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Dynamic NMR: A new procedure for the estimation of mixing times in the 2D EXSY experiments. A four‐site exchange system studied by 1D and 2D EXSY spectroscopy

A procedure for the estimation of the mixing time between the last two 90° pulses in the classic three‐pulse sequence NOESY/EXSY is proposed and tested and some considerations for the treatment of the two‐dimensional (2D) 1H 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 α‐[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) 1H 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.