DYNAMIC NMR - A NEW PROCEDURE FOR THE ESTIMATION OF MIXING TIMES IN THE 2D EXSY EXPERIMENTS - A 4-SITE EXCHANGE SYSTEM STUDIED BY 1D AND 2D EXSY SPECTROSCOPY

A procedure for the estimation of the mixing time between the last two 90 degrees pulses in the classic three-pulse sequence NOESY/EXSY is proposed and tested and some considerations for the treatment of the two-dimensional (2D) H-1 NMR exchange spectra are given. The rate constants are thus obtained with reasonable precision. This procedure was followed to obtain the 2D spectra of the model compound alpha-[bis(dimethylamino)methylene]-4-nitrophenylacetonitrile, which represents a four-site exchange system. The barriers to restricted rotations found in this compound were also determined from one-dimensional (1D) H-1 NMR spectra, which were processed with the iterative complete lineshape analysis (CLSA) method. The double-fit approach was incorporated in the CLSA method. It is shown that the results from the 2D dynamic NMR spectral studies corroborate those obtained by the CLSA double-fit method.