Vibrational analysis of acetate ion molecules and estimation of equilibrium constants for their hydrogen isotopic exchange reactions

The infrared spectra (4000-300 cm^-1) of CH₃COONa and its five isotopic substitutions including D- and ¹³C-labeled modifications suspended in KBr disks were measured at 80 and 290 K. Excellent resolutions were obtained by recording the spectra at low temperature. Complete vibrational assignments were established on the basis of correlations of group modes and the ¹³C shifts of the fundamentals. From a normal coordinate analysis a general valence force field involving 26 force constants has been determined, which reproduces 82 experimental frequencies with a root-mean-squares deviation of 3.3 cm^-1. The composition of the normal vibrations from the symmetry coordinates has been given in terms of the potential energy distribution which makes it possible to inspect the relation between the normal modes and the group vibrations. The valence force field was used to predict fundamental frequencies which were not available from experiments, and the reduced partition function ratios of acetate ion molecules were calculated by using the spectroscopic data including the predicted values of the fundamentals. Calculated equilibrium constants of some hydrogen isotopic exchange reactions between the acetate ion molecules are reported.