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

摘要

A nuclear magnetic doublehyphen;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,AandB.The lifetime (tgr;A) and spinmdash;lattice relaxation time (T1A) in SiteAare obtained through the study of the decay to a new equilibrium value of SignalAupon the sudden saturation of SignalB.The converse experiment permits the determination of tgr;BandT1B. 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 2hyphen;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 Gutowskymdash;Saika singlehyphen;resonance method and is particularly suited to the study of exchange rates slower than those accessible by the singlehyphen;resonance method.