Crossed-beam reaction of carbon atoms with hydrocarbon molecules .3. Chemical dynamics of propynylidyne (l-C3H; X(2)II(j)) and cyclopropynylidyne (c-C3H; X(2)B(2)) formation from reaction of C(P-3(j)) with acetylene, C2H2(X(1)Sigma(+)(g))1 Sigma+g)

The reaction between ground state carbon atoms, C(P-3(j)), and acetylene, C2H2(X(1) Sigma(g)(+)), is studied at three collision energies between 8.8 and 45.0 kJ mol(-1) using the crossed molecular beams technique. Product angular distributions and time-of-flight spectra of C3H at m/e = 37 are recorded. Forward-convolution fitting of the data yields weakly polarized center-of-mass angular flux distributions decreasingly forward scattered with respect to the carbon beam as the collision energy rises from 8.8 to 28.0 kJ mol(-1), and isotropic at 45.0 kJ mol(-1). Reaction dynamics inferred from the experimental data and ab initio calculations on the triplet C3H2 and doublet C3H potential energy surface suggest two microchannels initiated by addition of C(P-3(j)) either to one acetylenic carbon to form s-trans propenediylidene or to two carbon atoms to yield triplet cyclopropenylidene via loose transition states located at their centrifugal barriers. Propenediylidene rotates around its B/C axis and undergoes [2,3]-H-migration to propargylene, followed by C-H bond cleavage via a symmetric exit transition state to l-C3H(X (2) parallel to(j)) and H. Direct stripping dynamics contribute to the forward-scattered second microchannel to form c-C3H(X B-2(2)) and H. This contribution is quenched with rising collision energy. The explicit identification of l-C3H(X (2) Pi(j)) and c-C3H(X B-2(2)) under single collision conditions represents a one-encounter mechanism to build up hydrocarbon radicals in the interstellar medium and resembles a more realistic synthetic route to interstellar C3H isomers than hitherto postulated ion-molecule reactions. Relative reaction cross sections to the linear versus cyclic isomer correlate with actual astronomical observations and explain a higher [c-C3H]/[l-C3H] ratio in the molecular cloud TMC-1 (approximate to 1) as compared to the circumstellar envelope surrounding the carbon star IRC+10216 (approximate to 0.2) via the atom-neutral reaction C(P-3(j))+C2H2(X C-1(g)+). (C) 1997 American Institute of Physics.