norbornyl, campholenyl and decalin derivatives. Four steric and three electronic parameters were used to describe each molecular structure. Odour was coded by a binary variable. The neural network was used to classify the compounds into two groups and to predict their odours (sandalwood or non-sandalwood). The results obtained were compared with those given by discriminant analysis, and found to be better. The most important descriptors were revealed on the basis of correlation analysis. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 2 Ill.
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  • 46
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 631-636 
    ISSN: 0894-3230
    Schlagwort(e): chlorine transfer ; N-chlorosuccinimide ; amino compounds ; kinetics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---A kinetic study of the reactions of N-chlorosuccinimide (NCS) with glycine (Gly), sarcosine (Sar), 2-methylalanine (2MA), proline (Pro) and pyrrolidine (Pyr) was carried out. The reactions were found to be first order with respect to both NCS and the amine or amino acid and order -1 in proton concentration. In order to calculate the experimental activation parameters, the effect of temperature on the reaction rates was studied. The ionic strength and buffer concentration were found to have no effect on the rate constant. A reaction mechanism involving Cl+ transfer from NCS to the amine or amino acid to form an N-chloro compound is proposed © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 6 Ill.
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  • 47
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 577-584 
    ISSN: 0894-3230
    Schlagwort(e): carbenium ions ; reduction ; NAD(P)H analogues ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Hydride transfer mechanisms of the reductions of xanthylium ion by NAD(P)H analogues (i.e. BNAH, HEH and AcrH2) were investigated. Both the kinetic observations and an analysis of thermodynamic driving forces for each mechanistic step in all the possible mechanisms indicate that the reductions are initiated by a rate-determining electron transfer, followed by a fast hydrogen atom abstraction. The mechanism of the reductions of 9-phenylxanthylium and triphenylmethylium ions by BNAH were also investigated and are similarly discussed. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 4 Ill.
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  • 48
    ISSN: 0894-3230
    Schlagwort(e): peroxyoxalate reaction intermediate ; chemiluminescent decomposition ; kinetics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The peroxyoxalate system undergoes one of the most efficient chemiluminescence reactions and is the only one considered to involve an intermolecular chemically initiated electron exchange luminescence mechanism, with proven high efficiency. Several reactive intermediates have been proposed, which, upon interaction with a fluorescent activator, lead to excited-state generation. The synthesis and spectral characterization of 4-chlorophenyl O,O-hydrogen monoperoxyoxalate (1), a compound analogous to one of the proposed reactive intermediates, was recently reported. Here the results of a kinetic study on the chemiluminescent decomposition of this peracid 1, catalyzed by oxygen bases (potassium hydroxide, potassium tert-butoxide and potassium p-chlorophenolate) or nitrogen bases [pyridine, imidazole and 1,8-bis(dimethylamino)naphthalene] are presented. Based on the dependence of the observed rate constants on the base concentration, kinetic schemes are proposed for the catalyzed decomposition of 1 and rate constants are assigned to specific reaction steps. The results obtained with the nitrogen bases give further support in favor of 1,2-dioxetane dione as the reactive intermediate in the peroxyoxalate reaction. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 7 Ill.
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  • 49
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 637-645 
    ISSN: 0894-3230
    Schlagwort(e): 3-nitropyrazole ; structure ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The molecular and crystal structure of 3-nitropyrazole was determined by X-ray analysis. The triclinic unit cell contains 12 molecules which form four hydrogen-bonded (N - H···N) trimers. Each trimer comprises of pseudo-ring in a flattened envelope distorted towards a chair conformation. The crystal packing consists of layers formed by centrosymmetric related trimers joined through C - H···O interactions. Ab initio calculations were performed on 3(5)-nitro- and 4-nitropyrazole and their corresponding protonated forms up to the MP2/6-31G** level of theory. The origin of the difference in aqueous basicities between both nitropyrazoles is discussed. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 4 Ill.
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  • 50
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 669-674 
    ISSN: 0894-3230
    Schlagwort(e): azoles ; diiodine complexes ; basicity ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The diiodine basicity (a soft Lewis basicity) of 15 azoles (imidazoles, pyrazoles and triazoles) was measured by means of the formation constant of the diiodine-azole complexes in heptane at 298 K. The preferred sites of diiodine fixation are the nitrogens N-3 in imidazoles, N-2 in pyrazoles and N-4 in 1,2,4-triazoles. The diiodine basicity decreases with (i) the number of ring nitrogens, (ii) benzofusion, (iii) field electron-withdrawing effects of substituents on N-1 and (iv) for pyrazoles only, steric effect of substituents on N-1. In imidazoles and 1,2,4-triazoles, the lengthening and branching of alkyl groups on N-1 increase significantly the basicity, and 1-(adamant-1-yl)imidazole is the most basic of the azoles studied. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Tab.
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  • 51
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 755-767 
    ISSN: 0894-3230
    Schlagwort(e): substitution ; singlet-triplet gaps ; carbenes ; vinylidenes ; density functional calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---NDensity functional theory calculations of multiplet splittings are presented that agree closely with experimental measurements for six carbenes and six vinylidenes. The calculations are further analyzed to gauge the relative importance of different factors influencing the stabilities of the different spin and electronic states. In the carbene series, with halogen substituents, orbital rehybridization effects and charge redistribution effects are large. The magnitude of π-conjugation (back-bonding) is calculated to be only moderately larger (6-8 kcal mol-1) for singlets than for triplets based on natural bond orbital-derived conjugation energies. In the vinylidene series, substituion effects are primarily associated with through-space and through-bond inductive stabilization effects, especially hyperconjugation; π-conjugation effects are found to be small © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 2 Ill.
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  • 52
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 768-776 
    ISSN: 0894-3230
    Schlagwort(e): Diels-Alder addition ; butadiene ; thiocarbonyl oxides ; heterodienophiles ; heterocumulene ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Diels-Alder reactions of various thiocarbonyl S-oxides and thiocarbonyl S,S-dioxides with buta-1,3-diene were studied by semiempirical AM1 and PM3 methods. Calculations show that the reactions of such unsymmetrical heterodienophiles pass through asynchronous transition states (TSs) with C=S π* involved in the reaction. Calculated activation energies indicate that the reactivity of these heterodienophiles, viz. R2C=S, R2C=SO and R2C=SO2, decrease gradually with successive addition of oxygen atoms to the thiocarbonyl sulfur. This is in good agreement with the experimental observations. The predictions based on LUMO and deformation energies show that the above reactivity trend corresponds to a gradual destabilization of LUMO of the dienophile and increase in deformation energy of both diene and dienophile with increase in the number of oxygen atoms around sulfur. Thiophosgene and their S-oxides in Diels-Alder reactions are found to be less reactive than the parent analogues. Monosubstituted (Z/E)-sulfines react with buta-1,3-diene to form cis and trans products through closely lying TSs. Calculations predict that (E)-sulfine has a higher reactivity than the Z-isomer, in reasonable agreement with experimental results. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 4 Ill.
