Theoretical study of the reaction of Si+ with C3H2

A theoretical study of the (SiC3H)(+) and (SiC3H2)(+) species has been carried out, Two different models, MP4 at MP2 geometries and QCISD(T) at B3LYP geometries, have been employed. Significant differences are encountered when spin contamination is relatively high. Our calculations predict that the global minimum of the (SiC3H)(+) system is a cyclic isomer, derived from protonation of the SiC3 ground state. The proton affinities of the three lowest-lying isomers of SiC3 have been computed, obtaining relatively high values in all cases. The lowest-lying (SiC3H2)(+) species has a linear carbon backbone and can be formally derived from the bonding of Si+ to vlnylidenecarbene (1-C3H2) through an electron lone pair. The cyclic isomer obtained from cyclopropenylidene (c-C3H2) is also quite stable, lying only about 9-12 kcal/mol above the ground state. For the reaction of Sif with c-C3H2, charge transfer is endothermic, whereas production of SiC3+ is slightly exothermic and exhibits a small barrier. The preferred channel is formation of cyclic SiC3H+, since it is clearly exothermic and barrier-free. In the case of the reaction of Si+ + 1-C3H2 charge transfer is also endothermic and the path leading to linear SiC3+ involves a high energy barrier. There are two possible competitive processes which are barrier-free: production of linear SiC3H+ and formation of cyclic SiC3H+ through a previous isomerization into a cyclic SiC3H2+ species. Therefore, the reactions of Sit with both c-C3H2 and 1-C3H2 are feasible in the interstellar medium and consequently possible sources of precursors of SIC3.