Theoretical study of the reaction of S+ with acetylene

A theoretical study of the reaction of S+ with acetylene has been carried o ut 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 simila r results, the B3LYP results differ considerably. The G2(P) procedure (whic h 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 experimen t. G1, G2 and CCSD(T) methods predict a small activation barrier for this r eaction (about 2-4 kcal mol(-1)) on the quartet surface. Intersystem crossi ng, leading to the doublet surface, is feasible and the reaction proceeds o n this surface without any barrier. Therefore the reaction of S+ ions with acetylene could take place simultaneously by two different mechanisms: dire ct formation of products on the quartet surface and through the doublet sm face via intersystem crossing. (C) 1999 Elsevier Science B.V. All rights re served.