Kinetics of tautomerisation of thiouracils and cognate species at low temperatures: theory versus experiment.

Hydrogen-atom tunnelling is an important component in some chemical reactions particularly at low temperatures ≤300 K. Recent experiments by Rostkowska et al. [H. Rostkowska, L. Lapinski and M. J. Nowak, Intramolecular Hydrom-Atom Tunnelling in Matrix-Isolated Heterocyclic Compounds: 2-Thiouracil and Its Analogues, Phys. Chem. Chem. Phys., 2024, 26, 23944-23950.] showed that higher energy monomeric conformers of thiouracil and cognate species (thiols) prepared on neon and argon matrices at 3.5 K spontaneously reverted to the lower energy conformer (thiones) presumably by hydrogen-atom quantum mechanical tunnelling. We have shown that these observations can be rationalised by carrying out chemical kinetic calculations employing canonical variational transition state theory with tunnelling effects on these systems in the gas-phase. We show that tunnelling is totally dominant in these systems from 300 K down and discount the possibility of adventitious water contaminating the experimental observations.