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 ethylene, C2H4(X(1)A(g)), was studied at average collision energies of 17.1 and 38.3 kJmol(-1) using the crossed molecular beams technique. Product angular distributions and time-of-flight spectra of m/e = 39 were recorded. Forward-convolution fitting of the results yields a maximum energy release as well as angular distributions consistent with the formation 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 ethylene 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 rotates in a plane roughly perpendicular to the total angular momentum vector around its C-axis, undergoes ring opening to triplet allene, and decomposes via hydrogen emission through a tight transition state to the propargyl radical. The initial and final orbital angular momenta L and L' are weakly coupled and result in an isotropic center-of-mass angular distribution. A second microchannel arises from A-like rotations of the cyclopropylidene complex, followed by ring opening and H-atom elimination. In this case, a strong L-L' correlation leads to a forward-scattered center-of-mass angular distribution. The explicit identification of C3H3 under single collision conditions represents a single, one-step mechanism to build up hydrocarbon radicals. Our findings strongly demand incorporation of distinct product isomers of carbon atom-neutral reactions in reaction networks simulating chemistry in combustion processes, the interstellar medium, as well as in outflows of carbon stars, and open the search for the hitherto unobserved interstellar propargyl radical. (C) 1996 American Institute of Physics.