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  • 53
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 737-746 
    ISSN: 0894-3230
    Schlagwort(e): iminoxyl radicals ; spectra ; kinetics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---A series of new iminoxyl radicals, Et3C(C=NO·)Bu-tert (1), tert-C5H11(C=NO·) Bu-tert (2), (tert-C5H11)2C=NO· (3), Et3C(C=NO·)Ph (4), PhCH2CMe2(C=NO·)Bu-tert (5), PhCMe2(C=NO·)Bu-tert (6) and Me2CH(C=NO·)C5H11-tert (7), was characterized by EPR and in some cases visible spectra and decay kinetics in solution. Isolation of 1H and 3-tert-butyl-4,4-diethyl-5-methyl-4,5-dihydro-5-isoxazolol (9) among the decomposition products of 1 was consistent with a previously unrecognized mode of iminoxyl reaction in which the initial step is an internal β-H abstraction. Molecular orbital calculations on H2C=NO· indicate a C-N-O angle of about 140° and a 10 kcal mol-1 barrier to inversion. Rate constants for the reaction of tert-Bu2C=NO· (8) with a series of olefins were measured. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
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  • 54
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 27-32 
    ISSN: 0894-3230
    Schlagwort(e): Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: 1,3-Dihydro-1-oxido-3-methyl-1,2,3-benziodoxaphosphole 3-oxide (4) and 1-H-1-oxido-5-methyl-1,2,3-benziodoxathiole 3,3-dioxide (5) were used to cleave p-nitrophenyl diphenyl phosphate (PNPDPP) (2) in aqueous micellar cetyltrimethylammonium chloride (CTACI) in phosphate buffer at pH 8. The maximum pseudo-first-order cleavage rate constants (with 1·0×10-4 M 4 or 5 and 1·0×10-5 M 2) were 0·0016 s-1 for 4 ([CTAC1]=0·01 M) and 0·0013 s-1 for 5 ([CTAC1]=0·001 M). Reagents 4 and 5 were, respectively, 44 and 57 times less reactive toward PNPDPP than iodosobenzoate (1) under comparable conditions. Ab initio electronic structure calculations were carried out on 1, 4 and 5 and their protonated forms. Calculated structural parameters were compared with crystallographic data where possible. The computed atomic net charge on the oxido oxygens of 1, 4 and 5 was found to track the reactivity toward PNPDPP. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Ill.
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  • 55
    ISSN: 0894-3230
    Schlagwort(e): Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The 13C chemical shifts of eight series of para- or meta-substituted benzaldehyde 2-aminobenzoylhydrazones possessing both amide and imine functionalities were measured. The 13C chemical shifts were used to study the transmission of electronic substituent effects along the heteroaromatic side-chain of the substituted aromatic ring. In addition to the C=N bond, the benzoylhydrazones possess in their side-chain polarizable C=O and phenyl π-units. The benzylidenic ring-substituent chemical shifts were analysed by the dual substituent parameter approach to separate the inductive and resonance effects. The negative ρI and ρR values observed (i.e. reverse substituent effects) indicate a significant π-polarization of the C=N bond. The highly negative ρR values, especially those in the case of meta substitution, suggest a contribution from a marked secondary field-transmitted resonance effect. The results are compared with those obtained for other hydrazones or imines. Variation of the electron-withdrawing ability of the N2 substituent is seen to have a systematic effect on the ρI values. Reverse substituent effects are also observed at the C-1″ site of the 2-aminobenzoyl ring while C-4″ shifts show normal behaviour, consistent with the general concept of the π-polarization that each π-unit of the side-chain is polarized largely as a localized system. Accordingly, the π-polarization effect is seen efficiently to propagate also along a heteroaromatic chain. On the other hand, the C=O sites exhibit normal, although fairly slight, dependence on the benzylidenic substituent indicating an insignificant role of π-polarization at that site. The effects of the solvent, CDCl3 vs. DMSO-d6, on the ρI and ρR values are also considered. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 14 Ill.
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  • 56
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 107-112 
    ISSN: 0894-3230
    Schlagwort(e): 2-(2′-methoxy-3′-α-cumylphenyl)benzotriazole ; conformational preference ; light stabilizers ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---A pronounced conformational preference of the 3′-α-cumyl substituent in 2-(2′-methoxy-3′-α-cumylphenyl)benzotriazole is suggested from the results of molecular dynamics simulations. This suggestion is supported by both solution NMR spectral and solid-state x-ray crystallographic data. The orientation of the α-cumyl substituent may have implications on the relative performance of 3′-substituted 2′-hydroxyphenylbenzotriazole light stabilizers in polar media. © 1997 by John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Ill.
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  • 57
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 196-206 
    ISSN: 0894-3230
    Schlagwort(e): ethane-1,1-diol ; ethane-1,1,2-triol ; conformation ; ab initio study ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Ab initio optimizations at the HF/6-31G level and single-point calculations at the MP2/6-31G**//6-31G level were performed on ethane-1,1-diol and ethane-1,1,2-triol. Their conformational properties are discussed in terms of the anomeric effect, gauche effect and internal O-H interactions. The results showed a parallel behaviour with ethane-1,2-diol. The solvent effect was taken into account using the SCRF theory with a general cavity shape which is defined by the molecular surface. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 4 Ill.
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  • 58
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 233-241 
    ISSN: 0894-3230
    Schlagwort(e): surface effects ; radical anions ; benzoyl radical ; coupling of radicals ; surface catalysis ; electron transfer ; reactions in the double laye ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Benzaldehyde in THF reacts with lithium metal to give the expected reduction product, benzyl alcohol, plus benzoin and benzyl as minor products. The kinetics of the overall reaction as well as the partial rate coefficients of the several steps have been determined under various reaction conditions. It was found that adsorption on the surface and electron transfer from the lithium to benzaldehyde are the slow steps. The experimental results show interesting surface effects, and evidence for significant radical intermediates, which were characterized by their epr spectra and trapping experiments, results useful to gain insight into the mechanisms of these and other related reactions. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 6 Ill.
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  • 59
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 292-304 
    ISSN: 0894-3230
    Schlagwort(e): calixarenes ; bipyridine ; lanthanides ; luminescence ; preorganization ; solvent effect ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Based on MD simulations, the structures of Eu3+ and EuCl3 complexes of bipyridine-substituted calixarenes in different environments (in vacuo and in acetonitrile and water solutions) were modelled. The data account for the differences in their luminescence properties. The role of the calixarene platform, preorganization of the ligands, and an explicit account of counterions in different environments are analysed. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 6 Ill.
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  • 60
    ISSN: 0894-3230
    Schlagwort(e): aryliminodimagnesium ; condensation with NO2 and CO groups ; addition to CN group ; single electron transfer efficiency ; σ-complexation ability ; inter- and intramolecular competition ; bifunctional substrates ; positional effect of functional groups ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The reactions of aryliminodimagnesium [ArN(MgBr)2, IDMg] with p′-substituted p-cyanobenzophenones, 1-cyano-9-fluorenone, o-, m- and p-dicyanobenzenes and o-, m- and p-nitrobenzonitriles were examined, and the relative yields of products were referred to the previous results of electron spin resonance studies. The products of condensation with carbonyl and nitro groups and of addition to cyano groups were formed. From the variations of the yields caused by the molar ratio of the magnesium reagent versus substrates and by the substituents of both reactants, the preferred groups were determined. The groups of p- and m-substrates are consistent with the position of highest density of free electrons in the anion radicals, whereas those of the o-substrates are inconsistent. The consistency indicates competition of functional groups reflecting the relative ability of single electron acceptance from the reagent, whereas the inconsistency is ascribed to cooperation of neighbouring groups for σ-complexation with the Mg atom of the reagent. The categories of competition and cooperation are discussed in relation to fundamental features and general governing factors proposed for the IDMg reactions of monofunctional substrates. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 8 Ill.
