Restricted rotation around the methylene bridge of 5-(2-p-(chlorophenyl)benzimidazole-1-yl)methyl-4-(o-substitutedphenyl)-2,4-dihydro-[1,2,4]-triazole-3-thiones as evidenced by NMR, X-RAY and DFT studies and the importance of low energy rotational conformers

Abstract

The aim of this study was to clarify the restricted rotation around the –CH2–N bond of the compounds, 5-(2-(p-chlorophenylbenzimidazol-1-yl-methyl)-4-(o-methylphenyl)-2,4-dihydro-[1,2,4]-triazole-3 thione [1], 4-(o-florophenyl)- [2], 4-(o-chlorophenyl)- [3] and 4-(o-bromophenyl)- [4], which are some benzimidazole derivatives showing antioxidant properties. In NMR spectra of relevant compounds, because of the hindered rotation around the methylene bridge, methylene protons appear as an AB quartet in DMSO?d6. The high temperature NMR spectra (296.1 K, 308.1 K, 323.1 K and 332.1 K) for the compound 1 show similar behavior in DMSO?d6. The NMR spectra in different solvents point out the same manner. The preventing of free rotation in the methylene bridge caused by the existence of substituents at the second position of the phenyl group connected to triazole ring was supported by both the experimental results (NMR and X-ray) and the theoretical results in the gas phase. The hindered rotation of compound 1 also investigated by applying the polarizable continuum model (PCM) for different solvents (DMSO?d6 and C6D6), as did in NMR. Additionally, potential energy surface (PES) scan revealed that each compound has two possible conformers. © 2018 Elsevier B.V.