Hydrogen-atom-assisted processes on thioacetamide in -H matrix - formation of thiol tautomers.

TA has been isolated in low-temperature -H 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 -H, 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 NH. 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 (HC-Ċ(-SH)-NH), which has been detected tentatively for the first time. The second step is a barrierless H-atom induced H-abstraction from the -NH moiety of the HC-Ċ(-SH)-NH 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.