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.