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

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Abstract
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.