Structural Elucidation of Sulfur Derivatives of Benzimidazoles by Density Functional Calculations, and Vibrational and NMR Spectral Analyses

Abstract

The room temperature structural as well as vibrational (IR and Raman) and NMR (1H and 13C) spectral studies have been performed on 1-methyl-benzimidazole-2-thione, 2-(methylthio)benzimidazole, and 1-methyl-2- mercaptobenzimidazole tautomers at B3LYP/6-311++G** level of theory. 1-methyl-benzimidazole-2-thione is the most stable tautomer with significant energetic separations than the other two tautomers and with too high transition barriers to the other tautomers. This suggests that 1-methyl-benzimidazole-2-thione is the main species at the room temperature. Comparison of experimental and calculated room temperature vibrational and NMR spectra suggests the presence of the 2-(methylthio)benzimidazole and 1-methyl-2-mercaptobenzimidazole tautomers in the solid and solution phases as minor species.