Infrared Studies of Halide Ion Solvation in Methanol1

Abstract

Concentrated solutions of alkali halides in methanol at low temperature have infrared spectra in the O—H stretching region that are dominated by features assigned to solvent shared ion-pairs [graphic omitted]. Such units cannot form in concentrated solutions of tetra-alkylammonium halides, but their low-temperature infrared spectra nevertheless show up to six resolved O—H features. These are assigned to a range of anion solvates in which both primary and secondary solvation has been reduced relative to dilute solutions. On warming, these features rapidly broaden and merge into a single very broad feature. (Dilute solutions of CH3OH in CH3OD were used to avoid intermolecular coupling effects).

Solutions of tetra-alkylammonium halides in methylene chloride, on the addition of methanol gave mainly the mono-solvate at ambient temperatures, but on cooling, marked aggregation was observed, and several of the features obtained from methanolic solutions were again detected.

Addition of water in low concentrations resulted in rapid loss of features assigned to terminal methanol molecules and also of features assigned to primary solvent molecules. The rôle of these water molecules may be to form linked units of anions, or possibly, cyclic units containing fewer terminal solvent molecules and fewer primary methanol molecules.