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.