Preparation, NMR, Raman, and DFT/IGLO/GIAO-MP2 Study of Mono- and Diprotonated Thiourea and Theoretical Investigation of Triprotonated Thiourea1

Mono-, di-, and triprotonation of thiourea (H2N)2CS, was studied by low-temperature 1H, 13C, and 15N NMR spectroscopy in superacidic systems. In FSO3H/SO2ClF at -80 °C, thiourea is monoprotonated exclusively at the sulfur atom giving rise to [(H2N)2CSH]. The addition of SbF5 to this system increases the acidity of the solution and results in the observation of the diprotonated species [H3NC(SH)NH2]. No NMR evidence was found for triprotonation under these conditions, although a limited equilibrium should not necessarily be detected. The dication was isolated as its AsF6 salt at -64 °C and decomposes at room temperature to AsF5, HF, and [(NH2)2CSH]AsF6. The monoand the diprotonated AsF6 salts were characterized in the solid state by lowtemperature Raman spectroscopy, and vibrational assignments are given for both cations. The experimental results and spectroscopic data were confirmed by density functional theory methods at the B3LYP/6-31G* level. Whereas the monoand diprotonated ions are thermodynamically stable, the triprotonated ion is only kinetically stable. Deprotonation of triprotonated thiourea to the diprotonated species is exothermic by 76.3 kcal/mol but displays a high kinetic barrier (51.1 kcal/mol).