C-H and N-H bond dissociation energies of small aromatic hydrocarbons

A survey of computational methods was undertaken to calculate the homolytic bond dissociation energies (BDEs) of the C-H and N-H bonds in monocyclic a romatic molecules that are representative of the functionalities present in coal. These include six-membered rings (benzene, pyridine, pyridazine, pyr imidine, pyrazine) and five-membered rings (furan, thiophene, pyrrole, oxaz ole). By comparison of the calculated C-H BDEs with the available experimen tal values for these aromatic molecules, the B3LYP/6-31G(d) level of theory was selected to calculate the BDEs of polycyclic aromatic hydrocarbons (PA Hs), including carbonaceous PAHs (naphthalene, anthracene, pyrene, coronene ) and heteroatomic PAHs (benzofuran, benzothiophene, indole, benzoxazole, q uinoline, isoquinoline, dibenzofuran, carbazole). The cleavage of a C-H or a N-H bond generates a sigma radical that is, in general, localized at the site from which the hydrogen atom was removed. However, delocalization of t he unpaired electron results in similar to 7 kcal.mol(-1) stabilization of the radical with respect to the formation of phenyl when the C-H bond is ad jacent to a nitrogen atom in the azabenzenes. Radicals from five-membered r ings are similar to 6 kcal.mol(-1) less stable than those formed from six-m embered rings due to both localization of the spin density and geometric fa ctors. The location of the heteroatoms in the aromatic ring affects the C-H bond strengths more significantly than does the size of the aromatic netwo rk. Therefore, in general, the monocyclic aromatic molecules can be used to predict the C-H BDE of the large PAHs within 1 kcal.mol(-1).