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  • 61
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 305-310 
    ISSN: 0894-3230
    Schlagwort(e): non-covalent interactions ; host-guest complexes ; fluorinated phenyl compounds ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Complexation constants with the macrocyclic azoniacyclophane CP44 and phenyl guest compounds with at least four fluorine atoms or alternatively protons at the ring were obtained by NMR shift titrations in water. The fluorinated compounds show free energies of complexation which are smaller by ΔΔG=3·4-7·7 kJ mol-1 in comparison with the protonated compounds. The NMR shifts induced upon 100% complexation (CIS values) were obtained simultaneously from non-linear least-squares fitting and indicate intra-cavity inclusion in all cases. The CIS values agree roughly with screening constants calculated from aromatic ring current and linear electric field effects, the latter resulting from the permanent charges at the host compound. Molecular mechanics calculations (CHARMm) indicate that intracavity inclusion is possible with all compounds with negligible strain induced (〈1 kJ mol-1) in the macrocycle upon complexation. In contrast, α-cyclodextrin can accommodate fluorinated phenyl compounds only at the rim of the cavity without larger strain. Preliminary data with α-cyclodextrin, obtained by competitive UV-visible titration with methyl orange, indicate again a smaller association free energy (ΔΔG=1·-7 kJ mol-1) for pentafluorphenol compared with normal phenol as guest. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Ill.
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  • 62
    ISSN: 0894-3230
    Schlagwort(e): hydrophobic cavity characterization ; cyclophanes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The guest-binding behavior of two different cyclophane hosts, each being capable of providing a three-dimensionally extended hydrophobic cavity toward aromatic guests, was examined in aqueous media: a steroid cyclophane bearing four rigid cholate moieties and an octopus cyclophane having four flexible double-chain segments. Even though the binding constant for 2,7-dihydroxynaphthalene with the steroid cyclophane was comparable to that with the octopus cyclophane, the guest binding modes were very different from each other, as confirmed by 1H NMR spectroscopy. That is, the steroid cyclophane incorporates the guest into its rigid macrocyclic cavity with axial geometry whereas the octopus cyclophane provides a three-dimensional space created by the macrocyclic skeleton and the flexible hydrocarbon chains so that the long axis of the guest becomes more or less perpendicular to the molecular axis of the host upon complexation. Temperature-dependent molecular recognition by these hosts toward 8-anilinonaphthalene-1-sulfonate was examined by means of fluorescence spectroscopy. Characteristic differences in the guest-binding mode between these hosts were sensitively reflected in the thermodynamic entropy change on host-guest complexation and the temperature-dependent microscopic viscosity experienced by the guest at the binding site. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Ill.
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  • 63
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 254-272 
    ISSN: 0894-3230
    Schlagwort(e): π-π interactions ; self-assembly ; catenanes ; cyclophanes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The recent surge of interest in the control of molecular organization in both the solution state (i.e. self-assembly) and the solid state (i.e. crystal engineering) has led researchers to recognize increasingly the importance of weak non-covalent interactions. The design and synthesis of an efficient molecular construction set are dependent upon a very close interplay between x-ray crystallography and synthetic chemistry. π-π Stacking interactions between π-donors, such as hydroquinone, resorcinol or dioxynaphthalene residues, and π-accepting ring systems, such as bipyridinium or π-extended viologen units, can govern the self-assembly of a variety of complexes and interlocked molecular compounds in both the solid and solution states. Non-covalent bonding interactions (i.e. π-π interactions) can be considered as information vectors: they define and rule the self-assembly processes that lead to the formation of the desired molecular and supramolecular architectures, and thereafter they still govern the dynamic processes occurring within the self-assembled structures and superstructures. The manner in which such molecules and supermolecules can contribute to an understanding of non-covalent interactions at both structural and superstructural levels is described, with reference to numerous examples of self-assembly processes in synthesis, of dynamic processes in the solution state, and of the packing of molecules and molecular complexes in the solid state. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 31 Ill.
    Materialart: Digitale Medien
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  • 64
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 323-334 
    ISSN: 0894-3230
    Schlagwort(e): redox-switched amphiphiles ; ferrocene derivatives ; vesicles ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Thirty ferrocene derivatives were prepared and their ability to form vesicles in aqueous solution when oxidized was assessed. The compounds included alkyl ferrocenylmethyl ether derivatives of the form C10H9FeCH2OR in which R=octyl, dodecyl, tetradecyl, hexadecyl, octadecyl and eicosanyl. One single-tailed amine derivative, C10H9FeCH2NR2, R=octadecyl, was studied. Alkylferrocene derivatives had the form C10H9FeR in which R=butyl, decyl, tetradecyl, hexadecyl, octadecyl, eicosanyl and docosanyl. Sixteen symmetrical 1,1′-disubstituted ferrocenes were also studied. Three ethers were of the form C10H8Fe-1,1′-(CH2OR),2, R=tetradecyl, hexadecyl and octadecyl. Four corresponding dialkyl derivatives of the form C10H8Fe-1,1′-R2, R=decyl, tetradecyl, hexadecyl and octadecyl, were assessed. Finally, a range of 1,1′-disubstituted ferrocene derivatives were analyzed. These all had the form C10H8Fe-1,1′-(COR)2, for which R has the following identities: octyl, tridecyl, pentadecyl and heptadecyl (ketones); heptadecyloxy, 3-cholesteryl and 3-cholestanyl (esters); and two amides, R=NHC18H37 and N(C18H37)2. The alkyl and ether derivatives could be readily oxidized and formed vesicular aggregates upon sonication. The ketones, esters and amides could be oxidized but the ferricenium derivatives did not form stable aggregates. An interesting observation is that the aggregates formed were vesicular whether the ferrocene derivative had one or two alkyl tails. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
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  • 65
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 273-285 
    ISSN: 0894-3230
    Schlagwort(e): cation-π interactions ; calix[n]arenes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Non-covalent intermolecular forces have been recognized as a very important part of molecular interactions in complex biological systems. The fundamental functions of living matter such as transcription of genetic information in DNA, spatial arrangement of protein molecules, enzymatic functions or immunity system response are enabled owing to the presence of weak non-covalent forces based on hydrogen bonding interactions, van der Waals interactions, electrostatic interactions, hydrophobic effects, etc. Recently, among them so-called "cation-π" interactions have been proved to contribute to the overall binding process in various artificial or biological systems. Calix[n]arenes have emerged as an important family of molecules with promising applications in many branches of chemistry. Because of their suitable molecular preorganization with aromatic units being "concentrated' in a relatively small space, calix[n]arenes represent interesting compounds exhibiting an enhanced ability for cation-π interactions. The importance of such forces in calix[n]arene chemistry is demonstrated here on several recent examples. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 25 Ill.
