Computational Study of the H2CS and H2CO Reactions with CCH: Mechanisms and Kinetic Implications for Interstellar Chemistry

Abstract

The high abundance of the ethynyl radical (CCH) in the interstellar medium (ISM) makes its reaction with thioformaldehyde (H2CS) potentially important for understanding the chemistry of sulfur-containing molecules and small carbon chains in cold environments. In this work, we present a comprehensive theoretical investigation of this gas-phase reaction through an accurate characterization of the corresponding reactive potential energy surface (PES). To determine reaction rate coefficients and identify the most favorable reaction pathways under the low-temperature and low-pressure conditions of the ISM, kinetic simulations are performed. To assess the reliability of the theoretical approach and elucidate the role of sulfur substitution, the analogous H2CO + CCH reaction has been investigated at the same level of theory and directly compared with available experimental kinetic data. Despite the markedly richer and more interconnected PES of the sulfur system, both reactions are overwhelmingly dominated by a barrierless H-abstraction mechanism leading to HCS/HCO and acetylene, thus making the H2CS + CCH reaction an inefficient source of the C3H2S isomers under interstellar conditions. The comparison with the oxygen analogue reveals both common mechanistic features and important differences while validating the adopted computational protocol. Our study provides the first reliable kinetic characterization of the H2CS + CCH reaction, delivering rate coefficients suitable for inclusion in astrochemical databases.