The influence of alkaline earth ions on the structural organization of acetone probed by the noncoincidence effect of the ν(C=O) band: experimental and quantum chemical results

We have investigated the Raman noncoincidence effect (NCE = (nu) over bar (aniso) - (nu) over bar (iso), where (nu) over bar (aniso) and (nu) over bar (iso) are the anisotropic and the isotropic Raman frequencies) of the nu(C=O) band of acetone arising from the interactions of this solvent with the metal ions in acetone electrolytic solutions of alkaline earth metal (Mg, Ca, Sr, Ba) perchlorates. Assisted by the results of ab initio molecular orbital (MO) calculations carried out at the Hartree-Fock (HF) level with the 6-31 + G(2df, p) and LanL2DZ basis sets, we have been able to attribute the anisotropic and isotropic components of this band to the formation of acetone-metal ion clusters, (acetone) nM(2+), and to interpret its high and negative NCE, opposed to the positive NCE of the bulk liquid, as the consequence of the large separation between the higher frequency of the in-phase mode (active in the Raman isotropic spectrum) and the lower (average) frequency of the n - 1 out-of-phase modes (predominantly active in the Raman anisotropic spectrum). The negative sign of the NCE is compatible with the transition dipole coupling (TDC) mechanism. The comparison between the observed NCE for each electrolytic solution at the concentrations used in this study and those calculated for the different solvation numbers n of each (acetone)(n)M2+ cluster gives a clear indication of the highest stability of the hexa-coordinated cluster for the Mg2+ ion, but leaving uncertain (n = 6 or 8) this conclusion for the acetone clusters of the remaining M2+ ions. We have interpreted the observed and calculated decrease of the magnitude of NCE with the ion size through the ion polarizing power in the light of the ion effective charge and its distance (M2+ ...O=C) from the C=O oscillators.