Raman Spectroscopic Study on Acetic Acid Clusters in Aqueous Solutions:  Dominance of Acid−Acid Association Producing Microphases

Abstract

With the addition of water into liquid acetic acid, the CO stretching vibration band of acetic acid shows a high-frequency shift from 1665 to 1715 cm-1. This means that the hydrogen bond of the CO group of acetic acid is not as strong as those seen in liquid acetic acid or in CCl4 solution (in which the band appears at 1668 cm-1). A bent type hydrogen bond is accountable for this observation. On the other hand, the increase of acetic acid in water drastically decreases the intensity of the hydrogen-bonded O−H stretching Raman band of water at 3200 cm-1. This suggests that acetic acid breaks the hydrogen-bond networks of water. Low-frequency R(ν̄) spectra of acetic acid/water binary solutions are re-examined with new experimental data and ab initio molecular orbital analysis of intermolecular vibrational modes. The R(ν̄) spectrum of the aqueous mixture at x A = 0.5 bears 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 are acetic acid clusters. The calculated low-frequency Raman feature of a side-on type dimer with bent-type hydrogen bonds based on ab initio molecular orbital theory reproduces the observed Raman pattern nicely. Any evidence of the formation of stable acid−water pairs is not found in the low-frequency Raman spectra. Furthermore, an isosbestic point is seen in the region of 0.1 ≤ x A (mole fraction of acetic acid) ≤ 0.5, and another one is also observed in 0.5 ≤ x A ≤ 1.0. The observed spectra in the region of 0 < x A < 0.5 are reproduced simply by linear combinations of the pure water spectrum and the spectrum at x A = 0.5. These results strongly suggest the presence of the two microphases with homogeneously associated molecules: a water cluster phase and an acetic acid cluster phase. The spectral change in 0.5 < x A < 1.0 is attributed to the coexistence of the acetic acid cluster phase in aqueous environment and the acid associated phase characteristic of liquid acetic acid.

Synthesis of 2,5-Diformylfuran and Furan-2,5-Dicarboxylic Acid by Catalytic Air-Oxidation of 5-Hydroxymethylfurfural. Unexpectedly Selective Aerobic Oxidation of Benzyl Alcohol to Benzaldehyde with Metal/Bromide Catalysts

“…Such mixed-metal polymeric species have been isolated from acetic acid. [31] Recent observations [32] suggest that acetic acid/water solutions may be more complex, containing waterrich microphases. It is proposed that Co(III) aquo asc.wiley-vch.de Table 3. acetate and Co(III) aquo acetate bromide have structures similar to those shown in Figure 8 (a and b, respectively).…”

Infrared Photodissociation Spectroscopy of Protonated Formic Acid−Water Binary Clusters, H+·(HCOOH)n·H2O (n = 1−5). Spectroscopic Study of Ion Core Switch Model and Magic Number

“…Because a carboxylic acid molecule has a few groups capable of the hydrogen-bonding formation, its aggregates may show various kinds of intermolecular interaction. 6 In the crystals, carboxylic acid molecules are interlinked to form either a cyclic dimer or an infinite chain; the linkage manner depends on the substituent R. 7 Formic acid, HCOOH, is the simplest carboxylic acid. Formic acid molecules form an infinite chain in the crystal 7 and a cyclic dimer in the gas phase.…”

Hydrogen bonding of single acetic acid with water molecules in dilute aqueous solutions

“…The structures of the 4-and the 5-membered rings are consistent with those reported in the recent experimental work [21] ; however, they are not very stable in aqueous solution. The dominant molecular interactions in dilute aqueous solutions of HOAc are those between HOAc and water molecules, but it is totally different from the case in more concentrated solutions, where the leading role is played by the acid aggregates [15,19,20] . Therefore, much more efforts are needed to isolate acetic acid from dilute aqueous solutions of HOAc.…”

“…Recently, Johnson et al [13,14] attributed the observed vibrational sum frequency spectra (especially of HOAc/H 2 O solutions with high acid concentrations) to several species including the HOAc cyclic dimer. Moreover, Nishi et al [15] suggested the dominant species are large aggregates of HOAc molecules, which produce microphases.…”