Theoretical study of the reaction of S+ with C3H2

A theoretical study of the (SC3H2)+ species has been carried out. Two different models, complete MP4 at MP2 geometries and QCISD(T) at B3LYP geometries, have been employed. Our calculations predict that the global minimum is SCCCH2+(2B2), although a doublet cyclic isomer lies only about 5 kcal/mol higher in energy. The lowest-lying quartet is also SCCCH2+(4B2), followed by a nonplanar three-membered ring isomer. These theoretical results allow the development of thermodynamic arguments about the reaction pathways of the process S++C3H2. For the reaction of S+ with cyclopropenylidene (c-C3H2) production of cyclic SC3H+ is slightly endothermic and proceeds through a significant energy barrier. On the other hand, production of the linear isomer SCCCH+ is exothermic and there is at least a mechanism leading to this species that is barrier-free. However, the preferred channel seems to be charge transfer, since it is predicted to be more exothermic. In the case of the reaction of S+ with vinylidenecarbene (l-C3H2), charge transfer is not competitive since it is clearly endothermic. Production of cyclic SC3H+, although exothermic, seems to involve an energy barrier at the QCISD(T) level. Production of the linear SCCCH+ isomer should be the preferred channel, since it is more exothermic and there are two different mechanisms that are barrier-free. Therefore, it seems that only the linear SCCCH+ isomer can be produced from the reaction of S+ with both c-C3H2 and l-C3H2. Consequently, these ion-molecule reactions could be possible sources of precursors of C3S in space.