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  • 1
    Electronic Resource
    Electronic Resource
    The He(Iα) Photoelectron Spectra of the Perfluoroderivatives of Trisannelated Benzenes and Tetrakisannelated Cyclooctatetraenes (1984)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 67 (1984), S. 47-53 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The He(Iα) photoelectron (PE) spectra of tris(perfluorocyclobuta)benzene 4(F)3,3,4,4,7,7,8,8,11,11,12,12-Dodecafluorotetracyclo[8.2.0.02,5.06,9]dodeca-1,5,9-triene, tris(perfluorocyclopenta)benzene 5(F)1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-Octadecafluoro-2,3,4,5,6,7,8,9-octahydro-1H-trindene., tetrakis(perfluorocyclobuta)cyclooctatetraene 6(F)3,3,4,4,7,7,8,8,11,1l,l2,l2,l5,15,16,16-Hexadecafluoropentacyclo[12.2.0.02,5.06,9.010,13]hexadeca-1,5,9,13-tetraene., and of tetrakis(perfluorocyclopenta)cyclooctatetraene 7(F)1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12-Tetracosafluoro-1,2,3,4,5,6,7,8,9,10,11,12-dodeca-hydrotetracyclopenta[a,c,e,g]cyclooctene. are reported. A tentative assignment of the PE spectra is derived by empirical correlation with those of relevant reference compounds. The results suggest that 6(F) retains the D4h-conformation in the gas phase, i.e. A conformation with a planar cyclooctatetraene ring, as observed in the crystal. All four compounds exhibit a sharp increase of their first ionization energies, relative to the corresponding parent hydrocarbons, due to the perfluoro effect.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19840670107
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  • 2
    Electronic Resource
    Electronic Resource
    Conjugated Biradical Intermediates: Spectroscopic, Kinetic, and Trapping Studies of 2,2-Dimethyl-1,3-perinaphthadiyl (1985)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 68 (1985), S. 777-788 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The biradical 2,2-dimethyl-1,3-perinaphthadiyl (a) was generated from two different precursors, the naphthocyclopropane 1 and the azo compound 2, and from each by three different pathways (pyrolysis, direct photolysis, and triplet sensitization, Scheme 1). The combined evidence from flash photolysis, low-temperature spectroscopy, and product analyses provides a detailed mechanistic picture of the formation and decay of this reactive intermediate which is persistent at 77 K in the triplet ground state (3a) and rather long-lived (400 μs) at room temperature. When formed in its lowest singlet state (1a), the biradical is too short-lived to undergo intersystem crossing to 3a or bimolecular reactions. Thus, 3a is formed exclusively from the excited triplet state of the precursor compounds, 31* or 33*. The monomolecular decay of 3a is retarded by the spin barrier; 3a initiates the polymerization of acrylonitrile and is trapped by 3O2.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19850680328
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  • 3
    Electronic Resource
    Electronic Resource
    The Azo(Azoxy) Functionality as a π2 Component in Photo [2+2] cycloadditions ‘syn’- and ‘anti’-3,4-diazatricyclo[4.2.2.22,5]dodeca-3,7-diene, syntheses, photolyses, X-ray-structure analysis, and PE spectra (1988)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 71 (1988), S. 1937-1965 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The propensity of the N=N bond to undergo photo [2 + 2] cycloadditions has been further explored. In the specifically designed 1,5-azo/enes 1-3, no [2 + 2] cycloaddition has been observed upon either direct or sensitized excitation with light of various wave lengths at temperatures down to 77 K, in line with expectations based on X-ray (1: d = 2.71 Å, ω = 129°) and PE measurements (1: I1 = 8.00, I2 = 9.05 eV; 2; I1 = 8.00, I2 = 9.25eV). The steric/stereoelectronic demands for the participation of the N=N bond in pericyclic reactions are clearly more stringent than those for the C=C bond.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19880710813
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  • 4
    Electronic Resource
    Electronic Resource