    Materialart: Digitale Medien
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  • 66
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 335-342 
    ISSN: 0894-3230
    Schlagwort(e): hydrophobic interactions ; aqueous solutions ; living systems ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Evidence is provided for the enormous role of hydrophobic interactions in aqueous solutions. It is concluded that complementary functions of hydrophilic and of hydrophobic species are in continuous operation and that these are a conditio sine qua non for the existence of the liquid. With regard to the supermolecular aggregations, hydrophobic interactions are operative on the highest hierarchic levels of the system organization, i.e. at the interface and at the holes around dissolved hydrophobic species. It is further emphasized that water is essential both for the unity and for the differentiation of each living organism. With regard to its role for the differentiation of the body, the hydrophobic interactions are of paramount importance, notably those provided by amphipathic solutes. The cell membrane is considered to be the result of interactions between the highest hierarchic levels of intracellular and extracellular water. The hydrophobic double layer provides the barriers for the separation of intracellular and extracellular water and at the same time the connections for recognition and for exchange of information between them. The DNA structures obtain conservative boundary conditions for their surrounding water systems which do not freeze at -60 °C. These water systems are dynamically superior to all other parts of the water system of the organism. It is suggested that more knowledge about the abilities of liquid water can be obtained by giving appropriate attention to its properties within the living body. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 67
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 514-524 
    ISSN: 0894-3230
    Schlagwort(e): eterocycles ; donor/acceptor units ; push-pull conjugated molecules ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The synthesis and spectroscopic investigation of a number of push-pull ethenes in which the donor moiety is represented by a π-excessive five-membered heterocycle (pyrrole, indole and thiophene) and the acceptor group is a π-deficient heterocyclic azine ring (pyridine, pyrazine, pyrimidine, pyridazine) are described. The intramolecular charge transfer in both the neutral compounds and the corresponding N-alkylpyridinium triflates is discussed and confirmed on the basis of three different descriptors, ΔλHetPh,Δλ+n, and Δλsolv1solv2, that take into account the substitution of a phenyl with a heterocyclic donor ring, charge effects and solvatochromism, respectively. According to the ΔλHetPh descriptor, the intramolecular charge transfer in the described diheteroarylethenes increases upon increasing the electron-withdrawing capacity of the acceptor, sustained by the presence of either more than one nitrogen atom or the positive charge in the heterocyclic azine. The described pyridinium derivatives belong to the rarely investigated class of dimethine cyanine dyes. The response of the 13C and 15N NMR chemical shift data appears to be less clear because of the low sensitivity of the NMR probes to remote substitution. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 4 Tab.
    Materialart: Digitale Medien
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  • 68
    ISSN: 0894-3230
    Schlagwort(e): trianylmethanes ; 9-arylxanthenes ; ion and radical staleclitres ; amphihydric compounds ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Thermodynamic stability properties of 11 p-substituted trityl and seven 9-phenylxanthyl carbocations are reported in sulfolane and of their conjugate carbanions in DMSO. The cations are compared by calorimetric heats of hydride transfer from cyanoborohydride ion, their first and second reduction potentials, their pKR+s in aqueous sulfuric acid, 13C chemical shifts and free energies of methoxy exchange. Carbanions are compared by their heats and free energies (pKHA) of deprotonation and their first and second oxidation potentials. Radicals are compared by their oxidation and reduction potentials. Their bond dissociation energies are derived by alternative routes: from the carbocation and its reduction potential and from the carbanion and its oxidation potential. The various properties are correlated against each other and against appropriate Hammett-type substituent parameters. Correlations between the different measured properties reported here range from fair to excellent. Despite their importance as historic prototypes for the three trivalent oxidation states of carbon, trityl and xanthyl systems are atypical models for comparing transmission of electron demand in other series of carbocations, radicals or carbanions with significantly different structures. The 9-arylxanthyl series is especially poor because of its insensitivity to substituent effects. The effects of substituents on various properties which represent the stabilities of R+s correlate surprisingly well against those for corresponding R-s. Accordingly, compensating effects on the oxidation and reduction of a series of related R·s may lead to a nearly constant electron transfer energy and absolute hardness for the series. In contrast, the free energies for interconversion of the carbocations and carbanions which determine the gap between pKR+ and pKHA are very sensitive to structural change. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 69
    ISSN: 0894-3230
    Schlagwort(e): n-Butyllithium ; benzoic acid ; nucleophilic addition ; deprotonation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---An evaluation of a branching vs sequential mechanism for the reaction of benzoic acid with n-butyllithium favors the latter. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 70
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 347-350 
    ISSN: 0894-3230
    Schlagwort(e): benzene · · · hexafluorobenzene ; supramolecular synthon ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: The benzene · · · hexafluorobenzene stacking interaction was evaluated at several levels of theory. At the MP2/6-31G** level, it is estimated that the interaction is stabilizing by approximately 3.7 kcal mol-1. Lower levels of theory perform poorly on this system. This is a fairly strong non-covalent interaction, suggesting this motif may be a valuable supramolecular synthon. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 1 Ill.
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  • 71
    ISSN: 0894-3230
    Schlagwort(e): N-(aminoalkyl)-9-phenanthrenecarboxamides ; molecular structures ; folded conformations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The molecular structures of three tertiary N-(aminoalkyl)-9-phenanthrenecarboxamides were investigated in solution and the solid state by means of 1H NMR spectroscopy and x-ray crystallography. The tertiary amides exist as a mixture of E and Z isomers in solution and the aminoalkyl groups exist as a mixture extended and folded conformers. A crystalline N-(aminoethyl)amide was obtained as the pure Z isomer in which the phenanthrene and amide planes are nearly perpendicular and the aminoethyl group is folded over the less hindered face of the amide group. Rotation about the Ar-C(O) bond is slow in resolution rendering these molecules chiral on the NMR time-scale. As a consequence, the α-methylene protons display large diasterotopic splittings when the aminoalkyl group is syn to the amide carbonyl. Folded conformations place the Z and E aminoalkyl groups in the deshielding and shielding regions, respectively, of the phenanthrene rings, resulting in large differences in chemical shifts. © 1997 John Wiley & Sons, Ltd.
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  • 72
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 563-576 
    ISSN: 0894-3230
    Schlagwort(e): free energies of solvation ; chloroform ; water ; linear response ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Monte Carlo statistical mechanics simulations were used to compute absolute free energies of solvation in chloroform for 16 organic molecules. The intermolecular interactions were described by classical potential functions consisting of Coulomb and Lennard-Jones interactions. The partial charges for the solutes were derived from fitting to the electrostatic potential surfaces of ab initio 6-31G* wavefunctions. First, free energy perturbation (FEP) calculations yielded relative free energies of solvation. These were converted to absolute quantities through perturbations to the reference molecule, methane, which was annihilated. The average error in the FEP-computed free energies of solvation is 0·8 kcal mol-1. Then, a linear response equation, which contains terms proportional to the Lennard-Jones (van der Waals) and Coulombic components of the solute-solvent energy and to the solvent-accessible surface area of the solute, was optimized and reproduced both the FEP-calculated and experimental free energies of solvation with average errors of ca 0·5 kcal mol-1. In addition, an existing solute dataset for water, which had previously been fitted to the same equation, was expanded from 16 to 35 molecules. The fit of the Monte Carlo results for this set of molecules in TIP4P water to the experimental free energies of hydration yielded an average error of 0·8 kcal mol-1. Combination of the predictions of free energies of solvation in water and chloroform yields partition coefficients, log P, with an average error of 0·3-0·4 log unit. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 73
    ISSN: 0894-3230
