Acetate ion and its interesting solvation shell structure and dynamics

Abstract

The asymmetric stretch of the carboxylate group located near 1560 cm−1 in solutions of sodium acetate in water and 6M sodium chloride was investigated by linear and two-dimensional infrared spectroscopies. The linear IR spectra showed that the carboxylate asymmetric stretch central frequency and bandwidth are not significantly perturbed by the presence of a large amount of sodium chloride. Similarly, the frequency-frequency autocorrelation functions extracted from the time evolution of two-dimensional infrared spectra revealed a dynamical component having a picosecond time constant which is also not affected by the presence of high salt concentrations. A comparative study on the asymmetric stretch of the azide ion showed that the lack of change in the vibrational spectroscopy of the ion in the presence of large concentration of sodium chloride is particular to the acetate ion. Ab initio molecular dynamics simulations and a frequency map confirmed our experimental observations and linked the observed vibrational phenomenon to the hydrogen bond structure and dynamics of the acetate first solvation shell, which is governed by the thermal rotation of its methyl group (—CH3).