    Flash Photolysis of 5-Methyl-1,4-naphthoquinone in Aqueous Solution: Kinetics and mechanism of photoenolization and of enol trappingDedicated to our colleague Fabian Gerson who is venturing to new shores (1997)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 80 (1997), S. 1106-1121 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 5-Methyl-1,4-naphthoquinone (1) is a remarkable probe to study hydrogen and proton transfer reactions. The photoenol 4-hydroxy-5-methylidene naphthalen-1 (5H)-one (2) is formed in the ground state within 2 ps of excitation and with a quantum yield of unity, presumably through a conical intersection of the S0 and S1 hypersurfaces. In aqueous acid, enol 2 is hydrated to 5-(hydroxymethyl)naphthalene-1,4-diol 3 (X = OH, Scheme 1). The rate of hydration of 2 increases linearly with acid concentration from ca. 1.5 × 104 s-1 at pH 6 to reach a maximum value of 9 ×107 s-1 when the remaining carbonyl function is protonated, pKa(2+) = 1.1. Contrary to an earlier suggestion, the rate-determining step in the acid-catalyzed hydration of 2 is addition of water to the conjugate acid 2+. Pronounced acceleration of the decay rate of 2 by hydrazoic-acid buffers indicates competitive trapping of 2+ by the azide ion. In neutral-to-weakly-basic solutions, enol 2 reacts by ionization, pKa(2) = 6.5, and nearly diffusion-controlled condensation of the carbanionic species 2- with quinone 1. Proto-nation at the methylidene C-atom does not compete measurably with protonation on carbonyl O-atom, despite a Substsial thermodynamic basic for carbon Portoation of ca. 50 kJ mol-1 for 2 and 100 mol-1 for 2-.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19970800408
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  • 5
    Electronic Resource
    Electronic Resource
    Cyclooct-1-en-5-yne. Preparation, spectroscopic characteristics and chemical reactivityPresented at the Swiss Chemical Society meeting, 7.10.1977. (1978)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 61 (1978), S. 1663-1674 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cyclooct-1-en-5-yne (3) has been obtained as an isolable, but highly reactive hydrocarbon by oxidation of cyclooct-5-ene-1, 2-dihydrazone with lead tetraacetate (Scheme 2). Information regarding the structure and conformational mobility of 3 has been gained from the analysis of its 1H- and 13C-NMR. spectra and was found to agree with the results of force field calculations. Photolysis (206 nm) of 3 in solution has induced cleavage to butatriene and butadiene. The first band in the photoelectron spectrum of 3 (Iv 9.10 eV) is attributed to ionization from a rather delocalized orbital with predominant weight of the double bond p-AO's. Nevertheless, the high reactivity of 3 stems from the strained triple bond as evidenced by the reaction products obtained by pyrolysis, oxidation, Diels-Alder addition, and 1, 3-dipolar addition (Scheme 4).
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19780610514
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  • 6
    Electronic Resource
    Electronic Resource
    Competitive Formation of Ylide and Carbene Intermediates from 1π,π*-Excited α,β-Unsaturated γ,δ-Epoxyketones131st Communication of the series ‘Photochemical reactions’ 130th Communication see [1]. (1983)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 66 (1983), S. 2489-2500 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Laser flash photolyses (λ = 265mm) of the γ,λ-epoxyenones 1-3, 7 and 8, the α,β-unsaturated γ,δ-epoxy ester 6, and the epoxytriene 9 at ambient temperature produced short-lived transients with broad absorption maxima in the visible region, which are identified as carbonyl ylides. Comparison of the rather long-wavelength absorption maxima with the results of standard PPP SCF SCI calculations suggests that some degree of twisting is present in all the ylides studied. The lifetimes of the order of hundreds of ns of these intermediates and Stern-Volmer analysis of the trapping of the carbonyl ylide derived from 2 with CH3COOH provide conclusive evidence that the carbene products are not formed via the carbonyl-ylide intermediate (Scheme 3).
