The influence of ions on the structural organization of dipolar liquids probed by the noncoincidence effect. Experimental and quantum chemical results: the case of Mg+2, Ba+2, Ca+2.

Electrolytic solutions (M+X/Solvent) are of noteworthy importance in both technological [1] and biological solvation processes [2]. The mobility of ions and their activity within a solution strongly depend on its structure at molecular level and then on the specific interactions between ions and between ions and solvent molecules. Ion pairs (M+--X) and solvent clusters M+(S)n formation is expected as a consequence of these interactions. Recently we have investigated the liquid structure of electrolytic solutions of mono-valent (M+=Li+, Na+) salts in carbonyl solvents by making use of the Raman noncoincidence effect (NCE) of the solvent (C=O) band and, with the joint use of ab initio molecular orbital quantum-chemical calculations, we have been able to assess the formation of cluster species M+(S)n [3]. NCE, i.e. the difference between the spectral first moments, M, of the anisotropic and isotropic profiles of a Raman band of a totally symmetric vibrational mode, is indicated as a reliable probe of the liquid organization. We have found [3] that the observed large and negative NCE of the (C=O) band is a consequence of the formation of clusters species M+(S)n made of n C=O groups of solvent molecules (S) pointing towards the M+ cation in a tetrahedral (n=4, Li+) and octahedral (n=6, Na+) organization. Now we have extended these investigations to electrolytic solutions of bi-valent cations, Mg2+ and Ba2+ (in the future to Ca2+), to shed light on the effect of the increased strength of the electric field on NCE and assess, on a quantum chemical basis, the formation of M2+(S)n clusters. Our observations indicate a remarkable increase of the negative NCE (34.2 cm-1 in Mg2+/acetone and 21.1 cm-1 in Ba2+/acetone solutions) with respect to those found in the singly charged ions [3]. The NCE calculated for the species (acetone)nMg2+ with n=3,4 and 6 suggests the formation, in the Mg+2/acetone solution, of the latter cluster species (NCE= 35.1 cm-1) in which six carbonyl groups are pointing towards the Mg2+ ion in an octahedral organization.