A mechanistic study of the reactions of H, O (3p), and OH with monocyclic aromatic hydrocarbons by density functional theory

The chemistry of small aromatic hydrocarbons with radicals of relevance to high temperature combustion and low temperature atmospheric processes has been studied computationally using the B3LYP method and transition state theory (TST). The reaction of H, O (P-3), and OH with aromatic hydrocarbons can proceed by two mechanisms: hydrogen-atom abstraction or radical addition to the ring. The calculated free energies for the transition state barriers and the overall reactions show that the radical addition channel is preferred at 298 K, but the H-atom abstraction channel becomes dominant at high temperatures. The thermodynamic and kinetic preference for reactivity with aromatic hydrocarbons increases in the order O(P-3) < H < OH. K-atom abstraction from six-membered aromatic rings is more facile than from five-membered aromatic rings. However, radical addition to five-membered rings is thermodynamically more favorable than addition to six-membered rings. In general, the barrier heights and preferences for H-atom abstraction from sites within an aromatic hydrocarbon are well correlated with the corresponding C-H band dissociation enthalpies.