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

Abstract

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.

Quantum chemical studies on prototautomerism of 1H‐imidazo[4,5‐c]pyridine

“…These modes are observed in farinfrared and Raman spectra at 262 cm À1 (IR), 265 cm À1 (Ra); and 238 cm À1 (IR), 236 cm À1 (Ra), respectively. These assignments are also in agreement with values given in the literature [24].…”

“…The optimized geometry is compared with the structural parameters obtained from crystallographic analysis of similar molecules or units, benzimidazole [13], 5-ethoxy-1-azaindolizine [34], and 5-methyl-1H-imidazo [4,5-b]pyridine [24], pyrrole [35], and indole [36]. The conformation of the molecule is planar.…”

“…Later Lindon et al gave the 1 H NMR and 13 C NMR measurements of ICPY molecule [23]. Lorenc et al recorded the vibrational spectra, and molecular structure of 1H-and 3H-imidazo [4,5-b]pyridine and their methyl derivatives, by using DFT quantum chemical calculations [24]. In that study, X-ray data of 5-methyl-1H-imidazo [4,5-b]pyridine is also given.…”

XRD studies, vibrational spectra, and molecular structure of 1H‐imidazo [4,5‐b]pyridine based on DFT quantum chemical calculations

“…The structure of IPb molecule is similar to that of 5MIPb. [9] The pyridine ring is distorted. The valence angles between non-hydrogen atoms of pyridine are in the range of 115.52(18)-125.90(18) • , whereas the inter-atomic distances assume intermediate values between those for single and double bonds.…”

“…In our previous article on molecular structures, vibrational energy levels and potential energy distributions of 1H-imidazo [4,5-b]pyridine (1HIPb), 3H-imidazo [4,5-b]pyridine, 5-methyl-1H-imidazo [4,5-b]pyridine (5MIPb), 6-methyl-1H-imidazo [4,5-b]pyridine, and 7-methyl-3H-imidazo [4,5-b]pyridine were studied. [9] For our previous article we did not have the crystallographic data on the non-substituted 1HIPb, but the single crystal of this compound has been obtained by us and more precise discussion of the structural and vibrational properties are possible. Therefore, the present article compares the results of the X-ray diffraction (XRD) studies with those obtained from the molecular optimization, as well as infrared (IR) and Raman spectra recorded for the solid state with the theoretical calculations.…”