Quantum chemical evaluation of the astrochemical significance of reactions between S atom and acetylene or ethylene

Addition-elimination reactions of S atom in its ³P ground state with acetylene (C ₂H ₂) and ethylene (C ₂H ₄) were characterized with both molecular orbital and density functional theory calculations employing correlation consistent basis sets in order to assess the likelihood that either reaction might play a general role in astrochemistry or a specific role in the formation of S ₂ (X ³∑ _g ^-) via a mechanism proposed by Saxena, P. P.; Misra, A. Mon. Not. R. Astron. Soc. 1995, 272, 89. The acetylene and ethylene reactions proceed through C ₂H ₂S ( ³A″) and C ₂H ₄S ( ³A″) intermediates, respectively, to yield HCCS ( ²∏) and C ₂H ₃S ( ²A′). Substantial barriers were found in the exit channels for every combination of method and basis set considered in this work, which effectively precludes hydrogen elimination pathways for both S + C ₂H ₂ and S + C ₂H ₄ in the ultracold interstellar medium where only very modest barriers can be surmounted and processes without barriers tend to predominate. However, if one or both intermediates are formed and stabilized efficiently under cometary or dense interstellar cloud conditions, they could serve as temporary reservoirs for the S atom and participate in reactions such as S + C ₂H ₂S → S ₂ + C ₂H ₂ or S + C ₂H ₄S → S ₂ + C ₂H ₄. For formation and stabilization to be efficient, the reaction must possess a barrier height small enough to be surmountable at low temperatures yet large enough to prevent redissociation to reactants. Barrier heights computed with B3LYP and large basis sets are very low, but more rigorous QCISD(T) and RCCSD(T) results indicate that the barrier heights are closer to 3-4 kcal/mol. The calculations therefore indicate that S + C ₂H ₂ or S + C ₂H ₄ could contribute to the formation of S ₂ in comets and may serve as a means to gauge coma temperature. The energetics of the ethylene reaction are more favorable.