Crossed-beam reaction of carbon atoms with hydrocarbon molecules. III: Chemical dynamics of propynylidyne (l-C3H;X 2Πj) and cyclopropynylidyne (c-C3H;X 2B2) formation from reaction of C(3Pj) with acetylene, C2H2(X 1Σg+)

The reaction between ground state carbon atoms, C(3Pj), and acetylene, C2H2(X 1Σg+), 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(3Pj) 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 2Πj) and H. Direct stripping dynamics contribute to the forward-scattered second microchannel to form c-C3H(X 2B2) and H. This contribution is quenched with rising collision energy. The explicit identification of l-C3H(X 2Πj) and c-C3H(X 2B2) 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 (≈1) as compared to the circumstellar envelope surrounding the carbon star IRC+10216 (≈0.2) via the atom-neutral reaction C(3Pj)+C2H2(X 1Σg+).