Structure of 2-nitro-2’-hydroxy-5’-methylazobenzene: Theoretical and spectroscopic study

Abstract

Objectives. 2-Hydroxy-nitroazobenzenes comprise reagents for the synthesis of heteroaromatic compounds, in the molecules of which the benzene and azole cycles are annulated. These reagents are widely used in the production of chemical products for various industries. In particular, 2-2’-hydroxy-5’-methylphenylbenzotriazole is used as an effective photo stabilizer for polystyrene and polyethylene. A promising method for its preparation is the liquid-phase catalytic hydrogenation of 2-nitro-2'-hydroxy-5'-methylazobenzene (2NAB). The aim of the present study was to establish the structure of 2NAB in solutions of different composition.Methods. Theoretical calculations were carried out within the framework of the density functional theory at a temperature of 298.15 K for the gas phase at B3LYP/6-311++G(d, p) and M06-2X/6-311++G(d, p) levels; for hexane, 2-propanol, toluene at B3LYP/6-311++G(d, p) level using the conductor-like polarizable continuum model. An experimental study to determine the probable isomeric structure of 2NAB in various solvents, including sodium hydroxide (NaOH) and acetic acid (CH3COOH) additives, was carried out using infrared (IR) and ultraviolet (UV) spectroscopy.Results. The most probable structure of 2NAB isomers for the gas phase and a number of solvents was determined. Experimental and theoretical IR and UV spectra were obtained. The thermodynamic characteristics of the reaction of intramolecular proton transfer from –OH to –N=N– group in the gas phase were calculated.Conclusions. A comparison of the experimental and calculated results supports the conclusion that the cis-isomer should be considered most probable for the gas phase. For the studied solutions, a trans-isomer of 2NAB with hydrogen bonds formed between the hydroxyl group hydrogen and the β-nitrogen atom of the azo group of dye molecule should be considered as the most likely structure. In the studied individual and binary solvents, prototropic equilibrium is shifted towards the azo form of the dye, while intramolecular proton transfer is possible only in aqueous diethylamine and dimethylformamide solutions with additions of NaOH.