Raman spectroscopic studies and DFT calculations on NaCH3CO2 and NaCD3CO2 solutions in water and heavy water

Abstract

Sodium acetate and sodium acetate-d₃ solutions in water and heavy water were studied using Raman spectroscopy over a wide concentration range and from low wavenumbers (40 cm⁻¹) up to 4200 cm⁻¹. In the terahertz region the broad breathing mode Na–O at 189 cm⁻¹ was detected as well as a broad shoulder at 245 cm⁻¹ of the restricted translation band of acetate–water. Fundamental modes of CH₃CO₂⁻(aq) and acetate-d₃, CD₃CO₂⁻(aq), were assigned and discussed according to pseudo C_s symmetry. The vibrational isotope effect of the CH₃/CD₃ group was observed and the Teller-Redlich product rule confirmed the assignments. Additionally, band assignments of CH₃CO₂⁻ and CD₃CO₂⁻ in heavy water were reported and discussed. By changing from H₂O to D₂O, 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 CO₂ group for CH₃CO₂⁻ in water and heavy water was obtained at 1413.5 cm⁻¹ and 1418.6 cm⁻¹ respectively and for CD₃CO₂⁻ in water and heavy water, the symmetric stretch was obtained at 1407.5 cm⁻¹ and 1412.4 cm⁻¹, 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 CH₃CO₂⁻·nH₂O·mH₂O (n = 1–5, m = 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 CH₃CO₂⁻·5H₂O and CD₃CO₂⁻·5H₂O 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 νO–H bonds at ∼3670 cm⁻¹. In NaCF₃CO₂ solutions, however, dangling νO–H bonds could be observed at ∼3670 cm⁻¹ caused by the hydrophobic CF₃ group. Finally, the nature of the ion pairs formed between Na⁺ and acetate were discussed in NaCH₃CO₂(aq) and in concentrated solutions no contact ion pairs could be detected.