The Synthesis Company of San Francisco Mountain Logo

Abstract

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 aromatic molecules that are representative of the functionalities present in coal. These include six-membered rings (benzene, pyridine, pyridazine, pyrimidine, pyrazine) and five-membered rings (furan, thiophene, pyrrole, oxazole). By comparison of the calculated C−H BDEs with the available experimental values for these aromatic molecules, the B3LYP/6-31G(d) level of theory was selected to calculate the BDEs of polycyclic aromatic hydrocarbons (PAHs), including carbonaceous PAHs (naphthalene, anthracene, pyrene, coronene) and heteroatomic PAHs (benzofuran, benzothiophene, indole, benzoxazole, quinoline, isoquinoline, dibenzofuran, carbazole). The cleavage of a C−H or a N−H bond generates a σ radical that is, in general, localized at the site from which the hydrogen atom was removed. However, delocalization of the unpaired electron results in ∼7 kcal·mol-1 stabilization of the radical with respect to the formation of phenyl when the C−H bond is adjacent to a nitrogen atom in the azabenzenes. Radicals from five-membered rings are ∼6 kcal·mol-1 less stable than those formed from six-membered rings due to both localization of the spin density and geometric factors. The location of the heteroatoms in the aromatic ring affects the C−H bond strengths more significantly than does the size of the aromatic network. 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.

C−H and N−H Bond Dissociation Energies of Small Aromatic Hydrocarbons

Citations Over TimeTop 10% of 1999 papers

Abstract

Related Papers