Nuclear Magnetic Resonance Study of the Proton Transfer in Water

Abstract

Measurements of the nuclear relaxation in water are reported. The transverse relaxation rate (1/T2) of the proton resonance is pH dependent. The effect is shown to be due to a spin-spin splitting of the proton resonance by O17 (spin 5/2), which is only partially averaged out by proton exchange. The increase of relaxation rate is observable in natural water (0.037% O17), and becomes very appreciable in water enriched in O17. Additional information can be obtained by measuring relaxation rates in the presence of an rf field H1, using a method due to Solomon. A study of the width of the O17 resonance as a function of pH is in quantitative agreement with the results of the proton resonance. The observations provide a direct determination of the rate constants of the exchange reactions: H2O+H3O+→ lim k1H3O++H2O and H2O+HO−→ lim k2HO−+H2O.It is found that k1=(10.6±4)×109 liter mole−1 sec−1 and k2=(3.8±1.5)×109 liter mole−1 sec−1. The spin-spin interaction between H and O17 in water is determined as 92±15 cps. In the Appendices, theoretical equations for the exchange contribution to the relaxation rate are derived.