Vibrational spectra, X-ray and molecular structure of 1H- and 3H-imidazo[4,5-b]pyridine and their methyl derivatives: DFT quantum chemical calculations

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) A, Z = 8). The N+H group plays the role of a proton donor in a medium strong hydrogen bond of the type NH…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) A). 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. Copyright © 2007 John Wiley & Sons, Ltd.