Ion-Solvent Interactions in Acetonitrile Solutions of Lithium, Sodium, and Tetraethylammonium Perchlorate Using Attenuated Total Reflectance FTIR Spectroscopy

摘要

A vibrational spectroscopic study of ion solvation in LiClO_4-, NaClO_4-, and tetraethyammonium perchlorate- acetonitrile solution has been performed using attenuated total reflection FTIR spectroscopy. Spectra were curve fitted in the C ident to N stretching and Cimplied C - C ident to N bending regions of the solvent using a multiple-dimensional fitting routine that factors an entire set of solution spectra into their physically real basis spectra according to a user supplied mathematical model. The spectra of The solutions of tetraethylammonium perchlorate in acetonitrile were shown to be a linear combination of one physically real basis spectrum that accounts for self-associated acetonitrile and another that accounts for the spectrum of the acetonitrile molecules associated with the perchlorate anion. It is demonstrated for the first time that the C ident to N stretching band of acetonitrile molecules associated with ClO_4~- anions has a significantly different molar absorption coefficient from the same band for self-associated acetonitrile. The spectra of LiClO4 and NaClO4 solutions in acetonitrile were shown to be a linear combination of one physically real basis spectrum that accounts for the spectrum of self-associated acetonitrile and another that accounts for the spectrum of the acetonitrile molecules associated with both the cation and the anion. The fractional contribution of these physically real basis spectra to the solution spectra depends on the total electrolyte concentration, rather than on the extent of ion pairing. Two possible rationalizations for this phenomenon are discussed.