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  • 1
    Digitale Medien
    Digitale Medien
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 961-967 
    ISSN: 1434-193X
    Schlagwort(e): Diradicals ; Dioxygen trapping ; Diradical well ; Heat of formation ; Ab initio calculations ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The observation of practically identical activation parameters for the Cope rearrangement 2 → 6 and its “frustrated” counterpart 1 → 5 indicates a two-step mechanism for the reaction 2 → 6. Direct proof of this interpretation comes from trapping experiments, which demonstrate the intermediate formation of the diradicals 5 and 9. From the temperature and oxygen dependences of the trapping rates, recombination barriers of ΔH≠ = 11.5 and 6.5 kcal · mol-1 have been derived for 5 → 1 and 9 → 6, respectively. These results agree well with ab initio calculations (CASPT2).
    Materialart: Digitale Medien
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  • 2
    Digitale Medien
    Digitale Medien
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 1917-1928 
    ISSN: 0947-3440
    Schlagwort(e): Diradicals ; Supercritical fluid reactions ; O2 trapping ; Reaction force field ; Cage effect ; Cope reaction ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: van der Waals Complexes, Intermediates in Unimolecular ReactionsThermolysis of rac- and meso-1,1′,2,2′,3,3′ -hexamethyl-1,1′-biindenyl and (1E, 5E)-1,3,4,6-tetraphenyl-1,5-hexadiene in supercritical CO2 in the presence of oxygen allows to identify three types of intermediates, the free radicals (2 and 5) and the equilibrating complexes I and II consisting of radical pairs. From the oxygen dependence of the trapping and rearrangement rates it is shown, that the complex I is the primary thermolysis product, which in contrast to the complex II does not react with oxygen. The temperature dependance of these rates allows to calculate the enthalpy well of the complex II, which agrees well with predictions of our reaction force field.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 3
    Digitale Medien
    Digitale Medien
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 2155-2169 
    ISSN: 0947-3440
    Schlagwort(e): Diradicals ; NO Trapping ; O2 Trapping ; Trapping, NO and O2 ; Energy well ; Singlet-triplet splitting ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Resonance Energy of Diradicals.  -  1,8-NaphthoquinodimethaneFrom the racemization of 19c and the temperature and NO dependence of the trapping rate of 20b between 218 and 288°C the energy profile for the equilibrium 19b ⇄ 20b is derived. The singlet-triplet splitting of the diradical is 2.5 kcal · mol-1 with the triplet being the ground state. By comparison of the experimental reaction enthalpy with the analogous value for 25, 26 or a hypothetical model reaction with non-interacting radicals it is shown that the stabilizing interaction of the radicals in the singlet state of 20b is negligible making the stabilization of the triplet equivalent to the singlet-triplet splitting. By using literature data it is shown by an analogous analysis that the interaction of the radicals in the bisallyl diradical 28 and in the triplet state of the trimethylenmethane derivative 29 is also negligible whereas in the singlet state of 29 the interaction is strongly destabilizing.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 4
    Digitale Medien
    Digitale Medien
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 2171-2183 
    ISSN: 0947-3440
    Schlagwort(e): Diradicals ; Gas-phase kinetics ; NO and O2, trapping ; Racemization barrier ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The Racemization of Allene, a Two-Step ReactionFor four vinyl- and phenyl-substituted allenes racemization enthalpies have been determined, which are 2-6 kcal · mol-1 below the estimate derived from the parent system by correction for the additional radical stabilization of the intermediate allyldiradical 2(I), indicating the growing importance of the carbene type structure 2(II) for the intermediate.  -  The racemization of 1,3-dimethylallene 1b at temperatures 〉300°C in the presence of NO allows the trapping of an intermediate, which is considered to be the orthogonal allyl diradical 2b. From the trapping kinetics (307-388°C), the reversible bimolecular addition of NO at low temperatures (225-285°C) and the strength of the C—NO bond an enthalpy well for the singlet diradical of 2.7 kcal · mol-1 and a singlet-triplet splitting of 5.6 kcal · mol-1 is derived with the triplet being the groundstate.  -  The postulated intermediate diradical 2 can be stabilized electronically by phenyl and vinyl groups or by strain when incorporated in an eight-membered ring. Thermolyses of the optically active 1,2,5-cyclooctatriene 18 in the presence of oxygen allows to determine the enthalpy well of the corresponding diradical 25 to be 13.1 kcal · mol-1.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
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  • 5
    Digitale Medien
    Digitale Medien
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 1323-1327 
    ISSN: 0947-3440
    Schlagwort(e): Diradicals ; Dioxygen trapping ; Diradical well ; Supercritical fluids ; Radicals ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: From the O2-dependence of the trapping rate of 1,1-difluoro-2,3-diphenylcyclopropane in supercritical CO2 in the temperature range 110-180°C and the rates of its geometrical isomerization and racemization of the trans-isomer, the energy profile for the geometrical isomerization is derived. Assuming that 7 is a common intermediate, the temperature dependence of the ratio of the rates of isomerization/racemization leads to a lowering of 1.2 kcal · mol-1 for the activation energy for the disrotatory cyclization of the intermediate diradical compared to the conrotatory mode. The energy barriers associated with cyclization of the diradical amount to 2.1 and 0.9 kcal · mol-1, respectively.
