Solvation spectra. Part 54.—A low temperature infrared spectroscopic study of the solvation of ions in methanol

Abstract

Broad featureless bands in the O—H stretching region for CH₃OH in CH₃OD containing a range of 1:1 electrolytes become narrower on cooling below the glass point and usually display several features. Lithium and sodium salts up to ∼3 mol kg^–1 show narrow bands on the high-frequency side of the bulk methanol band that are assigned to solvent molecules bonded to the anions. The bulk methanol bands are broadened and shifted by the cations, but separate bands for the cation solvates are not resolved. At higher concentrations of salt, the anion bands are also shifted by the cations to give new high frequency bands assigned to solvent shared ion-pairs. Tetra-alkyl-ammonium salts in the 1 mol kg^–1 region give new bands characteristic of the anions only.

Sodium tetraphenylboron gives a narrow component at 3520 cm^–1 and a very broad component centred at ∼3360 cm^–1. The former is assigned to two O—H groups bonded to a single methanolic oxygen atom: the latter is assigned to oscillators in short chains or cyclic structures. A broad band at ∼3360 cm^–1 for other concentrated electrolyte solutions is assigned similarly.