Infrared spectral studies on preferential solvation of lithium ions in binary mixtures of acetonitrile and dimethyl formamide

Infrared spectra for solutions of lithium perchlorate in binary mixtures of acetonitrile and dimethyl formamide are reported in the C=N stretching and NCO deformation regions. Spectral parameters for the free and complexed components obtained after band fitting are analysed. It is noticed that both the bands increase in frequency on complexation. If the concentration of dimethyl formamide increases the complexed NCO band increases in intensity whereas the complexed C=N stretching band decreases. The intensity data is analysed in terms of the solvation numbers of the two solvents around the cations as well as competing solvation equilibria. It is concluded that the IR spectra can be successfully utilised for studies of preferential solvation of cations and there is definite evidence on the basis of the relative intensities of the complexed and free bands as well as solvation numbers that the composition of the solvation shells around the cations is richer in dimethyl formamide than in the bulk in the whole range of composition of the binary solvent mixtures studied.