Theoretical studies of [n]paracyclophanes and their valence isomers |
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Authors: | F Bockisch J -C Rayez H Dreeskamp D Liotard B Duguay |
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Institution: | (1) Laboratoire de Physicochimie Théorique, Université Bordeaux I, 351, cours de la Libération, F-33405 Talence Cédex, France;(2) Institut für Physikalische und Theoretische Chemie, Technische Universität Braunschweig, Hans Sommer Str. 10, W-3300 Braunschweig, Germany |
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Abstract: | Summary The valence isomerisations of benzene, 6]- and 7]paracyclophane to their Dewar benzene and prismane isomers are studied with the MNDO method using the unrestricted Hartree-Fock (UHF) and the configuration interaction (C.I.) approximations. The enthalpy of the reaction Dewar benzene benzene is H°
r
=–68.9 kcal/mol and the activation enthalpy is H°=27.9 kcal/mol (with C.I.). The reaction path hasC
2v
symmetry.The determination of several points of the lowest potential energy surface of 6]- and 7]paracyclophanes leads to a minimum reaction path having the same topology as for the potential energy surface of the nonbridged benzene. The only difference is a quantitative change in the energy values of the aromatic isomers due to the deformation introduced by the alkyl chain. For 6]paracyclophane, the activation enthalpy is H°=24.6 kcal/mol and the activation entropy is S
0=0.6 cal K–1 mol–1 calculated with C.I.The enthalpy of the reaction prismane Dewar benzene is H°
r
–32 kcal/mol and the activation enthalpy is H°19 kcal/mol. The highest molecular symmetry group common to both molecules isC
2v
, whereas the symmetry group of the reaction path is lowered toC
s
. Along this reaction path is located a biradicaloid intermediate, separated by low activation barriers from the products. No significant changes of the potential energy surfaces are found for the bridged n]prismanes and the n]Dewar benzenes.All the calculated values, reaction enthalpies, activation enthalpies and entropies, are in a good agreement with literature experimental data.This article is dedicated to Professor A. Pullman |
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Keywords: | [n]Paracyclophanes Dewar benzene isomers Prismane isomers Semiempirical methods Transition states Biradicaloid intermediates |
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