Theoretical analysis of the effect of intermolecular interaction on ir spectra of skatole

Abstract

543.42:547.753 The molecular structure and IR vibrational spectra of skatole in the isolated state, in aqueous solution, and in hexane have been calculated in the p) approximation. Characteristic spectral differences associated with the influence of both polar and nonpolar solvents and a hydrogen bond have been defined for the vibrational spectrum of skatole. Calculation of the isolated skatole molecule in the anharmonic approximation enabled the interpretation of the vibrational spectrum to be refined.Introduction. Indole derivatives are extremely physiologically active compounds that are widely used in medical practice. They can exert stimulating and inhibiting action on the nervous system and possess hemorrhagic, sedative, and radioprotective properties in addition to others [1,2]. For example, indole-3-carbinol facilitates detoxification of poisons, including those with carcinogenic properties, and reduces the threat of developing a series of hormone-dependent tumors of epithelial origin [3].Indole and skatole are toxic substances that are formed in the intestines as a result of the destruction of the tryptophan side chain by bacteria. Like phenol and cresol, they are detoxified in the liver where they are oxidized preliminarily into compounds containing a hydroxyl. Subsequent binding of indole and skatole to polar glucuronic acid makes these nonpolar (hydrophobic) compounds highly soluble and more easily eliminated from the organism [4,5]. Vibrational spectra (IR and Raman) of skatole and its radicals were calculated earlier in a harmonic approximation by the DFT method [6]. Results of the theoretical calculation of isolated skatole were analyzed and compared with experimental spectra in CCl 4 solutions.Considering that the properties of molecules in the gas phase and in solutions can differ considerably, the goal of the present work was to determine the effects of intermolecular interactions (IMI) on the structure and vibrational spectra of skatole. Furthermore, skatole exhibits hydrophobic properties and is poorly soluble in water. Therefore, the effects of IMI on the properties of skatole are conveniently studied in various types of solvents (polar and nonpolar).Normal vibrations and geometric parameters were calculated by the DFT method in the B3LYP/6-311++G(d,p) approximation [7]. The explicit and effective effects of IMI were analyzed for a model with a directly formed H-bond, i.e., for an intermolecular complex of indole with water (1:1), and for models of a self-consistent reactive field (SCRF) that enabled the effects of polar (water, dielectric permeability ε = 78.39) and nonpolar (cyclohexane, ε = 2.023) solvents to be considered.