Influence of ions on the structural organization of dipolar liquids probed by the noncoincidence effect: experimental and quantum chemical results.

The C=O stretching [nu(C=O)] Raman bands of the carbonyl solvents, S (acetone and acetophenone), in some electrolytic solutions of lithium and sodium salts (M(+)X(-)) are analyzed. The large and negative values of the noncoincidence effect (NCE=nu(ani)-nu(iso)) measured for the component of this band generated by the solvent-ion interactions are interpreted in the light of the results of ab initio quantum chemical calculations performed for clusters of type (S)nM(+) and also on the basis of the transition dipole coupling mechanism between pairs of nu(C=O) oscillators. The effects of the size of the ion M(+) and of the solvation number n on the NCE are analyzed. It is shown that the decrease of the NCE resulting from the change in the size of the ion M(+) from Li(+) to Na(+) is appreciably counterbalanced by the increase of the NCE arising form the change in the ion solvation number n from 4 for Li(+) to 6 for Na(+).