Raman spectroscopic study on acetic acid clusters in aqueous solutions: Dominance of acid-acid association producing microphases

The C=O stretching vibration band of acetic acid showed a high-frequency shift from 1665 to 1715/cm by adding water into liquid acetic acid, indicating that the hydrogen bond of the C=O group of acetic acid was not as strong as those in liquid acetic acid or in CCl₄ solution (in which the band appeared at 1668/cm). The increase of acetic acid in water decreased the intensity of the hydrogen-bonded O-H stretching Raman band of water at 3200/cm. Low-frequency R(v) spectra of acetic acid/water binary solutions were re-examined with new experimental data and ab initio molecular orbital analysis of intermolecular vibrational modes. The R(v) spectrum of the aqueous mixture at x_A = 0.5 had a very close resemblance to that of the acetic acid/methanol mixture with x_A = 0.5, indicating that the molecular complexes responsible for the Raman spectra were acetic acid clusters.