Infrared and proton magnetic resonance studies of the structures of methanol + aprotic solvent systems

Abstract

Solvent systems comprising methanol + a range of basic aprotic solvents have been studied over the complete mole fraction range. Stretching frequencies for the O—H group of methanol were measured in the fundamental and first overtone regions and discrete features were assigned to the following structures: [graphic omitted] where (bulk) represents two or more hydrogen-bonded methanol molecules and the underlining refers to the group detected.

The O—H proton resonance was always a single averaged line. However, a correlation between infrared band maxima and proton resonance shifts has been used to give specific shifts for the O—H protons in the structures shown above. These results, together with the experimental shifts, have been used to obtain some measure of the equilibria involved. In particular, for strongly basic aprotic solvents, the assumption that hydrogen bonding to these solvents is always complete leads to good reconstructions of the n.m.r. data over the complete mole fraction range.

A novel feature of this work is the recognition of the significance of terminal methanol molecules, [graphic omitted] referred to as (lone-pair)_free groups. To verify their formation, we added the proton donors trichloromethane and phenylacetylene to methanolic solutions of strong bases. The resulting down-field shifts were in good accord with our assignments.