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J. Phys. Chem. B 2008, 112, 7506–7514
Influence of Ions on the Structural Organization of Dipolar Liquids Probed by the Noncoincidence Effect: Experimental and Quantum Chemical Results Maria Grazia Giorgini,*,† Hajime Torii,‡ Maurizio Musso,§ and Giampaolo Venditti† Dipartimento di Chimica Fisica ed Inorganica, UniVersita` di Bologna, Viale del Risorgimento 4, I-40136 Bologna, Italy, Department of Chemistry, School of Education, Shizuoka UniVersity, 836 Ohya, Shizuoka 422-8529, Japan, and Fachbereich Materialforschung und Physik, Abteilung Physik und Biophysik, UniVersita¨t Salzburg, Hellbrunnerstraβe 34, A-5020 Salzburg, Austria ReceiVed: January 11, 2008; ReVised Manuscript ReceiVed: April 2, 2008
The CdO stretching [ν(CdO)] 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 ) ν˜ ani - ν˜ 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 ν(CdO) 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+. 1. Introduction Ionic solvation phenomena are of noteworthy importance in both industrial and biochemical processes. While the latter prevalently involve protic dipolar solvents, such as water, several industrial procedures for the preparation of electrochemical devices, such as secondary (rechargeable) batteries, prevalently involve the use of nonprotic dipolar solvents (or mixtures of solvents) for dissolving metal salts.1 The performance of these electrochemical devices relies upon the ionic conductivity of their electrolytic solutions, and this, in turn, depends on the structures of these solutions at the molecular level.2 Therefore, the knowledge of the nature and strength of ionssolvent and ionsion (contact ion pairs) interactions in electrolytic solutions is a subject of primary importance in this technological field. There are several references documenting the ability of vibrational spectroscopy to enlighten the nature of these kinds of interactions in electrolytic solutions.3–5 The main spectroscopic probe is the appearance of new bands in the vibrational (IR and Raman) spectrum upon dissolution of the salt into the solvent. These new bands are close in frequency to the bands of neat solvent and are attributable to the changes in the solvent normal-mode frequencies induced by the ion electric field or, expressed in a different way, attributable to the changes in solvent normal-mode frequencies within the structures of the ionssolvent associated species (clusters). The frequency shifts and the intensities of these new vibrational bands and their concentration dependences help assess the strength of the ionssolvent interaction and evaluate the solvation numbers, that is, the number of solvent molecules clustering around each ion in the first solvation shell.4 Among organic solvents, carbonyl compounds (such as acetone, N,N-dimethylformamide, and ethylene carbonate) and nitriles occupy a preferential position * Corresponding author. E-mail:
[email protected]. † Universita ` di Bologna. ‡ Shizuoka University. § Universita ¨ t Salzburg.
for the preparation of electrolytic solutions by virtue of their generally high dielectric constant. In the spectroscopic investigations of these solutions, the frequency and intensity changes upon salt solvation of the vibrational bands associated with the stretching of the CdO or CtN groups [ν(CdO) or ν(CtN)] have been frequently used.3–5 While the ν(CtN) mode exhibits, upon cluster formation, a blue shift in the Raman (and IR) spectrum caused by the charge transfer from the lone pair on the nitrogen atom to the metal ion, 6 both blue and red shifts of the ν(CdO) mode were observed in the Raman spectra of different carbonyl compounds (such as acetone and methylacetate).7 It is well-known that the ν(CdO) band of the liquid carbonyl compounds, around 1600-1800 cm-1, manifests a phenomenon known as the noncoincidence effect (NCE), which consists in the separations both between its IR and Raman peak frequencies (NCEIR-R ) ν˜ IR - ν˜ iso) and between its anisotropic and isotropic Raman components (NCE ) ν˜ ani - ν˜ iso), the latter being generally referred to as the Raman NCE. Even though first and most remarkably observed in carbonyl compounds, the NCE may be expected and has been indeed observed for several other compounds such as sulfoxides, sulfones, and alcohols.8–19 Investigations performed in recent years have established the idea that the NCE can be considered a valuable tool to probe the structural arrangement of liquids at the molecular level and to monitor the structural modifications induced by the presence of solvent molecules in chemical and isotopic mixtures10–14 and by the change in the thermodynamic conditions such as temperature, pressure, and nanoconfinement of the neat liquids.9,15–17 The reason that makes the NCE of a strongly IR active mode, such as ν(CdO), of a dipolar liquid a sensitive probe of its structure (molecular positions and orientations) resides in the fundamental intermolecular interaction mechanism at the basis of this phenomenon, that is, the transition dipole coupling (TDC) between neighboring identical oscillators. To be more specific, the magnitude and sign of this kind of intermolecular interaction, and consequently the magnitude and sign of the frequency
