Exchange Rates by Nuclear Magnetic Multiple Resonance. III. Exchange Reactions in Systems with Several Nonequivalent Sites

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Abstract
The nuclear magnetic multiple resonance method for the study of chemical 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 τK at any one site K. In this procedure the effective ``relaxation time'' τ1K and the equilibrium z magnetization, M0K (τ1K/T1K), are determined in experiments (Type I) in which the X signals at all other sites are completely saturated. The rate constants λKL and λLK of the individual exchange reactions (K⇄L) 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 =CH and —OH protons which does not involve the CH2 group in the keto form as an intermediate.