A combined crossed-beam, ab initio, and Rice-Ramsperger-Kassel-Marcus investigation of the reaction of carbon atoms C(3Pj) with benzene, C6H6(X 1A1g) and d6-benzene, C6D6(X 1A1g)

The reactions of atomic carbon, C(P-3(j)), with benzene, C6H6(X (1)A(1g)), and with d(6)-benzene, C6D6(X (1)A(1g)) were investigated at twelve collision energies between 8.8 and 52.5 kJ mol(-1) using the crossed molecular beams technique. Forward-convolution fitting of the data, high-level electronic structure calculations, and Rice-Ramsperger-Kassel-Marcus (RRKM) investigations on the singlet and triplet C7H6/C7D6 potential energy hyperface suggest that at low collision energies the chemical reaction dynamics are indirect and dominated by large impact parameters. As the collision energy increases, smaller impact parameters become more important, and the chemical dynamics is increasingly direct. At all collision energies, the reaction proceeds on the triplet surface via a barrierless addition of the carbon atom to form a bicyclic intermediate followed by ring opening of the initial collision complex to a seven-membered ring intermediate (cycloheptatrienylidene). The latter decomposes without exit barrier to the thermodynamically less stable 1,2-didehydrocycloheptatrienyl radical, C7H5((XB1)-B-2)+H, and its deuterated C7D5((XB1)-B-2)+D counterpart. The formation of a C7D6 adduct is observed as a second channel. The barrierless route for the destruction of benzene can help to model important pathways for the synthesis of higher polycyclic aromatic hydrocarbon derivatives in the interstellar medium, in outflows of dying carbon stars, in hydrocarbon-rich planetary atmospheres, as well as in oxygen-poor combustion flames. (C) 2002 American Institute of Physics.