10.1021/jp800252n CCC: $40.75 2008 American Chemical Society Published on Web 06/04/2008
Structural Organization of Dipolar Liquids Probed by the NCE splitting that it gives rise to (i.e., the NCE), depend on the molecular separation rij (through the inverse of its third power) and on the magnitude and the mutual orientation of the transition dipoles b µki and b µkj (for the kth oscillator) in a couple of distinct and identical interacting molecules (i,j). This means that the analysis of the magnitude and sign of the Raman NCE provides information on the liquid structure. In dipolar liquids and liquid mixtures structured only by dipolar forces, the prevalent occurrence of the parallel head-to-tail and the antiparallel sideby-side dipolar organizations (or any intermediate organizations between them) makes the NCE positive (e.g., +5.3 and +14.5 cm-1 in the ν(CdO) bands of neat liquid acetone13,14 and DMFA,11 respectively), whereas a sign inversion occurs in liquids structured by forces that make prevalent the occurrence of head-to-head or parallel side-by-side dipolar organizations (or also any intermediate organizations between them), as it happens in H-bond structured liquids such as methanol, where the NCE of the ν(C-O) band amounts to -4.0 cm-1.9,12,18 An even more remarkable sign inversion of the NCE, first noticed by Kecki and Sokolowska,19 is the one observed in the carbonyl spectral envelope of some electrolytic solutions of acetone as an additional ν(CdO) Raman band, which arises on the higher-frequency side of the main ν(CdO) band. It was interpreted19 as a manifestation of the mediation operated by the third body (the ion in the present case)20 which, by virtue of its polarizability, would induce the reorientation of the ν(CdO) transition dipoles in adjacent molecules from the prevalently antiparallel side-by-side orientation, as in neat acetone, to the prevalently parallel side-by-side orientation. Even though suggestive, this interpretation may hardly be straightforwardly inferred21 from the theoretical results [eqs 3.10 and 3.11 in ref 20a] of the third-body mediation theory. Therefore, it seems meaningful to reinvestigate the origin of the large and negative NCE observed for the ion-perturbed ν(CdO) band in electrolytic solutions of carbonyl solvents by taking advantage of improved instrumental performances and data treatment procedures and, additionally, of the joint use of quantum chemical calculations. In this paper, we report the results of an experimental investigation of the Raman isotropic and anisotropic profiles and then of the NCE of the ion-perturbed and unperturbed ν(CdO) bands observed in some electrolytic solutions of monovalent salts, M+X-, in carbonyl solvents, S. To assess the origin of the large and negative NCE of the ion-perturbed ν(CdO) bands, these findings will be interpreted in the light of the results of quantum chemical calculations (equilibrium geometry and vibrational frequencies) worked out on cluster species (S)nM+ formed of n () 3, 4, 6) CdO groups of solvent molecules (S) pointing toward the M+ cation. This comparison will suggest that the very large and negative NCEs observed for the cluster ν(CdO) band in all the studied electrolytic mixtures may be traced back to the frequency separation of the in-phase and the n - 1 (in some cases almost degenerate) out-of-phase ν(CdO) vibrational modes, where the latter have, in each cluster species, lower frequencies than the former. These investigations have been extended to electrolytic solutions obtained by varying both the salt ions (the cation M+ and anion X-) and the carbonyl solvent (S ) acetone and acetophenone). The purpose is to enlighten the way how the size of the metal ion M+, the size of the counterion X-, and the steric hindrances of the substituents to the CdO group in the solvent S may affect the NCE of the ν(CdO) vibrations. It will be clarified that the origin of the large and negative NCE of the ν(CdO) vibrations of a (S)nM+ cluster is the TDC
J. Phys. Chem. B, Vol. 112, No. 25, 2008 7507 mechanism between CdO groups which are oriented prevalently side-by-side around the metal cation M+. 2. Experimental 2.1. Sample Preparation. All electrolytic mixtures were prepared in dry box under inert (N2) atmosphere and transferred into quartz stopcock cells appropriate for high-vacuum operations where they were held during all the experiments to warrant against the leakage of the inert atmosphere and prevent the access of ambient water. The solvents acetone (spectroscopic grade, H2O content
