Raman Spectroscopic Studies on Aqueous Sodium Formate Solutions and DFT Calculations

Abstract

NaHCOO(aq) and NaDCOO(aq) solutions were measured using Raman spectroscopy from dilute to concentrated solutions at 23 °C in water and heavy water from 50 to 4300 cm−1. A concentrated NaHCOO solution in heavy water was also measured. The Raman band parameters of HCOO−(aq) and DCOO−(aq) such as peak position, full width at half maximum (fwhm), integrated intensities, and depolarization values were determined. From the Raman spectroscopic data, it was concluded that the HCOO−(aq) and DCOO−(aq) symmetry is lower than C2v and probably as low as C1. In contrast to the solution state, HCO2- (DCO2-) possess C2v symmetry in the gas phase and the DFT frequencies are given. DFT frequencies on a cluster of HCOO−/DCOO− with five implicit water molecules in the first sphere and placed in a polarizable continuum deviate not more than 1–2% from the measured ones. In the Raman spectrum in NaHCOO(aq), a band doublet at 2730 cm−1 and 2820 cm−1 occurs instead of a single band. The band doublet is due to Fermi resonance and results from the interaction of the overtone of the bending C–H mode, 2ν6 at 1382 cm−1 and ν1. The undisturbed C–H stretching mode, ν1 amounts to 2785 cm−1. In DCOO−(aq), a Fermi doublet was also observed at 2030.5 and 2116.5 cm−1, and the undisturbed wavenumber position amounts to 2101 cm−1. Furthermore, a solution of HCOO− in D2O showed slightly changed frequencies compared with the ones in water caused by the solvent isotope effect. Ion pairing between Na+ and HCOO− characterizes the Raman spectrum at high solute concentrations which are melt-like enabling direct contact between the ions. A NaHCOO solution with high amounts of LiCl added showed large perturbations of the HCOO− bands especially νsCOO− and δ COO− of HCOO−and revealed a stronger affinity of Li+ toward HCOO−. The ion pairs formed are most likely contact ion pairs between Li+ and HCOO− which have different stoichiometry of Li+: HCOO− such as 1:1 and 2:1. Graphical Abstract: [Figure not available: see fulltext.].