Structures and C-H bond energies of hydrogenated polycyclic aromatic hydrocarbons

Density functional calculations are employed to calculate C-H bond dissocia tion energies of sp(3) centers at different locations of a variety of polyc yclic aromatic hydrocarbons (PAHs). The computed bond dissociation energies D-e range from less than 80 kJ mol(-1) for a hydrogen atom bound to the ba sal plane of a PAH layer up to 300 kJ mol(-1) for a CH-H bond on a zigzag b order position. Besides this not unexpected dependence on the specific proc ess, we observe a pronounced scatter of dissociation energies with the size and shape of PAHs, which reflects the complex chemistry for graphitic carb on reactions. On the basis of the present treatment, we determine reasonabl e estimates for the hydrogenation energies of extended graphite sheets, whi ch have been considered as important in the modeling of carbon deposition b y pyrolysis of light hydrocarbons.