Reaction product identification from O(3P)+benzene, toluene, and 1,3,5-trimethylbenzene collisions in crossed molecular beams

Although absolute rate coefficients have been measured for a number of reactions of aromatic molecules with O(3P) atoms, very little is known about the mechanisms of these reactions. In order to identify the products of O atom–aromatic molecule reactions, the products of single reactive collisions between oxygen atoms and benzene, toluene, and 1,3,5,-trimethylbenzene have been observed in crossed molecular beam experiments. The products were detected with a quadrupole mass spectrometer. Two product paths were observed for O+benzene: (1) the O atom–benzene adduct, which is most likely phenol, and (2) carbon monoxide and a C5H6 hydrocarbon which is probably 3-penten-l-yne. An additional path involving the formation of (methyl-substituted) benzaldehyde and H2 was observed for the reaction with methyl-substituted benzenes. Only the adducts, the benzaldehydes, and the olefins were observed directly. The identities of the corresponding products were inferred from the difference between the mass-to-charge ratio of the observed product and that of the C6H6O, C7H8O, and C9H12O adducts. Arguments based on electronic state symmetry indicate that the carbon monoxide-olefin path involves the decomposition of vibrationally excited phenol in its ground electronic state. This state is reached by a spin-forbidden transition from the triplet reactant state. The benzaldehyde products can be explained by a mechanism analogous to one of the reaction paths of O(3P)+ethylene.