Ion-Solvent Interactions in Acetonitrile Solutions of Lithium Iodide and Tetrabutylammonium Iodide Andrew K. Mollner, † Paula A. Brooksby, † John S. Loring, ‡ Imre Bako, § Gabor Palinkas,* ,§ and W. Ronald Fawcett* ,† Department of Chemistry, UniVersity of CaliforniasDaVis, DaVis, California 95616, Department of Land, Air, and Water Resources, UniVersity of CaliforniasDaVis, DaVis, California 95616, and Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri ut 59/67, H-1025 Budapest, Hungary ReceiVed: October 20, 2003; In Final Form: January 28, 2004 The vibrational spectra of acetonitrile solutions containing lithium iodide and tetrabutylammonium iodide have been studied using ATR-FTIR spectroscopy. The focus of interest was the effect of the iodide anion on the important vibrational bands of acetonitrile. The main effect of the LiI electrolyte is to shift the CtN stretching frequency in the blue direction due to interaction of the Li + cation with the electronegative CtN group. However, a small red shift of the CtN stretching mode due to the interaction of I - with the methyl group of acetonitrile is found in both electrolytes. On the other hand, the symmetic and asymmetric stretching modes of the methyl group are significantly red shifted in the presence of LiI. Ab initio quantum chemical calculations were carried out to determine the optimum location of the Li + and I - ions in ion-solvent complexes containing a varying number of solvent molecules. These calculations correctly predict the direction of the observed shifts for all the principal bands of acetonitrile except the methyl stretching modes. Introduction Vibrational spectroscopy is a very useful tool for studying ion-solvent interactions in electrolyte solutions. 1 In the case of aprotic solvents such as acetonitrile, dimethyl sulfoxide, and acetone, FTIR spectroscopy is the most convenient method especially when it is used in the attenuated total reflection (ATR) mode. In previous work, ATR-FTIR spectroscopy was used to study ion-solvent and ion-ion interactions in a variety of perchlorate solutions in acetonitrile; these included the alkali metal perchlorates, 2 the alkaline earth metal perchlorates, 3,4 and other divalent metal perchlorates. 3 The analysis of the spectro- scopic data was significantly improved recently 5 by carefully considering the correction of ATR data for distortion due to varying effective path length, and by applying factor analysis 6 to spectral data obtained as a function of electrolyte concentra- tion. When data for acetonitrile solutions containing LiClO 4 , NaClO 4 , and tetraethylammonium perchlorate were examined, 7 it was shown that the CtN stretching frequency is affected not only by the nature of the cation of the electrolyte but also by the anion. Although the latter effect is subtle, its recognition was a major step forward in the interpretation of electrolyte effects on the solvent’s vibrational properties in these systems. In the present paper, a detailed study of the FTIR spectra of acetonitrile solutions of LiI is reported. By changing the anion of the electrolyte from perchlorate to iodide, it was anticipated that a more careful examination of the anion’s role in spectral features would be possible. Experiments were also carried out in tetrabutylammonium iodide (TBAI). In this way, the role of the small Li + in determining spectral features could be more carefully assessed by comparison with spectra obtained in the presence of the large TBA + cation. Ab initio quantum mechan- ical calculations were carried out to describe the important features of the IR spectrum. By using an electrolyte with monatomic ions, the number of atoms associated with an ion- solvent cluster was kept at a minimum. The initial results of these calculations are also presented in this paper and compared with the experimental spectroscopic data. Experimental Methods Anhydrous acetonitrile (AcN) (99.8% < 0.005% water) was purchased from Aldrich and was not purified further. LiI (Sigma, 99%) and TBAI (Aldrich) were dried in the dark and under vacuum at 140 °C for 24 h. Manipulations of the salts and acetonitrile were done inside a nitrogen filled glovebox. All solutions were prepared by weight. A 20 cm 3 pycnometer (calibrated with water at 25 °C) was used to calculate the densities of a series of five electrolyte solutions in AcN with known weights. These calibration curves were used to convert the sample solution weights to molarity. A Mattson Galaxy 3000 RS-1 spectrometer was used for all infrared measurements. The spectrometer was operated at ambient pressures in an environment that had been purged significantly free of CO 2 and atmospheric water (PUREGAS Air Dryer, model CDA 1120). The spectrometer was equipped with a Ge/KBr beam splitter, a mechanical interferometer having cubic mirrors, a water cooled Globar ceramic source, and a DTGS detector operating at room temperature. A custom built overhead ATR cell (University of California, Davis) was designed to fit into the sample compartment of the Mattson spectrometer; it permitted use of a trapezoidal CdTe internal reflection element (Spectral Systems, 45° and 50 × 20 × 3 mm). All pATR spectra were obtained using a resolution of 1 cm -1 and analyzed by taking the negative logarithm of the ratio of sample and background single beam spectra. The empty ATR * To whom correspondence should be addressed. E-mail: palg@chemres.hu (G.P.); wrfawcett@ ucdavis.edu (W.R.F.). † Department of Chemistry, University of California, Davis. ‡ Department of Land, Air, and Water Resources, University of California, Davis. § Hungarian Academy of Sciences. 3344 J. Phys. Chem. A 2004, 108, 3344-3349 10.1021/jp037174y CCC: $27.50 © 2004 American Chemical Society Published on Web 03/30/2004