Tautomerization Kinetics of Thiouracils at Low Temperatures Driven Solely by Quantum‐Mechanical Tunnelling: Predictions of Suitable Candidates for Future Experiments

ABSTRACT In the past two decades numerous experiments have been conducted on the kinetics of reactions occurring at low temperatures on inert matrices. In particular, unimolecular proton‐transfer photo‐reactions of compounds containing a carbon–sulfur double bond, or thione group C═S, can lead to the formation of higher energy isomers or tautomers with a thiol linkage C─S─H, and their fate as they revert can be easily followed by IR spectroscopy. Using density functional theory quantum chemical calculations, but based on gas‐phase conditions, we investigate the barriers and chemical reaction rates of the return of these high energy isomers of 2‐thiouracil, 4‐thiouracil, 2,4‐dithiouracil and 6‐aza‐2,4‐dithiouracil to their original parent structures at temperature of 10‐300 K. We show that quantum mechanical tunnelling is the dominant process rather than thermal reactions and consider other candidates, with substituent fluoro, methyl, perfluoromethyl and amino groups, whose lifetimes, particularly those with 1‐ of a few hours, would be suitable for laboratory testing and which would provide more benchmarks for validating the theoretical calculations with the ultimate interest of deepening our understanding of reactions on water–ice grains in the interstellar medium.