PREPARATION, NMR, RAMAN, AND DFT IGLO/GIAO-MP2 STUDY OF MONOPROTONATED 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 sys tems. In FSO3H/SO2ClF at -80 degrees C, thiourea is monoprotonated exc lusively at the sulfur atom giving rise to [(H2N)(2)CSH](+). The addit ion of SbF5 to this system increases the acidity of the solution and r esults 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. T he dication was isolated as its AsF6- salt at -64 degrees C and decomp oses at room temperature to AsF5, HF, and [(NH2)(2)CSH]+AsF6-. The mon o- and the diprotonated AsF6- salts were characterized in the solid st ate by low-temperature Raman spectroscopy, and vibrational assignments are given for both cations. The experimental results and spectroscopi c data were confirmed by density functional theory methods at the B3LY P/6-31G level. Whereas the mono- and diprotonated ions are thermodyna mically stable, the triprotonated ion is only kinetically stable. Depr otonation of triprotonated thiourea to the diprotonated species is exo thermic by 76.3 kcal/mol but displays a high kinetic barrier (51.1 kca l/mol).