A Theoretical Study of the S + C3H Reaction: Potential Energy Surfaces

A theoretical study of the potential energy surfaces of the S + c -C 3 H and S + l -C 3 H reactions has been carried out. The most important local minima and 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) electronic energies has been used to calculate accurate relative energies. The lowest lying state is a nearly linear SCCCH ( 2 A‘) species, but a SC 3 H ( 2 A‘) species, with a cyclic C 3 H group attached to sulfur, is 17.1 kcal/mol higher in energy. The reaction appears to be exothermic for the production of SC 3 ( 1 Σ + ) + H and SC ( 1 Σ + ) + C 2 H ( 2 Σ + ). The reaction mechanisms are rather involved for the products may be generated from SCCCH ( 2 A‘) or imply some other intermediates. The energy profiles for the interaction of sulfur with both l -C 3 H and c -C 3 H 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 each case. The vertical energy gap between the ground and the lowest-lying states has been determined through the MRCI method for each reaction intermediate. The S + C 3 H reaction could indeed be a source of SC 3 ( 1 Σ + ), but its efficiency would depend on the SC 3 + H/SC + C 2 H branching ratio.