Preparation, NMR, Raman, and DFT/IGLO/GIAO-MP2 study of mono- and diprotonated thiourea and theoretical investigation of triprotonated thiourea

Mono-, di-, and triprotonation of thiourea (H2N)(2)CS, was studied by low-temperature H-1, C-13 and N-15 NMR spectroscopy in superacidic systems. In FSO3H/SO2ClF at -80 degrees C, thiourea is monoprotonated exclusively at the sulfur atom giving rise to [(H2N)(2)CSH](+). 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](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 AsF6- salt at -64 degrees C and decomposes at room temperature to AsF5, HF, and [(NH2)(2)CSH]+AsF6-. The mono- and the diprotonated AsF6- 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).