Study of Moderately Rapid Chemical Exchange Reactions by Means of Nuclear Magnetic Double Resonance

A nuclear magnetic double-resonance method for the determination of chemical exchange rates has been developed. The method is applicable to systems in which a nuclear spin is reversibly transferred between two nonequivalent sites, A and B. The lifetime (τA) and spin—lattice relaxation time (T1A) in Site A are obtained through the study of the decay to a new equilibrium value of Signal A upon the sudden saturation of Signal B. The converse experiment permits the determination of τB and T1B. A number of data for cross checks are furthermore obtained through the study of the recovery of the signals upon the release of various combinations of saturating rf fields. A simple theory based on the Bloch equations as modified by McConnell to incorporate the effects of chemical exchange is given. Experimental results on the hydroxyl proton exchange in the system salicylaldehyde and 2-hydroxyacetophenone are well described by this simple theory. The present method, which can readily be extended to systems with several sites, offers a complement to the Gutowsky—Saika single-resonance method and is particularly suited to the study of exchange rates slower than those accessible by the single-resonance method.