Energetics of the homolytic C-H and C-Cl bond cleavages in polychlorobenzenes: The role of electronic and steric effects
- Florida State Univ., Tallahassee, FL (United States)
Geometries of (poly)chlorosubstituted benzenes and phenyl radicals are optimized at the BLYP/6-311G{sup **} level of theory. The radicals, which are all planar and of the {sigma}-type, possess geometries that are influenced by both electronic and indirect steric effects. The total energies of the species under study are quantitatively analyzed with simple additive schemes involving the chlorine-chlorine and chlorine-trivalent carbon interactions. Comparisons with the few available experimental data reveal that the computed C-H and C-Cl bond dissociation energies (BDEs) of benzene and its chloro-derivatives are systematically too low by ca. 5 kcal/mol and that the experimental C-Cl BDE of 1,3-dichlorobenzene is most probably in error. The substituents are predicted to facilitate the homolytic C-Cl bond cleavage by up to 6.6 kcal/mol while making the C-H cleavage less favorable by as much as 3.8 kcal/mol. The trends in BDEs are readily accounted for by a superposition of electronic end steric effects. In all cases, the C-Cl bond cleavages are found to require significantly less energy than the C-H ones, implying kinetic control of the aryl radical formation in the course of pyrolysis of (poly)chlorobenzenes. 19 refs., 2 figs., 3 tabs.
- OSTI ID:
- 505167
- Journal Information:
- Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory, Journal Name: Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory Journal Issue: 5 Vol. 101; ISSN 1089-5639; ISSN JPCAFH
- Country of Publication:
- United States
- Language:
- English
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