Combined use of complete lineshape analysis of 1D spectra subjected to reference deconvolution and linear prediction, 2D-EXSY spectra and a double fitting method for the study of chemical exchange. Application to an eight-site exchange system

Abstract

Obtaining unknown NMR parameters from experimental spectra of exchanging systems is, from a mathematical point of view, associated with the solution of an inverse problem. Inverse problems are usually solved by iterative procedures and calculations may not always converge and the solutions are rarely unique. To attain reliable rate constants we adopted the following experimental–computational scheme: acquirement of perfect NMR lineshapes for complete lineshape analysis (CLSA) by elimination of undesirable effects due to magnetic field inhomogeneity through reference deconvolution and linear prediction; the combined use of 2D exchange spectroscopy (2D-EXSY) and 1D CLSA for the determination of the rotation rate constants; and application of the double fit method, which is based on the knowledge that the exchange parameters depend on thermally activated processes. In this paper, we demonstrate the advantages of this combined approach for the determination of precise and unique sets of rate constants and activation parameters. We applied this scheme to study hindered rotations in α-[bis(dimethylamino)methylene]-β-oxophenylacetonitrile. This compound is an example of exchange between eight potential sites. Activation parameters of the eight rotations in that molecule were calculated and are discussed. Copyright © 1999 John Wiley & Sons, Ltd.