    Schlagwort(e): inclusion complexation ; cyclobis (paraquat-p-phenylene) ; cyclophanes ; substituted aromatics ; cooperative non-covalent cavity ; external interactions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The cooperative nature of non-covalent interactions which give rise to inclusion complexes involving cyclobis(paraquat-p-phenylene), 14+, and related cyclophane derivatives, 24+-44+, with substituted 1,4-phenyl and 4,4′-biphenyl guests has been studied by spectroscopic techniques and ab initio and semiempirical molecular orbital methods. Inclusion complex formation and stability are primarily determined by the combination of two main interaction modes involving aromatic stacking of the guest within the cyclophane cavity and external interactions between guest side arms and the exterior of the cyclophane. A balance between cavity and external forces results in supramolecular association and is shown to change depending upon the functionality and substitution of the guest. Cavity binding was probed using 1,4-phenyl and 4,4′-biphenyl guests, where for the 1,4-phenyl guests the primary basis for energy stabilization with 14+ is found to be short-range stabilizing electrostatic forces complemented by small amounts of polarizability and charge-transfer. In contrast, the cavity binding between substituted 4,4′-biphenyl guests and 14+ is determined by almost equal contributions of polarizability and electrostatics. The effect of solvent is shown to have only a small effect on the computed geometry of 14+ complexes, but its impact upon binding energies is substantial. The first solvation shell of the cyclophanes is computationally approximated by 12 acetonitriles and satisfies the requirements of the 16 relatively acidic protons on the bipyridinium groups. Good correlations between the computed (with solv ation) and experimental 14+ binding energies are found. The degree of linear correlation improves substantially when the comparison between computed and experimentally observed binding energies is restricted to structurally similar (number of aromatic rings, number of substituents and position of substitution) molecular guests. Furthermore, computed molecular properties, such as polarizability, maximum hardness, softness and electronegativity of the isolated guests, correlate well with 14+ binding energies based upon the same requirement of guest similarity. The non-covalent forces associated with the external cyclophane interactions were studied with guest molecules built from symmetrical 1,4-extensions of hydroquinone composed of aliphatic or ethyleneoxy side arms. In particular, side arm length and functionality, and the position and type of heteroatoms along the chain, were systematically varied to define the external interactions between the guest side arms and different host cyclophanes. Specifically, the ethyleneoxy linkages are shown to provide a large chelate and cooperative effect which direct the binding with 14+. In order to probe further the special geometric and electronic character of 14+, we have synthesized and tested a new supramolecular host, 24+, similar to 14+ but where a pentacycloundecane unit replaces one of the xylyl groups. Both experimental and computed data on the new host emphasize the ideal geometry and electronic nature of the 14+ molecular receptor for aromatic guests. The inclusion complexes discussed in this paper are important not only because they, or similar entities, are the main components of many rotaxanes, catenanes and other switchable molecules, but because the intermolecular interactions involved, such as electrostatics, polarizability and charge-transfer, are ubiquitous in supramolecular chemistry. The information reported on the specific interactions involving the 14+-44+ molecular receptors with substituted aromatic guests can also be extended by analogy to many systems of broad interest. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 10 Ill.
    Materialart: Digitale Medien
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  • 74
    ISSN: 0952-3499
    Schlagwort(e): bovine α-chymotrypsin ; bovine basic pancreatic trypsin inhibitor (Kunitz-type inhibitor) ; serine proteinase:Kunitz inhibitor complex ; crystal structure ; Chemistry ; Biochemistry and Biotechnology
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Medizin
    Notizen: The crystal structure of bovine α-chymotrypsin (α-CHT) in complex with the bovine basic pancreatic trypsin inhibitor (BPTI) has been solved and refined at 2.8 Å resolution (R-factor=0.18). The proteinase:inhibitor complex forms a compact dimer (two α-CHT and two BPTI molecules), which may be stabilized by surface-bound sulphate ions, in the crystalline state. Each BPTI molecule, at opposite ends, is contacting both proteinase molecules in the dimer, through the reactive site loop and through residues next to the inhibitor's C-terminal region. Specific recognition between α-CHT and BPTI occurs at the (re)active site interface according to structural rules inferred from the analysis of homologous serine proteinase:inhibitor complexes. Lys15, the P1 residue of BPTI, however, does not occupy the α-CHT S1 specificity pocket, being hydrogen bonded to backbone atoms of the enzyme surface residues Gly216 and Ser217. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 5 Ill.
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  • 75
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 3-21 
    ISSN: 0894-3230
    Schlagwort(e): Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Monochloroalkanes were studied using molecular mechanics and parameters were developed and incorporated into MM3. The new force field reproduces well molecular structures, conformational energy differences and vibrational frequencies. The compounds studied were methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, neopentyl, cyclopentyl and cyclohexyl chloride, 4-tert-butyl-1-chlorocyclohexane and 1-chloroadamantane. The overall rms error for the averaged C - C and C - Cl bond lengths for the above set of compounds excluding sec-butyl chloride, neopentyl chloride and 1-chloroadamantane is 0·0047 Å, and the rms error for the vibrational frequencies is 377thinsp;cm-1. The moments of inertia (rg) are approximately 1% greater than the microwave values (ra0). Thermodynamic quantities are also compared with those observed using various experimental methods. © 1997 John Wiley & Sons, Ltd.
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  • 76
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 138-144 
    ISSN: 0894-3230
    Schlagwort(e): amide-alcohol solvent systems ; densities ; volumetric behaviour ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Densities were measured for the N-methylformamide-(C1-C10)alkan-1-ol and N,N-dimethylformamide-(C1-C10)alkan-1-ol solvent mixtures at 298·15 K over the whole miscibility range (0〈X1〈1). The excess volumes (VE), which are discussed in terms of intermolecular interactions, are negative for the mixtures with the smallest alkanols in the series and increase with increase in the chain length. Comparison with other amide-alcohol mixtures previously investigated (formamide and pyrrolidin-2-one) indicates that the excess volumes are strongly influenced by the size of the alcohol and to a lesser extent by the nature of the amide. In contrast with the amide-water system, neither the amide size nor its degree of substitution plays an important role in the amide-alcohol systems investigated. © 1997 by John Wiley & Sons, Ltd.
    Zusätzliches Material: 5 Ill.
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  • 77
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 159-166 
    ISSN: 0894-3230
    Schlagwort(e): diphenylmethanols ; chloramine-B ; oxidation ; kinetics, HCl medium ; ruthenium(III) ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The kinetics of oxidation of six para-substituted diphenylmethanols (Y-DPM, where Y=H, Cl, Br, NO2, CH3 and OCH3) by sodium N-chlorobenzenesulphonamide [chloramine-B (CAB)] in the presence of HCl and catalysed by RuCl3 in 30% (v/v) methanol medium was studied at 35 °C. The experimental rate law is rate=k′[CAB]0[DPM]x0[RuCl3]y[H+]z, where x, y and z are fractions. Addition of reaction product, benzenesulphonamide (BSA), retards the reaction. An increase in the dielectic constant of the medium decreases the rate. Rate studies in D2O medium showed that the solvent isotope effect k′(H2O)/k′(D2O)=0·53. Proton inventory studies were carried out using H2O-D2O mixtures. The rates correlate satisfactorily with the Hammett σ relationship and the plot is biphasic. The reaction constant ρ is -2·8 for electron-releasing groups and -0·31 for electron-withdrawing groups at 35 °C. The activation parameters ΔH
    Zusätzliches Material: 5 Ill.
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  • 78
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 182-186 
    ISSN: 0894-3230
    Schlagwort(e): enol of acetoacetic acid ; relative stability ; computational study ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Ab initio molecular orbital calculations at the MP2/6-311+G**//MP2/6-311+G** level show that the enol tautomer of acetoacetic acid in which the ketone group is enolized, 2, is more stable than its isomer in which the carboxylic acid group is enolized, 3, by 11·3 kcal mol-1. This difference may be attributed to a difference in the strength of resonance interactions between the carbonyl and hydroxyl groups of the enols: the carbonyl group in both isomers is conjugated with two hydroxyl groups, but in the acid enol, 3, both interactions are vinylogous, whereas in the ketone enol, 2, one of the vinylogous effects is replaced by a stronger direct interaction. © 1997 by John Wiley & Sons, Ltd.
