Near infrared spectral studies on interactions of CH<inf>3</inf> groups with halide ions

Near infrared (NIR) spectra in the CH3 stretching first-overtone region for methyl iodide, acetonitrile, nitromethane and dimethyl sulphoxide are discussed in terms of normal mode and local mode descriptions. NIR spectra in this region for solutions of several alkali metal halides and tetraalkyl ammonium halides in these solvents are also reported. It is found that the relative intensity of the cooperative excitation band with respect to the pure overtone excitation band reduces considerably on increase in the concentration of the electrolytes in solution. The results are discussed in terms of the formation of anion-molecular complexes as well as intermode coupling due to kinetic energy and anharmonicity effects. © 1993 Indian Academy of Sciences.