Nuclear magnetic resonance studies on interactions of halide ions with acetonitrile

Abstract

NMR spectra for ¹H and ¹³C nuclei of acetonitrile are reported for solutions of alkali metal and tetraalkylammonium halides. Chemical shifts and relaxation times for a series of concentrations at five temperatures between 25°C and 60°C are analyzed to gain an insight into the mode of anion-molecule interactions. The various salts considered include LiBr, LiI, NaI, tetrabutylammonium bromide (TBAB) and tetrabutylammonium iodide (TBAI). It is found that the chemical shift moves towards lower field on addition of the electrolyte and to higher fields on increasing the temperature of the system. An increase in the concentration of the electrolyte and a decrease in temperature increase the concentration of complexed species which is reflected in the measured chemical shifts. Earlier conclusions on the linear variation of the changes in the ¹H chemical shifts with concentration of the electrolytes are reviewed. The chemical shift data for solutions of tetrabutylammonium halides have been utilised for the determination of equilibrium constants for anion-molecule interactions and the values are found to be in the order I^- > Br^- which shows that the interactions are more of electrostatic type which depend on the extent of polarization of the charge on the halide ions. The relaxation times obtained from the line widths of ¹H, ¹³C and ⁸¹Br are analyzed on the basis of anion-molecular equilibria.