Molecular structure, vibrational energy levels and potential energy distribution of 1H-imidazo[4,5-b]pyridine, 3H-imidazo[4,5-b]pyridine, 5-methyl-1H-imidazo[4,5-b]pyridine, 6-methyl-1H-imidazo[4,5-b]pyridine and 7-methyl-3H-imidazo[4,5-b]pyridine were determined using density functional theory (DFT) at the B3LYP/6-31G(d,p) level. The optimised bond lengths and bond angles are in good agreement with the X-ray data of 5-methyl-1H-imidazo[4,5-b]pyridine obtained in the present work (Pbca space group; a = 8.660.2/, b = 11.078.2/, c = 11.078.3/Å, Z = 8). The N + H group plays the role of a proton donor in a medium strong hydrogen bond of the type N-H· · ·N, linking the N-atom of the pyridine with the adjacent molecule related by the symmetry operation: 1/2 − x, y − 1/2, z(N· · ·N = 2.869.25/Å). The presence of hydrogen bond is confirmed by appearance in the IR spectra of a very broad and strong contour in the 2000-3100 cm −1 range. The place of substitution of the methyl group at the pyridine ring influences the proton position of the NH group at the imidazole unit.
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