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  • 1
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1520-6904
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 1917-1928 
    ISSN: 0947-3440
    Keywords: Diradicals ; Supercritical fluid reactions ; O2 trapping ; Reaction force field ; Cage effect ; Cope reaction ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 2155-2169 
    ISSN: 0947-3440
    Keywords: Diradicals ; NO Trapping ; O2 Trapping ; Trapping, NO and O2 ; Energy well ; Singlet-triplet splitting ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 0947-3440
    Keywords: Diradicals ; Gas-phase kinetics ; Oxygen trapping ; Dynamic gas chromatography ; Rotational barrier, two-step ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 24 Tab.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 1361-1366 
    ISSN: 0947-3440
    Keywords: Diradicals ; Oxygen trapping ; Energy well ; Supercritic fluid reactions ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 2171-2183 
    ISSN: 0947-3440
    Keywords: Diradicals ; Gas-phase kinetics ; NO and O2, trapping ; Racemization barrier ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 102 (1969), S. 811-814 
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Die Synthese von 6.7-Diaza-tricyclo[3.2.2.02.4]nonen-(6) (4) gelingt ausgehend vom Cycloheptatrien-(1.3.5) mit einer Ausbeute von 45%. Bei 25° zerfällt 4 in Cycloheptadien-(1.4) und Stickstoff.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 108 (1975), S. 2923-2933 
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Photoelectron Spectra of Organic Compounds, VIII. 7,8-Dimethylenebicyclo[2.2.2]octa-2,5-diene and Related CompoundsThe photoelectron spectra of vicinal dimethylene derivatives of cyclohexane and cyclohexene bridged by a group with π-, Walsh, or n-orbitals (3-13) are discussed on the basis of a LCMO model within the HMO approximation. The homoconjugative interaction between the semicyclic diene and one other group is only slight; more pronounced effects are observed if an interaction with two other groups is possible, as in the case of 7,8-dimethylenebicyclo[2.2.2]octa-2,5-diene (3) or 5,6-dimethylene-7-oxabicyclo[2.2.1]hept-2-ene (12).
    Notes: Die Photoelektronenspektren von vicinalen Dimethylenderivaten des Cyclohexans und Cyclohexens, die durch eine Gruppe mit π-, Walsh- oder n-Orbitalen überbrückt sind (3-13), werden auf der Grundlage eines LCMO-Modells im Rahmen der HMO-Näherung diskutiert. Die homokonjugative Wechselwirkung zwischen dem semicyclischen Diensystem und einer weiteren Gruppe ist gering; ausgeprägtere Effekte treten auf, wenn wie im Falle des 7,8-Dimethylenbicyclo-[2.2.2]octa-2,5-diens (3) oder des 5,6-Dimethylen-7-oxabicyclo[2.2.1]hept-2-ens (12) eine Wechselwirkung mit zwei weiteren Gruppen möglich ist.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
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