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 (1 H and 13 C) spectral studies have been performed on 1-methyl-benzimidazole-2-thione, 2-(methylthio)benzimidazole, and 1-methyl-2mercaptobenzimidazole 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.

3a-(4-Chlorophenyl)-1-thioxo-2,3,3a,4-tetrahydroimidazo[1,5-a]quinazolin-5(1H)-one

“…BP86 /Jgauss-TZP2 level of theory was selected by combining a series of functionals and basis sets, validating them for some reference compounds. The calculation carried out in chloroform has provided a much better correlation of experimental 13 C chemical shifts with the calculated values for the thioamide form (NHC=S) than the iminothiol (N=CSH) tautomer. In detail, the experimental value 182.26 ppm is in line with the calculated 181.84 ppm assigned to C-10 of the thioamide form, whereas the value 162.72 ppm was obtained for the iminethiol form.…”

“…1 H and 13 C NMR spectra indicate the presence of compound 3 as a single form (Figure S2). In particular, the values 182.31 ppm and 163.44 ppm attributable to C-10 and C-4 respectively, suggest the thione tautomer, if referred to what is reported in the literature according to which the C=S nucleus resonates ca.…”

“…Knowing NMR spectroscopy is one of the most effective tools to study tautomerism and the predictive power of NMR DFT calculation, we decided to compare the experimental 13 CNMR data with the calculated values for both thioamide and iminethiol forms (Figure 1). BP86 /Jgauss-TZP2 level of theory was selected by combining a series of functionals and basis sets, validating them for some reference compounds.…”

“…Various investigations on tautomerism of synthetic imidazole-2-thiones have been reported by IR, UV, NMR, and X-ray spectroscopies [11]. Detailed studies were carried out for benzoimidazole-2-thione by experimental and computational approaches [12,13].…”

“…In the present study 3a-(4-chlorophenyl)-1-thioxo-2,3,3a,4-tetrahydroimidazo[1,5-a]quinazolin-5(1H)-one was synthesized from a precursor that we previously used for producing a three cycled fused quinazolin-4(3H)-one derivative [14]. The structural characterization of the title compound is described also in terms of possible tautomerism, supported by a comparison of experimental and density functional theory (DFT) calculated 13 CNMR spectra.…”