Raman spectroscopic studies and DFT calculations on NaCH₃CO₂and NaCD₃CO₂solutions in water and heavy water

Abstract

Sodium acetate and sodium acetate-d 3 solutions in water and heavy water were studied using Raman spectroscopy over a wide concentration range and from low wavenumbers (40 cm À1 ) up to 4200 cm À1 .In the terahertz region the broad breathing mode Na-O at 189 cm À1 was detected as well as a broad shoulder at 245 cm À1 of the restricted translation band of acetate-water. Fundamental modes of CH 3 CO 2 À (aq) and acetate-d 3 , CD 3 CO 2 À (aq), were assigned and discussed according to pseudo C s symmetry. The vibrational isotope effect of the CH 3 /CD 3 group was observed and the Teller-Redlich product rule confirmed the assignments. Additionally, band assignments of CH 3 CO 2 À and CD 3 CO 2 À in heavy water were reported and discussed. By changing from H 2 O to D 2 O, relatively strong H-bonding between the oxygen atoms of acetate causes a change in the vibrational energy levels of the dissolved acetate. The symmetric stretching mode of the CO 2 group for CH 3 CO 2 À in water and heavy water was obtained at 1413.5 cm À1 and 1418.6 cm À1 respectively and for CD 3 CO 2 À in water and heavy water, the symmetric stretch was obtained at 1407.5 cm À1 and 1412.4 cm À1 , respectively. Coupling of the intramolecular acetate bands is fairly extensive and therefore DFT calculations were carried out on discrete acetate-water (heavy water) clusters. The clusters with the general stoichiometry CH 3 CO 2 À $nH 2 O$mH 2 O (n ¼ 1-5, m ¼ 1) and n the number of first shell water molecules and m the second shell were considered and calculations at the B3LYP 6-311++G(3df,2pd) level were performed.The frequency calculations on CH 3 CO 2 À $5H 2 O and CD 3 CO 2 À $5H 2 O clusters supported the assignments of the fundamental modes. The geometrical parameters such as bond length and bond angles of acetate in solution state were obtained. The influence of acetate on the O-H stretching band of water was measured as a function of concentration in order to determine the influence of the methyl group on the structure of water. No enhancement of the water structure around the nonpolar methyl group could be detected nor the existence of dangling nO-H bonds at $3670 cm À1 . In NaCF 3 CO 2 solutions, however, dangling nO-H bonds could be observed at $3670 cm À1 caused by the hydrophobic CF 3 group. Finally, the nature of the ion pairs formed between Na + and acetate were discussed in NaCH 3 CO 2 (aq) and in concentrated solutions no contact ion pairs could be detected.

Identification of ion pairs in solution by IR spectroscopy: crucial contributions of gas phase data and simulations

“…2,4 This situation is typically encountered in vibrational spectroscopy where the help of quantum chemistry is widely appreciated. 5,6 Unfortunately, comparison between bulk experimental spectra and frequency calculations at high quantum chemistry level is not straightforward and is indeed sometimes questionable (poor modelling of the environment around the vibrational probe, simplistic corrections of harmonic frequencies...). In response to the need of benchmark data for each type of ion pair, gas phase investigations have recently proven their ability to provide IR spectra of type-selected pairs by mass-and conformer-selective techniques.…”

“…29 On the theoretical side, ion pair type distributions have been calculated at 1 M using a force field specifically designed for electrolyte solutions. 30 On the experimental side, dielectric relaxation spectroscopy 31,32 as well as Fourier-Transformed Infrared (FTIR) and Raman experiments 5,6 have detected signals assigned to SIPs, while X-ray absorption spectroscopy experiments in microjets 33,34 claim to have detected CIPs in the same concentration range. In order to better understand these phenomena, we now endeavour to provide type-resolved vibrational spectra of ion pairs between a carboxylate anion and an alkali cation.…”

On-line analysis and in situ pH monitoring of mixed acid fermentation by Escherichia coli using combined FTIR and Raman techniques

“…Figure 3b, c, and d show Raman spectra obtained at different stages during the anaerobic fermentation of E. coli (see 'Mixed acid fermentation of E. coli' for more details). In addition to the phosphate peaks, there were characteristic peaks from acetate [37,38] (at 928 and 1414 cm −1 ) and formate [37] (at 1349 cm −1 ). Reference spectra of 1 M ammonium acetate and potassium formate standards were also recorded as calibration to obtain formate and acetate concentrations of experimental Raman spectra.…”

Raman vibrational dynamics of hydrated ions in the low-frequency spectral region

“…ACCEPTED MANUSCRIPT 4 time. Although our previous focus was on water-anion H-bonds, the effect of the cations was also noted through their electric field effects, namely, by blue shifting the water-anion Hbond vibration frequencies.…”

“…[1][2][3][4] Characteristic low frequency modes can reveal information about ion hydration, ionic dissociation and ion-pair formation. [5] The understanding of the structure and dynamics of ion hydration is important, especially in the fields of chemistry and biology.…”