Liquid structure of acetic acid studied by Raman spectroscopy and Ab initio molecular orbital calculations

The liquid structure of acetic acid was studied experimentally and theoretically. Experimentally, the Raman spectra of acetic acid at various temperatures between 287 and 348 K were measured in the region 15-3700 cm^-1. Theoretically, ab initio molecular orbital calculations were performed on the Raman activities of seven cluster species of acetic acid molecules. The Raman spectrum (in R(v̄) representation) of crystalline acetic acid at 287 K shows six distinct bands in the 15-300 cm^-1 region. These bands broaden on the melting of the crystal, whereas their peak positions remain almost unchanged on melting. These spectral changes are reproduced in the case where the liquid spectrum mainly arises from a variety of sizes of chain clusters as the fragments of the crystalline networks. The C=O stretching band becomes broadened toward higher wavenumbers and exhibits an asymmetric shape with increasing temperature. The wavenumbers calculated for the C=O stretching vibrations suggest that the strongly hydrogen-bonded C=O groups of the chain clusters show the prominent C=O band and its asymmetric shape is due to the presence of weakly hydrogen-bonded C=O groups of the same cluster species. The spectral analyses in both the low wavenumber and the C=O stretching regions suggest that liquid acetic acid is mainly composed of the chain clusters, not the cyclic dimer. Assignments of the low-frequency Raman bands observed in the vapor and crystalline states are discussed on the basis of the calculated wavenumbers.