The S+(4 S)+SiH 2(1A1 ) Reaction: Toward the Synthesis of Interstellar SiS

We have performed a theoretical investigation of the S + (4 S) + SiH 2 (1 A 1 ) reaction, a possible formation route of the HSiS + and SiSH + cations that are alleged to be precursors of interstellar silicon sulfide, SiS. Electronic structure calculations allowed us to characterize the relevant features of the potential energy surface of the system and identify the reaction pathways. The reaction has two exothermic channels leading to the isomeric species 3 HSiS + and 3 SiSH + formed in conjunction with H atoms. The reaction is not characterized by an entrance barrier and, therefore, it is expected to be fast also under the very low temperature conditions of insterstellar clouds. The two ions are formed in their first electronically excited state because of the spin multiplicity of the overall potential energy surface. In addition, following the suggestion that neutral species are formed by proton transfer of protonated cations to ammonia, we have derived the potential energy surface for the reactions 3 HSiS + / 3 SiSH + +NH 3 (1 A 1 ).