Hydrogen-atom-assisted thione–thiol tautomerization of thiourea derivatives in <i>para</i>-H2 matrix

Thiourea (TU) and its N-methylated derivative, N-methyl thiourea (NMTU), were exposed to H atoms generated in cryogenic para-H2 matrices. The reactions were followed online by FT-IR spectroscopy. The freshly deposited matrices exclusively contained the more stable thione tautomeric forms. However, upon exposure to H atoms, the peaks belonging to the precursor molecules clearly decreased along with the simultaneous appearance of new signals. These new bands could be attributed to the corresponding higher-energy thiol forms (in the case of TU) and, tentatively, to an intermediate radical (in both the TU and NMTU experiments). The radicals are suggested to be the H-atom-addition products of the TU and NMTU thione precursors, with the addition occurring on the S atom. These intermediates may then react with another free H atom, leading to the formation of the more energetic thiol tautomers, following an H-atom-abstraction process. As such, these radicals act as the centerpiece of the reaction scheme, enabling the thione-thiol tautomerization. This H-atom-assisted process is similar to that observed for the related molecule, thioacetamide. The interpretation of the experimental results was supplemented by quantum-chemical computations, which predicted all the above-mentioned reactions to be barrierless. The presence of H atoms opens a barrierless pathway; thus, the process does not necessarily require activation through irradiation (e.g., broadband UV). These findings point to the ubiquitous nature of the facile hydrogenation/dehydrogenation of the S atom, implying that thione-thiol tautomerization may occur easily.