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  • 79
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 383-395 
    ISSN: 0894-3230
    Schlagwort(e): hydrogen bond strength ; organic groups ; isotopic substitution ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Although the electronic contribution to the strength of a H-bond is unaffected by isotopic substitution, the heavier mass of deuterium compared with protium lowers some of the vibrational frequencies in the complex. The binding energy of the complex, which includes zero-point and thermal vibrational energies, can thus be altered by several tenths of a kcal mol-1 by H/D substitution. Ab initio calculations are used to analyze this phenomenon in a number of common organic functional groups that are prone to form H-bonds: hydroxyl, carboxyl and amide, both self-complexing as homodimers and with water molecules as partners. It is found that any site of D-substitution increases the complexation energy; however, the bridging sites show a stronger preference for D over H than do the non-bridging, or terminal, sites. Hence D-bonding can be considered to be stronger than H-bonding in these functional groups. Of the groups considered, the energetic preference for D over H is greater in the hydroxyl group, so deuterium would be expected to gravitate toward solvent water molecules in isotopic scrambling experiments. The increments in H-bonding energy resulting from each site of substitution are addititve in cases of multiple substitution. © 1997 John Wiley & Sons, Ltd.
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  • 80
    ISSN: 0894-3230
    Schlagwort(e): molecular recognition ; non-aqueous titration ; hydrogen bonding ; proton transfer ; pKa shifts ; acid-base catalysis ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Whenever hydrogen bonding is involved in molecular recognition, the possibility of a proton transfer from the donor to the acceptor arises. In most cases the pKa of the donor is far enough above the pKa of the conjugate acid of the acceptor for it to be clear that no proton transfer will occur. However, as the difference between the donor and acceptor pKas decreases, it can become difficult to predict whether a proton transfer will occur. Since most hydrogen bond-driven molecular recognition is studied in low dielectric solvents, non-aqueous titrations can be used to measure the pKas and therefore predict proton transfers. In this paper three studies which involved non-aqueous titrations are summarized. The first deals with distinguishing simple proton transfer from host-guest complex formation. The second involves measuring pKa shifts upon host-guest complex formation. The last is a study of the catalysis of a phosphoryl transfer. In all three scenarios the non-aqueous titration method gave results which would have been difficult to obtain by other means, and which proved crucial for a complete understanding of the molecular recognition process. © 1997 John Wiley & Sons, Ltd.
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  • 81
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 405-413 
    ISSN: 0894-3230
    Schlagwort(e): non-covalent interactions ; thermodynamic hydrogen bond parameter ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---This paper describes how empirical free energy and/or enthalpy values for hydrogen binding strength are derived from thousands of corresponding measurements between H-bond donors and acceptors, mostly in carbon tetrachloride, and how they can be used to construct common scales or factor values (increments) also for other reactions involving electron donor and acceptor ability of functions. The corresponding databases and programs (HYBOT) allow one to predict thermodynamic values for experimentally unknown equilibria, including also ionophore complexes with crown ethers or cryptands. Applications in QSAR involve the prediction of lipophilicity from any structure on the basis of only two variables, e.g. 234 systems are described this way with a correlation coefficient r=0·96. Similarly, permeabilities and some biological properties such as narcotic activities of chemicals and anti-HIV-1 activity of some porphyrins are evaluated. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 2 Ill.
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  • 82
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 908-916 
    ISSN: 0894-3230
    Schlagwort(e): AM1-SM2.1 ; transition-state imbalance ; benzylidenemalonotriles ; nucleophilic additions ; Brønsted coefficients ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Semiempirical MO theoretical studies were carried out on the nucleophilic addition of phenoxide nucleophiles to 1,1-dicyano-2-arylethenes in the gas phase and in water using the AM1 method and the Cramer-Truhlar solvation model SM2.1. The gas-phase αn and βn values are 0·81 and 0·65, respectively, leading to the positive imbalance of I=0·16; the electric polarization and dispersion interactions of water incorporated in the SM2.1 model reduce both the αn and βn values to 0·61 and 0·36, giving I=0·25. The two Brønsted coefficients obtained theoretically in water agree satisfactorily with the experimental values (αncorr=0·55 and βn=0·35) obtained with amine bases. The small imbalance found both theoretically (I=0·25) and experimentally (I≈0·20) can be ascribed to (i) a near-zero distance factor, Δd=dp-dTS≈0 and (ii) the small extent of negative charge localization by strucutral reorganization in the transition state. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Ill.
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  • 83
    Digitale Medien
    Digitale Medien
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 121-126 
    ISSN: 0947-6539
    Schlagwort(e): crystal structure ; NMR spectroscopy ; proton transfer ; pyrazoles ; tautomerism ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The crystal structure of the title compound was determined by X-ray analysis at 200 K. Three independent molecules form a trimer joined by strong and linear N-H … N hydrogen bonds. There is another centrosymmetrically related trimer in the unit cell. Both tautomers (1H and 2H) are present in each trimer. Disorder of the NH protons involved in the N-H … N hydrogen bonds has been observed. Solid-state 13C CPMAS NMR was used to establish the dynamic nature of the NH-proton disorder, the title compound being the first example of proton transfer in a tautomeric mixture of pyrazoles with an equilibrium constant other than 1.
    Zusätzliches Material: 5 Ill.
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  • 84
    ISSN: 0947-6539
    Schlagwort(e): crystal structure ; cyclophanes ; electrochemistry ; porphyrazines ; tetrathiafulvalenes ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The pyrazinoporphyrazine system 13 (metal-free, zinc and copper derivatives) has been synthesised by tetramerisation of 2,3-dicyanopyrazine monomer unit 10. The structure of 13a-c has been established by 1H NMR spectroscopy, UV/Vis spectrophotometry, MALDI-TOF mass spectrometry, cyclic voltammetry and differential pulse voltammetry. The electrochemical redox behaviour of 13a-c is strongly solvent dependent. The expected two-stage oxidation of the tetrathiafulvalene (TTF) units of 13a-c was observed in a range of solvents; in addition, oxidation and reduction of the pyrazinoporphyrazine core of the metal-free derivative 13a was detected in benzonitrile. On excitation of 13 in the Q-band region no fluorescence was observed, which is presumably the consequence of intramolecular charge transfer between the TTF moieties and the excited state of the central porphyrazine. Molecular modelling studies on 13a and 13c are reported. During the course of this work, the novel TTF macrocycles 11 and 20 were synthesised; their X-ray crystal structures reveal severely bent TTF units, the conformations of which are discussed in detail. The X-ray crystal structures of the bis(1,3-dithiole) systems 15 and 18 have also been determined.
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  • 85
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 49-54 
    ISSN: 0894-3230
    Schlagwort(e): Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Various diallyl- and allylpropargylamines were pyrolyzed in a stirred-flow reactor at temperatures in the range of 330-500°C, pressures of 8-16 Torr and residence times of 0·4-2·4 s using toluene as carrier gas. The reaction products were propene, allene and aldimines. The allene to propene ratio changed considerably with the third substituent on the N atom. At conversions in the range 13-55%, the consumption of the amines showed first-order kinetics. The following Arrhenius parameters [A (s-1) and Ea (kJ mol-1)] were obtained from the rate coefficients: diallyl-tert-butylamine, log  A = 10·32 ± 0·12, Ea = 139 ± 2; diallylneopentylamine, log A = 12·87 ± 0·26, Ea = 168 ± 3; allylpropargyl-tert-butylamine, log A = 10·23 ± 0·18, Ea = 126 ± 2; allylpropargyltosylamine, log A = 13·05 ± 0.36, Ea = 197 ± 5; and allylpropargylphenylamine, log A = 12·10 ± 0·35, Ea = 162 ± 4. Polar, six-center cyclic transition states are suggested for the elimination of propene and allene, via non-concerted bond breaking-bond forming mechanisms. © 1997 John Wiley & Sons, Ltd.
