Cis--Trans Rotational Isomerism of Seleno-, Thio-, and Formic Acids and Their Dimers: Chemical Kinetics under Interstellar Conditions
Judith Wurmel, John M. Simmie
TL;DR
This work tackles cis–trans rotational isomerism (rotamerisation) of formic acid and its heavier analogues (thio- and seleno-formyl acids) and their dimers under interstellar conditions by computing kinetic rate constants across 10–300 K. A two-stage quantum-chemical framework is employed: initial IRC-based geometry/frequency calculations at B2PLYP-D3BJ/def2-TZVP with ISPE corrections, followed by high-level DLPNO-CCSD(T)/def2-TZVP refinements, with CVT/SCT tunnelling corrections (and QRC/ISPE where applicable) to obtain rate constants. The results show tunnelling dominates at low temperatures, with formic acid cis→trans isomerisation occurring very rapidly and effectively removing the cis form from the gas phase, while dimers exhibit much slower tunnelling. The findings provide upper-bound gas-phase kinetics to guide interpretation of cryogenic matrix experiments and imply that observed cis-HC(O)OH in the ISM must arise from routes other than simple cis→trans interconversion on grains, highlighting the importance of matrix effects and alternative formation pathways. Overall, the study clarifies how barrier width and tunnelling govern rotamerisation in these systems and delivers benchmarks for astrochemical modelling of interstellar ice-grain chemistry.
Abstract
Tunnelling reactions of molecules embedded on cryogenic noble-gas matrices are being used in fundamental studies of how reactivity varies with the nature of the supposedly inert matrix as well as pointers to the chemistry occurring in the interstellar medium on ice-grains. To these ends we present chemical kinetic rate constants for the \textit{cis} to \textit{trans} isomerisation of seleno-, thio- and monomeric formic acids and that of their three dimeric species, based on multidimensional calculations in the gas-phase, from 10~K to 300~K as a guide to the matrix reactions.