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 (TA) and spin-lattice relaxation time (TIA) 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 TB and TIB • 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. I The definition of these terms is somewhat arbitrary. In the following the term moderately rapid is applied to reactions involving lifetimes of the reacting species of the order of 0.1-10 sec.

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Articles you may be interested inFast simulation of dynamic two-dimensional nuclear magnetic resonance spectra for systems with many spins or exchange sitesThe nuclear magnetic multiple resonance method for the study of chellical exchange rates has been applied to systems in which a nucleus X is reversibly exchanged between three nonequivalent sites. A procedure is outlined, which is applicable to systems with any number of sites and permits the determination of the lifetime TK at anyone site K. In this procedure the effective "relaxation time" Tur and the equilibrium z magnetization, MoK(TIKITIK), are determined in experiments (Type I) in which the X signals at all other sites are completely saturated. The rate constants ]o..KL and ]o..LK of the individual exchange reactions (KpL) may then be determined in Type II experiments in which the signals of all sites excluding K and L are saturated. In Type II experiments only the equilibrium signal intensities need be measured.The method has been applied to the base-catalyzed proton exchange in the keto-enol system of acetylacetone and it was found that exchange between the olefinic CH and hydroxylic ---OH protons occurs merely as a consequence of the keto-enol transformation. Within the accuracy of the measurements no evidence was found for a proton exchange between the =CR and -OR protons which does not involve the CR, group in the keto form as an intermediate. * The earlier papers in this series are to be found in Refs. 1 and 2.t

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