Infrared spectroscopic studies of the solvation of aprotic solvents and ions in methanol

Abstract

The effect of the addition of various polar aprotic solvents (B) and electrolytes on the first overtone of the O—H stretching band (2v_OH) of CH₃OH in CD₃OD or CD₃OH in CD₃OD has been measured. The results for the solvent mixtures are discussed in terms of contributions from MeO—H⋯B oscillators and terminal methanol molecules; designated lone-pair free [(LP)_free] units. For the electrolytes, in addition to contributions from methanol molecules bound to the cations and anions, we consider contributions from (LP)_free groups and from other weakly bound methanol molecules, including possible contributions from non-hydrogen-bonded OH groups, (OH)_free. The results are compared with recent proton magnetic resonance studies and with similar studies of aqueous solutions. It is concluded that basic aprotic solvents and anions form fewer hydrogen bonds to methanol molecules than to water molecules. For example, dimethyl sulphoxide forms one hydrogen bond in methanol (two in water), triethyl-phosphine oxide forms two hydrogen bonds in methanol (three in water) and chloride ions form four hydrogen bands in methanol (six in water).