THE DESCRIPTION AND USE OF A SPIN-ECHO SPECTROMETER TO STUDY CHEMICAL EXCHANGE

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Abstract
A spin-echo spectrometer, suitable for studies of chemical exchange, is described. The performance of the apparatus is described and illustrated. From a Carr–Purcell sequence of pulses T2 measurements have been made using two alternative schemes for the correction of cumulative error in the width of π pulses. Agreement is satisfactory and the standard errors in each method are ± 5% of the average T2. The Meiboom and Gill method of phase-shifting the r.f. in the first pulse has the advantage of more flexibility in the choice of pulse intervals.A theory developed by Bloom, Reeves, and Wells for the dependence of T2 measured in a Carr–Purcell experiment on pulse intervals in the presence of a suitable exchange process has been tested experimentally for two molecules. The rate constants for the hindered internal rotation about the N=N bond in N,N -dimethylnitrosamine agree for the following sets of data: (a) a previous high-resolution study by Phillips, (b) a repeated high-resolution study made in this work, (c) spin-echo studies using the complete closed expression for a two site exchange process, and (d) the approximate expression derived for the limit of fast pulse repetition rates.A similar study by the spin-echo method of the rates of internal rotation about the C=N bond in N,N-dimethyltrifluoroacetamide does not agree with a previous high-resolution study. Since the chemical shift between the inequivalent methyl groups is so small, it is suggested that the rates of rotation in this molecule are much better evaluated by the spin-echo method.The spin-echo method can more easily take account of variations with temperature of the natural relaxation time and the chemical shift. The non-selective nature of an experiment with such a large H1 field remains a disadvantage at this time. The selective deuteration of sites unaffected by exchange is in general difficult enough to limit the scope of the spin-echo method when protons are observed.