Study of the Optical Spectra of 4-Hydroxy-3-Methoxibenzoic Acid
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STUDY OF THE OPTICAL SPECTRA OF 4-HYDROXY-3METHOXIBENZOIC ACID O. N. Tchaikovskaya, O. V. Vusovich, and A. V. Malikov
UDC 535.37
Using electronic spectroscopy and quantum chemistry methods, the electronic spectra of vanillic acid in various solvents are interpreted. In the ground electronic state, four protolytic forms of vanillic acid in water were detected: anionic, dianionic, cationic, and neutral. In water, vanillin and isovanillin in the ground state have the same spectral characteristics: the line position and intensity in their absorption spectra coincide. The absorption and fluorescence bands of vanillic acid shift toward shorter wavelengths compared to vanillin and isovanillin. The values of the minimum electrostatic potential indicate that vanillic acid has the deepest minimum in the region of the carbonyl oxygen atom. The study of the fluorescence spectra showed that the radiative properties of the ionic forms of vanillic acid are different. The data of quantum chemical calculations indicate that electronic transitions in the molecule are formed with the participation of charge transfer from the phenyl part of the molecule to the oxygen atoms of the methoxy and carbonyl groups. The presence of OH, OCH3, and carbonyl group in the structure of vanillic acid leads to the existence of a dianionic form, both in the ground and electronically excited states. Keywords: vanillic acid, absorption, fluorescence, photophysical processes, quantum chemical calculation.
INTRODUCTION In the last few years the urgency of research of lignin and its model compounds is determined by its importance for chemical processes of wood processing. Lignin as a wood component that is most difficult to utilize is formed during chemical wood processing at pulp-and-paper and hydrolytic enterprises [1]. Industrial wastes that have already been accumulated occupy considerable territories and are sources of environmental pollution, thereby deteriorating human life conditions. Simultaneously with the aforesaid, lignin can replace depleted stocks of traditional organic raw materials and can become the main source of necessary compounds because of the special features of its chemical composition and practical inexhaustibility of natural vegetative resources [2]. The important role in delignification is played by the ability of structural lignin fragments to acid ionization. The centers responsible for H-acidity are phenolic hydroxyl groups and to a lesser degree, carboxyl groups. In this regard, a powerful tool of acidity control of phenolic compounds is the change of the solvent composition. This is most clearly manifested in aqueous mixtures with dipolar aprotic solvents widely used in wood chemistry [3]. At the same time, the properties characterizing the reactivity of model lignin compounds in various media are little studied. In connection with the aforesaid, a high-priority problem nowadays is a study of protolytic equilibria with participation of lignin-related compounds in water, water-acetone, and water-dioxane systems based on mo
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