Low-frequency Raman spectra of crystalline and liquid acetic acid and its mixtures with water. Is the liquid dominated by hydrogen-bonded cyclic dimers?

Low-frequency Raman spectra of crystalline and liquid acetic acid, and acetic acid-water binary solutions with varying mole fractions of acetic acid (x(A)), are presented in the form of the relative scattering activities, R(<(nu)over bar>) spectra. The spectrum of scattered light I(<(nu)over bar>) from crystalline acetic acid at 285 K shows five distinctive bands. Upon the melting, however, the liquid exhibited conspicuous difference in its appearance, although the R(<(nu)over bar>)spectrum shows two peaks and a shoulder at similar wavelengths to those of the crystalline bands. The spectral change does not directly support the dominance of the cyclic dimers in the liquid. R(<(nu)over bar>;) spectra of acetic acid-water binary solutions suggest a three-state model composed of acetic acid associates, binary associates and water associates for the local structure of the mixtures. (C) 1998 Elsevier Science B.V. All rights reserved.