Deuterium–protium fractionation in solutions of methoxide ion in mixtures of methanol and dimethyl sulphoxide

Abstract

The fractionation factor of the methoxide ion (ϕ_OMe)has been determined in mixtures of methanol and dimethyl sulphoxide (DMSO). By using ¹H n.m.r. data of very high precision it has been possible to use solutions of three different atom fractions of deuterium making the usual assumption about the chemical shift of the counterion (Na⁺) unnecessary. ϕ_OMe has been found to decrease from 0.74 (MeOH) to 0.38 (75 mol % DMSO). The variation of ϕ_OMe is discussed in terms of increasing symmetry in the hydrogen bonds to the methoxide ion as the desolvation of the methoxide ion proceeds.