Thione–Thiol Tautomerization Induced by Hydrogen Atoms in Amorphous Solids

Low-temperature neat amorphous ices of thiourea (TU) and N-methyl thiourea (NMTU) were exposed to hydrogen (H) atoms during deposition, generated by an H atom beam source. New spectral features emerged, at positions close to the ones observed in similar experiments, recently reported for the related compound, thioacetamide. These new features appearing upon H atom bombardment were assigned to higher-energy thiol tautomeric forms of TU and NMTU. The mid-infrared (MIR) spectra of unprocessed amorphous TU and NMTU ices are also reported here for the first time. Both compounds, either in amorphous state or in crystals, adopt exclusively the thione form in neat ices. The spectra undergo irreversible changes when the ices are heated up to the crystallization temperature. The MIR spectra of crystalline samples show great similarity with the ones in microcrystalline state dispersed in KBr pellets reported in the literature. The simulated IR spectra provided by quantum-chemical computational methods are in good agreement with the experimental ones; the theoretical data also enabled us to perform the vibrational analysis of the samples. Based on the MIR spectra of the crystalline samples, those of the amorphous ices could also be interpreted. The appearance of the new thiol bands, upon H atom bombardment of the thione precursors, points to the universal nature of thione → thiol tautomerization of thioamides in the presence of H atoms, most probably taking place on the ice surface. Furthermore, we provide a mechanistic explanation of the experimental observations, initiated by the generally facile manner of H atom addition to the sulfur of the thione group.