ON INTRAMOLECULAR HYDROGEN BOND IN THE MOLECULE OF 2-NITRO-2’-HYDROXY-5’-METHYLAZOBENZENE

The 2-nitro-2'-hydroxy-5'-methylazobenzene molecule is characterized by an intramolecular hydrogen bond of the N...OH type. The parameters of this bond were calculated within density functional theory (DFT) and the continuum solvation model (cpcm) in the gas phase and individual protic and aprotic solvents: hexane, toluene, dimethylformamide, diethylamine, 2-propanol and water at 298.15 K. B3LYP, cam-B3LYP and D3B3LYP functionals and 6-311++g (d, p) basis set was used. The most energetically favorable form of existence of the 2-nitro-2'-hydroxy-5'-methylazobenzene molecule should be considered its cis-isomer, which forms an intramolecular hydrogen bond of the proton of the hydroxy group with the β-nitrogen atom of the –N=N– group. Such conformer is more coplanar both in the gas phase and in solution. The intramolecular hydrogen bond should be classified as moderately strong with a significant contribution of electrostatic component. The binding energy is in the range of 33 to 51 kJ/mol, the O-H...N flat angle is 138-143°, and the length does not exceed 1.7-1.8 Å. An increase in the dielectric constant of the solvent and its solvating ability leads to a weakening of the binding energy, but its length is no changed almost. Theoretical study of prototropic equilibrium showed that the activation barriers to proton transfer in the gas phase and solutions do not exceed 20 kJ/mol. However, the calculation of ΔG0298 and experimental UV spectra of 2-nitro-2’-hydroxy-5’-methylazobenzene in solutions of various compositions proves that a quinoid structure is not formed in any of the solvents studied. A comparison of the calculation results and spectroscopy data allows us to conclude that the cam-B3LYP functional, which takes into account the correction for dispersion interactions, most correctly describes the structure of the 2-nitro-2'-hydroxy-5'-methylazobenzene molecule in solution. For citation: Fedorova A.A., Lefedova O.V. On intramolecular hydrogen bond in the molecule of 2-nitro-2’-hydroxy-5’-methylazobenzene. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2025. V. 68. N 7. P. 74-84. DOI: 10.6060/ivkkt.20256807.7212.