Etude par Spectroscopic Infrarouge de la Solvatation Préférentielle du Cation Li+

Abstract

The analysis of the infrared spectra of ternary mixtures LiClO4–S1–S2 shows a preferential solvation of the Li+ ion by ammonia and methylamines (S2) in acetonitrile or tetrahydrofuran (S1) and by acetonitrile (S2) in nitromethane (S1). The appearance of wide bands in the "ion-cage" vibration region is related to the formation of different species [Li(S1)n − i(S2)i]+. The variation of their relative concentration when ρ = [S2]/[LiClO4] increases explains the frequency shift of the bands. The interaction energy Li+ ion – solvent molecule decreases when the number of S2 molecules in the first solvation shell increases and does not seem to depend on the dielectric constant of the surrounding medium. The mean composition of the first solvation shell is obtained from intensity measurements of absorption bands of the S2 molecules not bonded to the ions; in favorable cases (S2 = ND3, CH3CN) solvation number of the Li+ ion in the pure solvent could be estimated. The solvating power of the different bases used is shown to decrease in the order NH3 > CH3NH2 > (CH3)2NH > (CH3)3N > CH3CN > CH3NO2 and involves both donor–acceptor and ion–dipole interactions.