Applicability of the SQM Force Field Method to the Vibrational Spectra of Sodium Acetate

The applicability of the scaled quantum mechanical force field (SQM FF) method to the prediction of the
vibrational spectra of a charged molecule has been studied by the example of the acetate ion (CH3CO2-) in
sodium acetate for which an efficient empirical valence force field (SVFF) based on observed IR spectra of
six isotopomers of sodium acetate is available in the literature. Standard SQM FF calculations done on a free
acetate ion at the B3LYP/6-31G* level failed to give an acceptable estimation of even the most characteristic
features of the observed spectra, which can be exemplified by the gross overestimation of the frequency
separation of the νaCO2- and νsCO2- vibrations. In search for a better description, SQM calculations were
done for three simple structural models of sodium acetate, testing different QM methods. The results indicate
that in addition to taking into account the dielectric field effect of the surrounding medium, incorporation of
a Na+ counterion is necessary to achieve a realistic simulation of the IR and Raman spectra. Satisfactory
results were obtained with a bidentate Na−acetate complex by the SQM method coupled with a continuum
model at the B3LYP/6-31+G** level, whereas the use of the Onsager-type spherical cavity model and the
polarizable continuum model (PCM) were found preferable over SCI−PCM.