Magnetic resonance studies in aqueous systems. Part 2.—Chemical shift and relaxation time studies of hydroxylic protons in dilute alcohol + water mixtures

Abstract

N.m.r. chemical shifts and relaxation times have been measured for hydroxylic protons in a series of dilute alcohol + water mixtures at different temperatures. The shifts are largely influenced by hydrogen bonding between alcohol and water molecules, resulting in low field shifts in dilute alcohol solutions. Hydrocarbon groups also shift the hydroxylic proton resonance, but to high fields. This effect is relatively small, particularly for small and spherical alkyl groups like t-butyl, but increases with chain length of the alkyl group. Water molecules have restricted motion in dilute alcohol + water mixtures. One of the main effects in restricting the molecular mobility of water molecules, for the short alkyl chain alcohols studied, particularly methanol and isopropyl alcohol, is hydrogen bonding interactions between alcohol hydroxyl groups and water molecules. Water molecules surrounding alkyl chains also have restricted motion and the effect increases with alcohol chain length. Contrary to some current theories, alcohol–water hydrogen bonding is important in these mixtures and should be taken into account in the interpretation of relevant thermodynamic data.