A theoretical study of the S+C2H reaction:: Potential energy surfaces and dynamics

A theoretical study of the reaction of S with C2H has been carried out. This reaction is a possible step in the generation of sulfur-containing cumulenes in interstellar clouds and circumstellar envelopes. The potential energy surfaces were computed by means of the G2, G2(QCI), and CBS-Q methods in the case of local minima and saddle points. The energy profiles for the interaction of S and C2H in all states associated with the lowest energy electron configurations have received special attention. The MR-AQCC/aug-cc-pVTZ method was used as the basic level of computation; spin-orbit interactions and basis set superposition corrections were also taken into account. We found only two neatly attractive potential energy surfaces, corresponding to the (2)Pi (3/2) and (2)Pi (1/2) electronic states. We employed an approximate classical trajectory method to compute the capture rate. According to our computations the reaction is relatively fast, its rate for T = 300 K being not too far from the typical values for ion-molecule reactions. The main product should be SC2((3)Sigma (-))+H(S-2). However, we have encountered a slight decrease in the rate coefficient with decreasing temperature.