Intermolecular hydrogen bonds in acetic acid and its solutions

Abstract

The band at 896 cm⁻¹ in the Raman spectrum of acetic acid consists of at least two lines (873 and 896 cm⁻¹) whose intensities and depolarization factors differ strongly. A temperature increase leads to an increase in the intensity of the 873-cm⁻¹ line as compared to the intensity of the 896-cm⁻¹ line. Dilution of the acid with acetonitrile leads to a similar but much more pronounced effect. In solutions with CCl₄ and water, the relative intensity of the low-frequency line decreases; in aqueous solutions, the width of the 896-cm⁻¹ line passes through a maximum with 0.4 mol. fractions of the acid (for this concentration, the line width is larger by a factor of 2.5 than in a pure liquid), while in the solutions with CCl₄ it decreases smoothly (almost twofold with strong dilution: 0.05 mol. fractions). Experimental data are in agreement with the assumption that the 873- and 896-cm⁻¹ lines refer to the vibrations of the same atoms in a molecule and to associates with a free and an H-bonded atom of the oxygen of a C=O group, respectively. The difference in the frequencies and depolarization factors of the lines causes the differences in the distribution of an electron cloud in the molecule.