The structural organization of organic eklectrolytic solutions probed by the Raman noncoincidence effect: experimental and quantum chemical results

The structural organization of organic electrolytic solutions probed by the Raman noncoincidence effect: experimental and quantum chemical results. M. G. Giorgini1, H. Torii2, M. Musso3 1 Dipartimento di Chimica Fisica ed Inorganica, Università di Bologna, Viale del Risorgimento 4, I-40136 Bologna, Italy. 2 Department of Chemistry, School of Education, Shizuoka University, 836 Ohya, Shizuoka 422-8529, Japan. 3 Fachbereich Materialforschung und Physik, Abteilung Physik und Biophysik, Universität Salzburg, Hellbrunnerstraße 34, A-5020 Salzburg, Austria 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+, Ca2+, Sr2+, and Ba2+, 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, in Figure 1a) as compared with those found in the singly charged ions (e.g., –16.0 cm-1 in Na+/acetone) [3]. The NCE calculated for the species (acetone)nMg2+ with n = 3, 4 and 6 suggests the formation, in the Mg2+/acetone solution, of the n = 6 cluster species (NCE= 35.1 cm-1) in which six carbonyl groups are pointing towards the Mg2+ ion in an octahedral organization, as illustrated in Figure 1b. 1. J. McBreen, H.S. Lee, X. Q. Yang, X. Sun, J. Power Sources 89, 163 (2000). 2. D. Vaden, J. M. Lisy, Chem. Phys. Lett. 408, 54 (2005). 3. M. G. Giorgini, H. Torii, G. Venditti, M. Musso, J. Phys. Chem B, 112, 7506 (2008). Figure 1. a) Isotropic and anisotropic Raman profiles of the (C=O) band of acetone/M2+ solutions with M = Mg, Ca, Ba (in red, blue, and green, respectively), where B and C indicate the “bulk” and “cluster” components of the band, and those of neat liquid acetone (in black); b) Cluster of six acetone molecules pointing towards the Mg2+ ion in an octahedral organization.