    Zusätzliches Material: 7 Tab.
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 1361-1366 
    ISSN: 0947-3440
    Schlagwort(e): Diradicals ; Oxygen trapping ; Energy well ; Supercritic fluid reactions ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Concerted and Nonconcerted Reaction Path; Thermolysis of Dispiro[2.2.2.2]deca-4,9-dieneThe enthalpy profile for the equilibrium 1 ⇄ 2 ⇄ 3 has been determined by oxygen trapping. From the oxygen dependence at low (0.0005-0.2 bar) and high pressure (1-180 bar) between 130 and 190°C the enthalpy wells of the diradicals 3 and 2 have been derived (3 17.1, 2 3.7 kcal · mol-1). The observation of a nonconcerted path for the equilibrium 1 ⇄ 3 is important with respect to the understanding of the controlling factor of the concerted and nonconcerted path of a reaction. The existence of a favourable potential energy path obviously is not of prime importance.
    Zusätzliches Material: 5 Ill.
    Materialart: Digitale Medien
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  • 7
    ISSN: 0947-3440
    Schlagwort(e): Diradicals ; Gasphase kinetics ; Trapping, NO and O2 ; Supercritical fluid reactions ; Reaction forcefield ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Radical Recombination Enthalpy.  -  Thermochemistry of the Ringopening of CyclophanesBy thermolysis of the cyclophanes 1, 5, 8, 11, 14, 18, and 21 in the presence of NO or O2 the enthalpy profiles for the ring opening were determined from the temperature, NO, or O2 dependance of the trapping rates of the intermediate diradicals. The experimental enthalpy wells, which varied between 3 and 12 kcal · mol-1, can be simulated well with a reaction forcefield making use of the EVBH method of Malrieu et al. This result gives confidence that also meaningful recombination barriers of radicals can be obtained by this method.
    Zusätzliches Material: 18 Ill.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 8
    Digitale Medien
    Digitale Medien
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 1691-1695 
    ISSN: 0947-3440
    Schlagwort(e): Diradicals ; NO trapping ; Diradical well ; Heat of formation ; Bergman cyclization ; Hydrogen abstraction ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: 1,4-DidehydronaphthaleneThe energy profile of the Bergman equilibrium o-diethinyl-benzene (1 · 1,4-didehydronaphthalene (2) has been established from the temperature and NO dependence of the trapping rate of the diradicals 2 which leads to a heat of formation for 2 of ΔHf0 = 152.9 · 1.4 kcal · mol-1. Activation parameters for the hydrogen abstraction of 2 and 1,4-didehydrobenzene (5) have been derived from the rate of the naphthalene and benzene formation, when 1 and cis-3-hexene-1,5-diyne (4) are heated in the presence of methanol, toluene, and 1,4-cyclohexadiene.
    Zusätzliches Material: 4 Ill.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 9
    ISSN: 0947-3440
    Schlagwort(e): Diradicals ; Gas-phase kinetics ; Oxygen trapping ; Dynamic gas chromatography ; Rotational barrier, two-step ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Energy Well of Diradicals, VII. - Conjugative-Stabilized Trimethylenemethane Derivatives. Geometry Dependence of the Singlet-Triplet SplittingFor the three 2,1′-bis-allyl diradicals 3-5 the singlet-triplet splitting has been determined by the oxygen-trapping technique. In agreement with theory the value for the planar diradical is large (〉14 kcal · mol-1) whereas for the orthogonal geometry the energy gap is small (6.3 kcal · mol-1). In all cases a triplet groundstate is observed. From the rotational barrier of the exo methylene groups in 6 it is shown that the interconversion of the planar and orthogonal singlet states have activation barriers (6-9 kcal mol-1) which are responsible for their kinetic stability. In contrast to 6, where the formation of the orthogonal diradical proceeds by way of the planar diradical 3, the formation of the analogous orthogonal diradical 29 from homofulvene 17 is a concerted process. This difference is an important observation with respect to the fundamental understanding of concerted and non-concerted reactions.
    Zusätzliches Material: 24 Tab.
    Materialart: Digitale Medien
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  • 10
    Digitale Medien
    Digitale Medien
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 1329-1331 
    ISSN: 0947-3440
    Schlagwort(e): Diradicals ; NO- and O2-trapping ; Diradical well ; Heats of formation ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: From the NO- and O2-dependence of the trapping rate of 1,6-cyclodecadiyne (3) in the gas phase and in supercritical CO2 in the temperature range 160-290°C, the enthalpy profile for the equilibrium 3 ⇌ 4 has been established. For the diradical 4, a heat of formation of 116.2 kcal · mol-1 is obtained, which agrees well with the expectation value derived by the abstraction of two hydrogen atoms from hexahydronaphthalene (5).
    Zusätzliches Material: 3 Ill.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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