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 (xA), are presented in the form of the relative scattering activities, R(ν̄) spectra. The spectrum of scattered light I(ν̄) 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(ν̄) 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(ν̄) 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.