Hydrogen-Atom-Assisted\nTautomerization on Solid SurfacesThe\nCase Study of Thioacetamide

Amorphous thioacetamide (TA) ice was prepared by deposition\non\na low-temperature substrate in an ultrahigh-vacuum simulation chamber,\nexposing the sample to a beam of hydrogen atoms. The structural changes\nwere monitored by reflection–absorption FT-IR spectroscopy.\nThe spectral data unambiguously evidence the formation of the higher-energy\nthiol tautomeric forms of TA upon interaction with H atoms, resulting\nfrom the more stable thione tautomer, which is otherwise exclusively\npresent in unprocessed ice. The regeneration of the thione species\nfrom the thiol tautomeric forms occurs upon heating the sample above\n100 K. Quantum-chemical computations were conducted to confirm the\nconclusions drawn from the experimental results. According to the\ntheoretical findings, the thiol molecules, formed on the surface of\namorphous ice upon H-atom-assisted thione → thiol tautomerization,\ncannot survive in the crystalline material. A mechanism for the process\nat the molecular level is also proposed. This work provides the first\nexample of H-atom-assisted tautomerization occurring on the surface\nof amorphous ice without the action of UV light as the source of energy.