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19830660814
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  • 7
    Electronic Resource
    Electronic Resource
    Photoisomerization pathways of 8,16-methno[2.2]metacyclopane-1,9-diene. A model case for adiabatic electrocyclic ring closure in the excited singlet state (1984)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 67 (1984), S. 305-317 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title compound 1b ideally meets the theoretical requirements for the occurrence of an adiabatic photoisomerization in the lowest excited state (11b*) and, indeed, the predominant primary photoreaction observed is the conversion to its fluorescent valence isomer 10b, 10c-methano-cis-10b 10c-dihydropyrene (11a*). The mechanism for the formation often previously observed photoproduct 8b, 9a-dihydro-9H - cyclopropa[e]pyrene (4a) has been analyzed in some detail (Scheme). Below - 30°C the reaction path consists of a three quantum process (two di-π-methane rearrangements and photochemical 1,7-H shift) involving two thermally stable, but light-sensitive isomers 8,11b-methanocyclodeca[cde]naphthalene (2b) and 9H -cyclohepta[def]-phenanthrene (3b). At room temperature the rearrangement 2b→4a proceeds with a single excitation step bypassing the ground state intermediate 3b. Finally, upon prolonged irradiation of (4a), the methylene group is lost to yield pyrene. Compound 2b completes the series of all possible adducts of methylene to a C=C bond of pyrene.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19840670137
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  • 8
    Electronic Resource
    Electronic Resource
    Photochemical Ring Slippage of Bis(pentafluorophenyl)titanocene: Reaction kinetics and matrix isolation of the primary photoproduct (1988)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 71 (1988), S. 1858-1867 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Laser flash photolysis and matrix-isolation techniques were applied to elucidate the photochemistry of the orange complex bis(pentafluorophenyl)titanocene ( = bis(η5-cyclopentadienyl)bis(pentafluorophenyl)titanium- (IV), Cp2TiIV{C6F5}2, Cp = η5-C5H5, 1) which is used as a polymerization photo-initiator. The primary photo-reaction of 1 is the formation of a highly reactive blue isomer X with unit quantum yield. In carefully dried and degassed benzene solution, the photoisomer X rearranges to starting material 1 with a first-order reaction, k = 5 · 103 s-1 at room temperature. X is highly reactive towards H2O, MeOH, acetone, MeCN, MeNO2, butane-1,4-diyl diacrylate, 2,2,6,6-tetramethylpiperidine N-oxide, CO, O2, and N2; absolute bimolecular rate constants range from 106 to 109 M-1 · s-1. The primary photoisomerization 1→X is tentatively ascribed to a cyclopentadienyl ring slippage from η5 to a lower hapticity, a process that opens up coordinative unsaturation.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19880710803
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  • 9
    Electronic Resource
    Electronic Resource
    Transient Radicals Formed by Electron Transfer between inorganic ions and excited aromatic molecules in polar solvents (1993)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 76 (1993), S. 1319-1331 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Fluorescence quenching of aromatic molecules by inorganic anions has been the subject of many investigations, yet the nature of the quenching mechanism is not fully understood. The fluorescence-quenching rate constants correlate with electrochemical data, but the radicals expected to form upon transfer of an electron to the excited aromatic molecules have escaped observation. We report the first observation of radical-ion species formed by electron-transfer quenching with inorganic anions in acetonitrile. A decisive step leading to formation of separated radical ions is the trapping of the primary charge-transfer complex by a second inorganic ion.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19930760319
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  • 10
    Electronic Resource
    Electronic Resource
    Triplet Energy of 2, 2-Dimethylisoindene from Electron-Energy-Loss Spectroscopy and Photoinduced Triplet Energy Transfer (1994)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 77 (1994), S. 1541-1548 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The excited electronic states of 2, 2-dimethylisoindene (1) have been studied by electron-energy-loss spectroscopy. Its vertical gas-phase triplet (13B2), and singlet (11B2) excitation energies are 1.61 and 3.19 eV, respectively. The excited states are thus lowered by 0.49 eV and 1.21 eV, respectively, when compared to the corresponding states of (all-E)-octatetraene, which serves as a reference compound. These shifts are partially reproduced by ZINDO calculations. The spectra give no evidence for a 21Ag state below the 11B2 state, but this lack of observation does not exclude its existence. The lowest triplet state T1(1) was further characterized by flash photolysis. T1(1) was observed as a transient intermediate, λ ≤ 350 nm, with a lifetime of 8 m̈s in degassed hexane. The adiabatic excitation energy of T1(1) was bracketed to the range of 1.1 ± 0.1 eV by energy-transfer experiments. Relationships between the energies of the lowest excited singlet and triplet states of 1 and the lowest excited doublet state of its radical cation \documentclass{article}\pagestyle{empty}\begin{document}${1}^{+\kern0pt {.}}$\end{document} - essentially a non-Koopmans' state - are discussed.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19940770609
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