ACCURATE RATE CONSTANTS FOR CHEMICAL-EXCHANGE FROM IMPROVED WEIGHTED LINEAR-LEAST-SQUARES ANALYSIS OF MULTIPLE 1D-EXSY DATA

The 1D-EXSY pulse sequence 90 degrees-t(1)-90 degrees-t(m)-90 degrees- t(2)(obs), with a minimum number of judiciously chosen t(1) values and a series of mixing time t(m), can provide NMR data for measurement of site-to-site rate constants in multisite systems undergoing chemical exchange. A condition can be specified for optimizing the choice of t( 1) values. The data are analyzed by an improved weighted linear-least- squares fitting procedure that uses matrix algebra to help evaluate th e weighting factors. This method is capable of providing highly accura te rate constants and realistic error estimates, without an inordinate demand on instrument time. Three experimental applications, to E/Z me thyl exchange in N,N-dimethylpropionamide, to H-E/H-Z/H-Solvent proton exchange in acrylamide, and to methoxy exchange between stereoisomeri c 4,6-dimethyl-2-methoxy-1,3-dioxanes and methanol, are demonstrated. (C) 1996 Academic Press, Inc.