Proton Nuclear Magnetic Relaxation Studies on Water: The Rates of Acid- and Base-Catalyzed Proton Exchange

Abstract

The rates of acid- and base-catalyzed exchange of protons in water have been redetermined. Rather than employ 17O-enriched water, as in Meiboom's method, we have taken advantage of our observation that T2 is frequency dependent in order to simplify the analysis. The acid-catalyzed rate constant for proton exchange, k1, was found to be 7.9×109 liter mole−1·sec−1 and the base-catalyzed rate constant, k2, to be 4.5×109 liter mole−1·sec−1. The activation energy for k1 was 2.6 kcal, while that for k2 was 2.7 kcal. Our results are compared with those of others as well as with the related anomalous conductance of water. Further studies on the properties of water are suggested by the observation that T2 is frequency dependent.