HYDROGEN-BONDING EFFECTS ON OXYGEN-17 CHEMICAL SHIFTS

The effects of solvents on the oxygen-17 chemical shifts of water and acetic acid have been investigated. The effects of added water on the 17O shifts of methanol and acetone and the shift changes upon titration of acetic acid have also been studied. The results are interpreted in terms of fast equilibrium between various plausible hydrogen-bonded complexes. It is found that breaking of hydrogen bonds involving the oxygen atom of a hydroxyl group leads to an upfield shift of the 17O resonance. Proton donation has an effect of –12 ppm and proton accepting an effect of –6 ppm on the 17O shift. These values are tentatively rationalized in terms of the relative contribution of valence bond resonance structures describing the hydrogen bond. The major effect seems to be due to the charge-transfer structure -O−H-+O Its contribution is estimated to be between 6.3 and 10.5% depending on the value taken for the 17O shift of H3O+ and −OH. The solvent effects on the 17O chemical shift of acetic acid clearly reveal the features of the equilibrium between the complex species present in these solutions. The calculated equilibrium constants are in agreement with values obtained from nmr studies of other nuclei (1H and 13C). Upon titration a negative shift is observed and, if plotted against pH, a well-defined titration curve is obtained. Previous study of the dilution shift of acetone in water has been extended. The observed behavior cannot be described by one equilibrium constant and the existence of two equilibria in this system is suggested. Oxygen-17 magnetic resonance appears to be a suitable method for studying hydrogen-bonded systems. © 1969, American Chemical Society. All rights reserved.