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  • 86
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 67-75 
    ISSN: 0894-3230
    Schlagwort(e): benzenediazonium ion ; host-guest complexation ; dediazoniation ; crown ethers ; ring size ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The host-guest complexation and the kinetics of the thermal dediazoniation of benzenediazonium tetrafluoroborate in the presence of unsubstituted and (di)benzene- and dicyclohexane-substituted crown ethers containing 4-10 oxygen atoms were studied by UV spectrophotometry in 1,2-dichloroethane at 40°C. The complexation equilibrium constants K and the stabilizing ability of the complexation (k2/k1) were calculated by a kinetic method. Complexation in the gas phase was observed and characterized by fast atom bombardment mass spectrometry (FAB-MS). All complexing agents except 12-crown-4 formed 1:1 complexes [crown ether-PhN2]+ under FAB conditions. The complexation caused a hypsochromic shift Δλmax in the UV spectrum of the benzenediazonium salt, which was largest for hosts containing six oxygen atoms. The thermodynamic and kinetic stability were much greater for insertion-type complexes containing six or more oxygen atoms in the host molecule than for the charge-transfer complexes formed with 15-crown-5. In contrast, 12-crown-4 destabilized benzenediazonium ion owing to the increase in homolytic dediazoniation. 21-Crown-7 was the strongest complexing and stabilizing agent for benzenediazonium ion; with a larger hole size in the host the effects weakened. The effects of benzene and cyclohexane substituents in crown ethers on the thermodynamic and kinetic stability were small compared with the effects of the number of oxygen atoms. © 1997 by John Wiley & Sons, Ltd.
    Zusätzliches Material: 2 Ill.
    Materialart: Digitale Medien
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  • 87
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 91-96 
    ISSN: 0894-3230
    Schlagwort(e): nucleophilic substitution reactions ; cycloalkylmethyl arenesulfonates ; positive ρXZ values ; secondary kinetic isotope effects ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Nucleophilic substitution reactions of cycloalkylmethyl arenesulfonates (CmH2n-1CH2OSO2C6H4Z) with anilines (XC6H4NH2) in methanol at 65.0 °C were studied. The reactivity order (n=4〉6〉7〉5) reflects largely the order of steric effect of the ring size (SEs term) except for n=5, which exhibits the least reactivity. This reversal of the order for n=5 is considered to result from large rate retardation due to polar effect of the ρ*σ* term. Application of the Taft equation to the rate data for n=5 and 6 gives ρ*=17·4 and S=2·3 with correlation coefficient of 0·90. The σ* values for n=4 and 7 are estimated to be -0·23 and -0·11, respectively. The positive ρXZ values of ca 0·3 are consistent with previous results for the reactions at primary reaction centers. © 1997 by John Wiley & Sons, Ltd.
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  • 88
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 113-120 
    ISSN: 0894-3230
    Schlagwort(e): ethylene oxide ; potential energy surfaces ; ab initio calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Ab initio calculations, including electron correlation, were employed to compute the geometries and energies of all stable C2H4O species, as well as four transition states along the potential surfaces connecting oxirane to the unimolecular dissociation products. The calculations indicate that the primary step in the major reaction observed experimentally is the isomerization of oxirane along the ground-state potential surface to acetaldehyde. Calculations indicate that the experimental reaction products are derived from unimolecular decomposition on the acetaldehyde S0 surface (CH4+CO) or, after intersystem crossing, along the lowest triplet state of acetaldehyde (CH3+HCO). Additional pathways connecting oxirane to a number of less energetically favorable ring-opened or fragmentation products are also presented. © 1997 by John Wiley & Sons, Ltd.
    Zusätzliches Material: 3 Ill.
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  • 89
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 129-137 
    ISSN: 0894-3230
    Schlagwort(e): electron transfer ; outer-sphere electron transfer ; mechanism ; nucleophile ; electrophile ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: Most organic bond-forming reactions can be described as nucleophile-electrophile combinations. For many of these reactions there have been discussions as to whether the bond formation occurs together with the electron transfer or whether electron transfer and bond formation take place in separate steps. This discussion is reviewed and it is shown how to distinguish between the different processes based on kinetic analysis. © 1997 by John Wiley & Sons, Ltd.
    Zusätzliches Material: 12 Ill.
    Materialart: Digitale Medien
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  • 90
    ISSN: 0894-3230
    Schlagwort(e): cyclic sulfamates ; nucleophiles ; reactivity ; kinetics ; mechanism ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The kinetics of the reaction of four X-3-(p-tolylsulfonyl)-1,2,3-benzoxathiazole 2,2-dioxides (X=5-Cl, 5-Br, 5-NO2 and 6-NO2) with hydroxide ion and imidazole in aqueous acetonitrile and aqueous ethanol solutions were investigated at various pressures. The volumes of activation were all found to be negative and consistent with a bimolecular reaction involving considerable solvent electrostriction. No significant dependence on the solvent composition was found. © 1997 by John Wiley & Sons, Ltd.
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  • 91
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 221-228 
    ISSN: 0894-3230
    Schlagwort(e): solvent effects ; hydroxide ; oximates ; phosphorus(V) esters ; nucleophilicity ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Second-order rate constants of reactions of OH-, 2,3-butanedionemonooximate and 2-hydroxybenzaldoximate ions with aryl phosphate, phosphinate and thioarylphosphinate esters go through minima with decreasing water content of aqueous acetonitrile, tert-butyl alcohol and N-methyl-2-pyrrolidone. For reactions in H2O-MeCN the solvent effects are analyzed in terms of activity coefficients of the anionic nucleophiles and transition states. In the drier solvents partial desolvation of the nucleophiles increases rates. Nucleophilicities of several oximates and inorganic anions are compared in water. © 1997 John Wiley & Sons, Ltd.
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  • 92
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 152-158 
    ISSN: 0894-3230
    Schlagwort(e): electron transfer ; 14N/15N isotope effect ; radical cation ; phenothiazine ; electron paramagnetic resonance ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: An appreciable equilibrium isotope effect has been observed for electron transfer from phenothiazine (PT) to the radical cation of its 15N-substituted analogue ([15N]PT+·), i.e.PT+[15N]PT+· ⇋ K PT+·+[15N]PTvia electron paramagnetic resonance analysis of the mixed radical cations formed from mixing the [15N]phenothiazine radical cation hexachloroantimonate and phenothiazine in acetonitrile (K=0·77±0·10 at 25 °C), and by physical separation of the neutral phenothiazines from the radical cation salts in the equilibrium mixture (K=0·83±0·10 at 25 °C). Infrared and Raman spectra of [14N]- and [15N]phenothiazines and their radical cations were measured to assign the vibrational frequency shifts caused by the heavy-atom substitution and radical cation formation, which gave an estimate of the enthalpy change of 441·7 J mol-1 for the electron transfer process. These results reveal that 15N substitution of phenothiazine decreases appreciably the ionization potential of the molecule, making it easier to lose an electron to form the corresponding radical cation in solution. © 1997 by John Wiley & Sons, Ltd.
