Energetics and Site Specificity of the Homolytic C−H Bond Cleavage in Benzenoid Hydrocarbons: An ab Initio Electronic Structure Study

Electronic structure calculations carried out at the BLYP/6-311G**
level of theory accurately predict the
dissociation energy of the C−H bond in benzene. The analogous
energies of the homolytic C−H bond cleavage in
the other nine polycyclic aromatic hydrocarbons (PAHs) are found to be
governed almost entirely by steric factors,
the hydrogens from congested regions of the PAHs being removed
preferentially. The removal of hydrogens is
accompanied by highly regular changes in the molecular geometries,
namely a widening of the ipso bond angle by
ca. 6.0° and a concomitant shortening of the adjacent C−C bonds by
ca. 0.02 Å. These observations suggest an
almost complete localization of the unpaired σ electrons on single
carbon atoms and the separation of the local σ and
π effects in the aryl radicals under study. This localization is
confirmed by the computed charges and spin populations
of atoms in the phenyl, 1-naphthalenyl, and 2-naphthalenyl radicals.
In contrast with their UHF counterparts, the
UBLYP electronic wave functions are only mildly spin
contaminated.