Crossed beams reaction of atomic carbon, C(P-3(j)), with d(6)-benzene, C6D6(X(1)A(1g)): Observation of the per-deutero-1,2-didehydrocycloheptatrienylradical, C7D5((XB2)-B-2)

The reaction of atomic carbon, C(P-3(j)) with per-deutero benzene, C6D6 is investigated at an average collision energy of 32.1 kJ mol(-1) using the cr ossed molecular beams technique combined with a universal mass spectrometri c detector. Product angular distributions and time-of-flight spectra of C7D 5 and C7D6 are recorded. Forward-convolution fitting of our time-of-flight data (TOFs) and laboratory angular distribution (LAB) together with high le vel electronic structure calculations on the singlet and triplet C7D6 poten tial energy surfaces are consistent with the formation of the per-deutero-1 ,2-didehydrocycloheptatrienyl radical, C7D5. No C7D6 adduct is found experi mentally. Our investigations indicate that the carbon atom attacks the benz ene molecule face without an entrance barrier to form an initial complex. T his undergoes a ring opening to give triplet cycloheptatrienylidene as a C7 D6 intermediate. The latter fragments without exit barrier via a C-D bond r upture to yield the per-deutero-1,2-didehydrocycloheptatrienyl isomer, C7D5 , and a D atom. This barrierless route for the destruction of benzene may b e involved in the synthesis of higher cyclic hydrocarbon derivatives in the interstellar medium, in outflows of dying carbon stars, in hydrocarbon-ric h planetary atmospheres, as well as in oxygen-poor combustion flames. (C) 1 999 American Institute of Physics. [S0021-9606(99)01513-5].