NATURE OF WATER SPECIES IN WATER-AMMONIA SOLUTIONS AS INFERRED FROM PROTON AND OXYGEN-17 NUCLEAR MAGNETIC RESONANCE OBSERVATIONS

H2O proton shifts as a function of H2O concentration in liquid NH3 at 29.6 ± 0.2° are well interpreted in terms of an equilibrium between monomeric and dimeric forms of H2O. The intrinsic shift for the average H2O proton in the monomeric species is -268.2 ± 0.2 Hz relative to TMS while for the dimeric species it is -416 ± 12 Hz. The equilibrium constant for formation of dimer is K2 = (1.26 ± 0.15) X 10-2. 17O shifts in H217O-NH3 liquid mixtures at room temperature when compared with 17O shifts in H217O-acetone and H217O-(CH3)3N suggest that the shielding at the 17O nucleus may be principally determined by the degree to which oxygen lone-pair electrons act as donors to solvent protons in hydrogen bond formation. Such interactions may be more important than heretofore considered. The temperature coefficient of the H2O proton shift in very dilute solutions of H2O in liquid NH3 is 13.5 × 10-3 ppm/deg and appears to be a property of a proton in the N---H-O hydrogen bond in this system. The temperature coefficient for the proton shift in liquid NH3 is 2.7 × 10-3 ppm/deg.