Theoretical study of the reaction of Si+ with C3H2

A theoretical study of the (SiC3H)(+) and (SiC3H2)(+) species has been carr ied out, Two different models, MP4 at MP2 geometries and QCISD(T) at B3LYP geometries, have been employed. Significant differences are encountered whe n spin contamination is relatively high. Our calculations predict that the global minimum of the (SiC3H)(+) system is a cyclic isomer, derived from pr otonation of the SiC3 ground state. The proton affinities of the three lowe st-lying isomers of SiC3 have been computed, obtaining relatively high valu es in all cases. The lowest-lying (SiC3H2)(+) species has a linear carbon b ackbone and can be formally derived from the bonding of Si+ to vlnylideneca rbene (1-C3H2) through an electron lone pair. The cyclic isomer obtained fr om 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, charg e transfer is endothermic, whereas production of SiC3+ is slightly exotherm ic and exhibits a small barrier. The preferred channel is formation of cycl ic 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 pa th leading to linear SiC3+ involves a high energy barrier. There are two po ssible competitive processes which are barrier-free: production of linear S iC3H+ 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 possibl e sources of precursors of SIC3.