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 (H 2 N) 2 CS, was studied by low-temperature 1 H, 13 C, and 15 N NMR spectroscopy in superacidic systems. In FSO 3 H/SO 2 ClF at −80 °C, thiourea is monoprotonated exclusively at the sulfur atom giving rise to [(H 2 N) 2 CSH] + . The addition of SbF 5 to this system increases the acidity of the solution and results in the observation of the diprotonated species [H 3 NC(SH)NH 2 ] 2+ . 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 AsF 6 - salt at −64 °C and decomposes at room temperature to AsF 5, HF, and [(NH 2 ) 2 CSH] + AsF 6 - . The mono- and the diprotonated AsF 6 - salts were characterized in the solid state by low-temperature 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 mono- and 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).