Liquid Structure of Acetic Acid Studied by Raman Spectroscopy and Ab Initio Molecular Orbital Calculations

Abstract

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(ν̄) 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.

Raman spectra of organic acids obtained using a portable instrument at −5 °C in a mountain area at 2000 m above sea level

“…1). The broad band at 1659 cm −1 is assigned to the C O stretching vibration in acetic acid in the solid state, which is shifted by about 6 cm −1 downwards compared to liquidstate measurements from the work of Nakabayashi et al [34] The weak bands between 1200 and 1500 cm −1 are assigned to CH 3 deformation and C-O stretching modes, respectively. [35] The strong to medium bands at 629, 599 and 452 cm −1 can be assigned to the ρCO 2 vibrations.…”

Abraham model correlations for estimating solute transfer of neutral molecules into anhydrous acetic acid from water and from the gas phase

“…The structure of neat acetic acid has been studied several times, with agreement that liquid acetic acid exists as linear associates [86][87][88]. Unlike the cyclic dimer, there are free -OH groups in the linear associates, see Figure 3, that are free to interact with external solutes that are hydrogen-bond bases.…”

Raman spectral investigations on the binary system (acetic acid N,N‐dimethyl formamide)

“…In the pure AA spectrum, the prominent Raman band observed around 1666 cm 1 can be ascribed to the strongly hydrogen-bonded C O groups of the long-chain cluster as the fragments of the crystalline network. 5 The asymmetric components observed in the region 1700-1740 cm 1 are assigned to the weakly hydrogenbonded C O groups of either the long-or short-chain species. 5 The C O stretching mode of AA has been known to be strongly affected by the surroundings.…”

“…5 The asymmetric components observed in the region 1700-1740 cm 1 are assigned to the weakly hydrogenbonded C O groups of either the long-or short-chain species. 5 The C O stretching mode of AA has been known to be strongly affected by the surroundings. The prominent C O stretching band broadens and shifts to a lower wavenumber with a small addition of DMF at a D 0.9.…”

“…From the ab initio MO calculation for the C O stretching modes of the chain clusters, it has been suggested that the prominent C O stretching band shifts to a higher wavenumber with decreasing aggregate size. 5 The upshift of C O stretching upon dilution can be understood in terms of self-association as in the case of acetone. 18 The observed upshift in C O stretching vibration may be caused by self-association with a smaller aggregation size.…”