Hydrogen-atom-assisted processes on thioacetamide in para-H2 matrix – formation of thiol tautomers

Abstract

TA has been isolated in low-temperature para-H₂ matrices and it has been exposed to H atoms. In accordance with previous experimental results, TA exclusively exists in its more stable thione tautomeric form in the freshly deposited matrix. However, upon H atom generation, the bands belonging to the precursor start decreasing with the simultaneous appearance of new bands. By comparing the position of these new peaks with earlier results obtained in para-H₂, they can be undoubtedly attributed to the presence of the higher-energy thiol tautomeric forms of TA. No other products could be observed, except for NH₃. Quantum-chemical computations have been invoked to understand the mechanism behind the observed thione–thiol tautomerization assisted by H atoms. Accordingly, tautomerization starts with barrierless H-atom addition on the S atom of the thione resulting in the formation of the intermediate 1-amino-1-mercapto-ethyl radical (H₃C–Ċ(–SH)–NH₂), which has been detected tentatively for the first time. The second step is a barrierless H-atom induced H-abstraction from the –NH₂ moiety of the H₃C–Ċ(–SH)–NH₂ radical. A comprehensive mechanism for tautomerization is proposed based on the experimental and theoretical results. Although earlier studies showed the possibility of TA thione–thiol tautomerization in cryogenic matrices achieved by energetic UV irradiation, the present study points out that this can also take place through a barrierless pathway by simply exposing the TA molecules to H atoms. As such, this is the first evidence for the occurrence of such a reaction in a matrix-isolated environment. The current results also help us better understand the mystery behind thione–thiol tautomerization in low-temperature environments.