ON THE PROTON MAGNETIC RESONANCE SHIFT DUE TO HYDROGEN BONDING

In order to investigate further the nature of the proton resonance shifts accompanying hydrogen bonding, and their relation to the properties of the hydrogen bond, the resonance shift of the acceptor molecule chloroform, interacting in turn with a variety of donor molecules, was measured. The donor liquids chosen were (Et)3N, (Et)2O, PrF, (Me)CO, and EtCN. These are all virtually non-associated in the pure state and may be expected to form simple 1:1 hydrogen-bonded complexes with chloroform, whose existence was confirmed by measuring the binary freezing-point phase diagrams. The difference between the proton shift of chloroform at infinite dilution in each of the donor solvents and at infinite dilution in inert hydrocarbon solvents was taken as the "association" shift representing the difference between chloroform in the hydrogen bonded complex and unassociated chloroform. These shifts correlate well with the expected hydrogen bond strengths in each system and the relative donor strength of the individual solvent molecules. The proton resonance technique appears as a promising method for measuring these properties.