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  • 93
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 311-322 
    ISSN: 0894-3230
    Schlagwort(e): enantioselective binding ; gas chromatography ; chiral selection ; permethyl-β-cyclodextrin ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Stochastic molecular dynamics simulations were used to determine the enantiomer retention orders of moderately polar analytes binding to permethylated β-cyclodextrin, a popular chiral stationary phase used in gas chromatography. It is found that averaging over multiple trajectories, each of which are lengthy, is required to faithfully reproduce experiment. From the simulations we find the major binding domain to be the interior of the macrocycle rather than the exterior with most analytes having a preference for associating to the primary rim rather than to the secondary rim. It is also found that the intermolecular forces responsible for holding the complexes together are the short range dispersion forces, and that the enantiodifferentiating forces of the competing diastereomeric complexes are dominated by the van der Waals contributions to the intermolecular energy. © 1997 John Wiley & Sons, Ltd.
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  • 94
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 445-455 
    ISSN: 0894-3230
    Schlagwort(e): electronegativity model ; polar ground-state effects ; bond dissociation energies ; radicals ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---Homolytic bond dissociation energies are a composite of the radical stabilization energies (RSE) of the product radicals and the polar ground-state stabilization energies (PSE) of the reactant molecules. Substituent effects on the PSE are rationalized in terms of changes in the difference of group electronegativities. Thus, the PSE is composed of a bond polarity term, which measures the contribution due to the change in the electronegativity difference between the atoms in the bond, which is broken, and a polar relaxation term, which accounts for the substituent-dependent group electronegativity changes in the remaining bonds. A semi-quantitative model based on Pauling's bonding theory is suitable to assess the direction and relative magnitude of such effects. For the cleavage of benzylic and related bonds, the polar relaxation energy can be neglected (one-bond approximation) to allow the use of correlation analyses and substituent σ 〉parameters for the interpretation of aryl substituent effects on the PSE. Accordingly, the plots of the PSEs versus σ substituent parameters should be linear, curved or parabolic depending on the electronegativity difference of the atoms in the bond being broken (ΔEN); moreover, the slopes (ρ values) should increase linearly with ΔEN. The predicted dependences of the PSEs on aryl substituents are compared with known experimental results and with the data obtained from semiempirical calculations. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 6 Ill.
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  • 95
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 343-346 
    ISSN: 0894-3230
    Schlagwort(e): charge-transfer complexes ; intrinsic (gas-phase) basicities ; stability ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The standard Gibbs energy changes for the formation of 1:1 charge-transfer complexes between molecular iodine and several bases in solution at 25·0 °C [ΔG0l2(soln)] were determined experimentally. The bases were 2,2,2-trifluoro- ethylamine, cyclopropylamine and 1,1,3,3-tetramethylguanidine. These [ΔG0l2(soln)] values, determined in n-heptane, and also that for the formation of the 1:1 complex between 2,2,2-trifluoroethanethiol and molecular iodine in dichloromethane, were found to follow with excellent precision the correlation equations linking ΔG0l2(soln) and the intrinsic (gas-phase) basicities of N(sp3), N(sp2) and S(sp3) bases. These findings strongly support previous conclusions regarding the relationship between gas-phase and solution reactivity data. © 1997 John Wiley & Sons, Ltd.
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  • 96
    ISSN: 0894-3230
    Schlagwort(e): nucleophilic addition ; electron transfer ; carbonylmetallate salts ; donor-acceptor interactions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The isostructural pentacarbonylmetallate anions M(CO)-5 (M=Mn and Re) react with a series of N-methylpyridinium cations (Py+) to yield products of nucleophilic addition [NA=Py-M(CO)5] or of one-electron redox reaction [ET=Py·+M(CO)·5]. The partitioning of the reaction along the two reaction pathways is controlled by steric factors and the electronic structure of the pyridinium cation, with cations which form stable, delocalized radicals favoring the ET pathway. The central metal also plays a role in determining the stoichiometry, and the NA pathway is favored by the rhenate anion and ET by the manganate analogue. Rates of both reactions correlate with the driving force for electron transfer, and the differing reaction pathways are not distinguished on the basis of linear free energy relations, previously discussed by Bordwell and co-workers. The contact ion pair [Py+, M(CO)-5] is identified as the critical precursor for both electron transfer and nucleophilic coupling. Based on these observations, it is proposed that the rates and mechanisms of these interionic reactions are controlled by donor-acceptor bonding in the transition states, which in turn is directly related to the charge-transfer interactions extant in the ion-pair intermediate. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 7 Ill.
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  • 97
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 590-592 
    ISSN: 0894-3230
    Schlagwort(e): iron diprotonated [1.1]ferrocenophane ; superacid ; NMR spectroscopy ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---[1.1]Ferrocenophane (1) has been shown to catalyze the photochemical splitting of water and iron diprotonated [1.1]ferrocenophane has been postulated as a reactive intermediate in this process. However, it has previously not been possible to observe the species directly by, e.g., NMR spectroscopy, since under the conditions it rapidly eliminates hydrogen. By employing a special ion generation technique, 1 has been pronated in superacid (FSO3H-SO2CIF) at -125°C. Under these conditions, the cation formed has a long lifetime and its structure and dynamics have been studied by NMR spectroscopy. The ion observed is iron diprotonated [1.1]ferrocenophane (2), which undergoes rapid degenerate rearrangements. 1H1H NOESY experiments suggest that in 2 the cyclopentadienyl rings in the ferrocene units are tilted relative to each other. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 98
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 473-474 
    ISSN: 0894-3230
    Schlagwort(e): Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---No Abstract.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 99
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 871-878 
    ISSN: 0894-3230
    Schlagwort(e): 9-(α-bromo-α-arylmethylene)fluorenes ; thiolate substitution ; acetonitrile solvent ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The substitution of 9-(α-bromo-α-arylmethylene fluorenes by MeS- and p-TolS- ions in 80% MeCN-20% H2O is a second-order reaction. With MeS-, for the change of the α-aryl group, Hammett's ρ=1·07 in MeCN. The reaction rate decreases on increasing the water content of the medium. The reactions proceed by the AdN-E route and no competitive SN1 reaction was observed. The expected influence of the changes in the substituent, solvent, nucleophile and nucleofuge on the competition between the AdN-E and SN1 reactions was analyzed. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 4 Tab.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 100
    Digitale Medien
    Digitale Medien
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 10 (1997), S. 917-924 
    ISSN: 0894-3230
    Schlagwort(e): nitrenium ion ; pyridinium salts ; photochemistry ; singlet states ; triplet states ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Physik
    Notizen: ---The photochemical reactions of N-(methylphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate were studied to find evidence of photodecomposition to a nitrenium ion reactive intermediate. Stable products were formed that were consistent with a singlet-state methylphenylnitrenium ion precursor. The methoxy and chloro adducts. N-methyl-p-anisidine and 4- (and 2-)chloro-N-methylaniline, were identified and quantified by high-performance liquid chromatographic analysis. A hydride shift from the N-methyl group of the nitrenium ion is also proposed based on the detection of aniline which would result from hydrolysis of the iminium ion rearrangement product. The rate constant for this rearrangement is estimated to be 108 s-1. The reduction product, N-methylaniline, is produced, and is believed to form, at least in part, from the hydrogen atom abstractions of the triplet nitrenium ion. This is supported by the results of triplet sensitized irradiations. Laser flash photolysis studies yielded the transient spectrum of a long-lived intermediate absorbing at 470 nm. This transient species is believed to be the cation radical of N-methylaniline. © 1997 John Wiley & Sons, Ltd.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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