Preparation, NMR, Raman, and DFT/IGLO/GIAO-MP2 Study of Mono- and Diprotonated Thiourea and Theoretical Investigation of Triprotonated Thiourea 1 George A. Olah,* ,² Arwed Burrichter, ² Golam Rasul, ² Karl O. Christe, ²,‡ and G. K. Surya Prakash* ,² Contribution from the Loker Hydrocarbon Research Institute and Department of Chemistry, UniVersity of Southern California, UniVersity Park, Los Angeles, California 90089-1661, and Hughes STX, Phillips Laboratory, Edwards Air Force Base, California 93524 ReceiVed January 23, 1997 X Abstract: 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). Introduction Thiourea, the thiocarbonyl analogue of urea, is one of the simplest organic molecules containing a thioamide group, and its structure and properties have been studied extensively by various experimental and theoretical techniques. 2,3,4 In bio- chemistry, considerable interest has been focused on the role of the thioamide group, as it is a fundamental building block in the skeleton of thiopurines and thiopyrimidines. 5,6 Extensive work has been done on the protonation of thiourea in aqueous solutions. It has been known since the middle of the 19th century that thiourea forms 1:1 compounds with strong acids such as HCl and H 2 SO 4 . 7 There are two possible structures for monoprotonated thiourea, the sulfonium form I and the am- monium form II, and the preferred site of protonation has remained controversial for a long time. 8 On theoretical grounds the sulfonium structure I should be preferred because of its resonance stabilization. However, on the basis of the infrared spectra of thiouronium salts in the solid state, Spinner had suggested N-protonation of thiourea. 9 On the other hand, 1 H NMR of the salts in solution indicated protonation on the sulfur atom. 10,11 In subsequent UV and IR studies of thiouronium salts, the vibrational frequencies were reassigned in favor of the sulfonium structure I. 12,13 Conductivity studies of thiourea in ClSO 3 H were also interpreted in terms of monoprotonation on the sulfur atom. 14 Semiempirical SCF calculations have been used to demonstrate that S-protonation is energetically favored over N-protonation. 15 Recently, Murgich et al. have used 14 N nuclear quadrupole resonance (NQR) spectroscopy to support S-protonation of thiourea in solution. 16 Although in the solid state the thiouronium monocation exists in the S-protonated form I, as shown by X-ray diffraction data, 17 the behavior of thiourea in strong acids has not been clearly established. Birchall and Gillespie studied the proton NMR spectra of thiourea in water, CF 3 COOH, H 2 SO 4 , and FSO 3 H. 11 In H 2 O and CF 3 COOH solutions at 25 °C, only one line due to the NH 2 group was observed. In the more acidic solvents H 2 SO 4 and FSO 3 H, a new resonance appeared at about δ 1 H 5.0 which was attributed to the CdSH + group and the NH 2 group signal disappeared. The observations were attributed to diprotonation, with the first proton being attached to sulfur and exchanging only very slowly with the solvent and the second proton being attached to nitrogen and exchanging rather rapidly with the solvent. Cryoscopic and conductometric studies by Paul et al., however, showed that ² University of Southern California. ‡ Hughes STX. X Abstract published in AdVance ACS Abstracts, April 15, 1997. (1) Considered: Chemistry in Superacids. Part 30. For Part 29, see: Olah, G. A.; Rasul, G.; Prakash, G. K. S. Chem., Eur. J., in press. (2) Treuter, M. R. 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(17) Feil, D.; Loong, W. S. Acta Crystallogr. 1968, B24, 1334. 3 4345 J. Am. Chem. Soc. 1997, 119, 4345-4352 S0002-7863(97)00221-7 CCC: $14.00 © 1997 American Chemical Society