Ab Initio and Statistical Rate Theory Exploration of the CH (X²Π) + OCS Gas-Phase Reaction

Abstract

The first theoretical results regarding the gas-phase reaction mechanism and kinetics of the CH (X2Π) + OCS reaction are presented here. This reaction has a proposed importance in the removal of OCS in regions of the interstellar medium (ISM) and has the potential to form the recently observed HCS/HSC isomers, with both constitutional isomers having recently been observed in the L483 molecular cloud in a 40:1 ratio. Statistical rate theory simulations were performed on stationary points along the reaction potential energy surface (PES) obtained from ab initio calculations at the RO-CCSD(T)/aug-cc-pV(Q+d)Z//M06-2X-D3/aug-cc-pV(Q+d)Z level of theory over the temperature and total density range of 150–3000 K and 1011–1024 cm–3, respectively, using a Master Equation analysis. Exploration of the reaction potential energy surface revealed that all three pathways identified to create CS + HCO products required surmounting barriers of 16.5 kJ mol–1 or larger when CH approached the oxygen side of OCS, rendering this product formation negligible below 1000 K, and certainly under low-temperature ISM conditions. In contrast, when CH approaches the sulfur side of OCS, only submerged barriers are found along the reaction potential energy surface to create HCCO + S or CO + HCS, both of which are formed via a strongly bound OCC(H)S intermediate (−358.9 kJ mol–1). Conversion from HCS to HSC is possible via a barrier of 77.8 kJ mol–1, which is still −34.1 kJ mol–1 below the CH + OCS entrance channel. No direct route from CH + OCS to H + CO + CS was found from our ab initio calculations. Rate theory simulations suggest that the reaction has a strong negative temperature dependence, in accordance with the barrierless addition of CH to the sulfur side of OCS. Product branching fractions were also determined from MESMER simulations over the same temperature and total density range. The product branching fraction of CO + HCS reduces from 79% at 150 K to 0.0% at 800 K, while that of HCS dissociation to H + CS + CO increases from 22% at 150 K to 100% at 800 K. The finding of CO + HCS as the major product at the low temperatures relevant to the ISM, instead of H + CS + CO, is in opposition to the current supposition used in the KIDA database and should be adapted in astrochemical models as another source of the HCS isomer.