Theoretical study of the reaction of S+ with acetylene

A theoretical study of the reaction of S+ with acetylene has been carried out using the G2 procedure and the CCSD(T) method with the cc-pVTZ basis set on B3LYP/cc-pVTZ geometries. Whereas G2 and CCSD(T) methods provide similar results, the B3LYP results differ considerably. The G2(P) procedure (which employs spin-projected MP values for computation of the G2 energy) gives results which are in agreement with the B3LYP ones. Production of SC2H+ is shown to be exothermic at all levels of theory, in agreement with experiment. G1, G2 and CCSD(T) methods predict a small activation barrier for this reaction (about 2–4 kcal mol−1) on the quartet surface. Intersystem crossing, leading to the doublet surface, is feasible and the reaction proceeds on this surface without any barrier. Therefore the reaction of S+ ions with acetylene could take place simultaneously by two different mechanisms: direct formation of products on the quartet surface and through the doublet surface via intersystem crossing.