DFT studies of the structure and vibrational and NMR spectra of 1-(2-methylpropenyl)-2-methylbenzimidazole

Synthesis of two new 1-(2-methylpropenyl)-2-methylbenzimidazoles by reaction of 2-methylbenzimidazole with 3-chloro-2-methylpropene using a strong base as a catalyst is described. Gas chromatography–mass spectrometry (GC–MS) allowed the characterization of the structural isomers: the slightly more stable 1-(2-methyl-1-propenyl)-2-methylbenzimidazole (51%) and a less stable 1-(2-methyl-2-propenyl)-2-methylbenzimidazole (49%). The results of theoretical calculations indicate that the difference in the energy between the two structural isomers is 3.21 kcal mol−1 at the B3LYP/6-311+G∗∗ level. The structures were confirmed by 1H and 13C Nuclear Magnetic Resonance, elemental analysis and spectroscopic methods such as FT-Raman, FT-IR and UV–VIS. The experimental results were supported by performing DFT calculations for energies, geometries, vibrational frequencies and shieldings constants using 6-311+G∗∗ basis sets and B3LYP functional. The theoretical data have satisfactorily reproduced the experimental results. The consistency and efficiency of the GIAO method used to calculate absolute shielding of the studied compounds were checked by the analysis of statistical parameters and were found to be in excellent agreement with experimental values. This correlation has been used for unambiguous NMR signal assignments for studied compounds.