A mechanistic study of the reactions of H, O ((3)p), 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 b een studied computationally using the B3LYP method and transition state the ory (TST). The reaction of H, O (P-3), and OH with aromatic hydrocarbons ca n 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 prefer red at 298 K, but the H-atom abstraction channel becomes dominant at high t emperatures. The thermodynamic and kinetic preference for reactivity with a romatic hydrocarbons increases in the order O(P-3) < H < OH. K-atom abstrac tion from six-membered aromatic rings is more facile than from five-membere d aromatic rings. However, radical addition to five-membered rings is therm odynamically more favorable than addition to six-membered rings. In general , the barrier heights and preferences for H-atom abstraction from sites wit hin an aromatic hydrocarbon are well correlated with the corresponding C-H band dissociation enthalpies.