The C-H bond dissociation energies of polycyclic aromatic hydrocarbons

PM3 molecular orbital calculations were carried out to estimate C-H bond dissociation energies (C-H BDEs) for thirteen polycyclic aromatic hydrocarbons (PAHs). It was found that the C-H BDE depends only slightly on the position in the molecule. The half-occupied orbital at the radical center or the dehydrogenation site tends to acquire the maximum degree of a character by taking a quasi-linear geometry with shortened C-C bonds. It seems likely that a small variation in the C-H BDE is primarily determined by the local structure near the dehydrogenation site. A solo hydrogen is a hydrogen atom bonded to the carbon atom with no adjacent C-H bonds. This type of hydrogen atom is a little more reluctant to dissociate than others, possibly because carbon atoms to which they are bonded are constrained by two adjacent benzene rings.