Vibrational spectroscopic studies and DFT calculations on NaCH3CO2(aq) and CH3COOH(aq)

Abstract

Aqueous solutions of sodium acetate, NaCH₃CO₂, and acetic acid, CH₃COOH, were studied using Raman and infrared spectroscopy. The spectra were recorded over a large concentration range, in the terahertz region and up to 4000 cm⁻¹. In the isotropic Raman spectrum in R-format, a polarized band at 189 cm⁻¹ was assigned to the ν₁Na–O stretch of the hydrated Na⁺-ion and a shoulder at 245 cm⁻¹ to the restricted translation band, ν_sO–H⋯O* of the hydrated acetate ion, CH₃CO₂⁻(aq). The CH₃CO₂⁻(aq) and the hydrated acetic acid, CH₃COOH(aq), possess pseudo C_s symmetry. Geometrical parameters for the species in the gas phase and for CH₃CO₂⁻(aq) and CH₃COOH(aq) are reported. Characteristic bands for CH₃CO₂⁻(aq) and CH₃COOH(aq) were assigned under the guidance of the DFT vibrational frequency calculations and discussed in detail. In aqueous NaCH₃CO₂ solutions, at high concentrations, no contact ion pairs could be detected, but instead solvent separated ion pairs were found. In LiCH₃CO₂(aq), however, contact ion pairs are formed which is indicated by the appearance of a shoulder at 939 cm⁻¹ and the shift of the symmetric stretching mode of the –CO₂⁻ group to higher wavenumbers.