CROSSED-BEAM REACTION OF CARBON-ATOMS WITH HYDROCARBON MOLECULES .1. CHEMICAL-DYNAMICS OF THE PROPARGYL RADICAL FORMATION C3H3 (X(2)B(2)), FROM REACTION OF C(P-3(J)) WITH ETHYLENE, C2H4(X(1)A(G))

The reaction between ground-state carbon atoms, C(P-3(j)), and ethylen e, C2H4(X(1)A(g)), was studied at average collision energies of 17.1 a nd 38.3 kJmol(-1) using the crossed molecular beams technique. Product angular distributions and time-of-flight spectra of m/e = 39 were rec orded. Forward-convolution fitting of the results yields a maximum ene rgy release as well as angular distributions consistent with the forma tion of the propargyl radical in its X(2)B(2) state. Reaction dynamics inferred from the experimental data indicate two microchannels, both initiated by attack of the carbon atom to the pi-orbital of the ethyle ne molecule via a loose, reactant like transition state located at the centrifugal barrier. Following C-s symmetry on the ground state (3)A '' surface, the initially formed triplet cyclopropylidene complex rota tes in a plane roughly perpendicular to the total angular momentum vec tor around its C-axis, undergoes ring opening to triplet allene, and d ecomposes via hydrogen emission through a tight transition state to th e propargyl radical. The initial and final orbital angular momenta L a nd L' are weakly coupled and result in an isotropic center-of-mass ang ular distribution. A second microchannel arises from A-like rotations of the cyclopropylidene complex, followed by ring opening and H-atom e limination. In this case, a strong L-L' correlation leads to a forward -scattered center-of-mass angular distribution. The explicit identific ation of C3H3 under single collision conditions represents a single, o ne-step mechanism to build up hydrocarbon radicals. Our findings stron gly demand incorporation of distinct product isomers of carbon atom-ne utral reactions in reaction networks simulating chemistry in combustio n processes, the interstellar medium, as well as in outflows of carbon stars, and open the search for the hitherto unobserved interstellar p ropargyl radical. (C) 1996 American Institute of Physics.