SPIN-ECHO NMR STUDIES OF CHEMICAL EXCHANGE. I. SOME GENERAL ASPECTS.

A method is described for calculating the exchange rate from the observed dependence upon the RF pulse separation of the apparent decay constant. The method was applied to the hindered internal rotation of N,N-dimethyltrichloroacetamide and dimethylcarbamyl chloride as a function of temperature. The resultant activation energies of 14.6 and 14.0 kcalmole and frequency factors of 3.5 x 10 to the 12th power and 7.1 x 10 to the 10th power, respectively, are appreciably higher than previously reported values. Comparative determinations were carried out using the high resolution NMR methods. The relatively large differences in the results from the various methods and from different laboratories indicate that systematic errors are present, several possibilities for which are discussed. A simple arrangement is described for generating the Carr-Purcell pulse sequence with three waveform generators and two pulse generators.