A theoretical study of the S+C3H reaction: Potential energy surfaces

A theoretical study of the potential energy surfaces of the S + c-C3H and S + l-C3H reactions has been carried out. The most important local minima an d saddle points have been located at the B3LYP/6-311G** and QCISD/6-311G** levels, whereas a G2-style method based on QCISD(T)/6-311+G(3df,2p) electro nic energies has been used to calculate accurate relative energies. The low est lying state is a nearly linear SCCCH ((2)A') species, but a SC3H ((2)A' ) species, with a cyclic C3H group attached to sulfur, is 17.1 kcal/mol hig her in energy. The reaction appears to be exothermic for the production of SC3 ((1)Sigma (+)) + H and SC ((1)Sigma (+)) + C2H ((2)Sigma (+)). The reac tion mechanisms are rather, involved for the products may be generated from SCCCH ((2)A') or imply some other intermediates. The energy profiles for t he interaction of sulfur with both l-C3H and c-C3H through the lowest lying doublet and quartet states have also been computed by means of the CASSCF method. We have found at least two neatly attractive electronic states in e ach case. The vertical energy gap between the ground and the lowest-lying s tates has been determined through the MRCI method for each reaction interme diate. The S + C3H reaction could indeed be a source Of SC3 ((1)Sigma (+)), but its efficiency would depend on the SC3 + H/SC + C2H branching ratio.