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Study of chemical exchange kinetics by nuclear magnetic resonance Inglefield, Paul T.

Abstract

A study of the rates of hindered internal rotation about the C—N bond in N,N-dimethyltrifluoroacetamide and N,N-dimethyldeuteroformamide has been made using the nuclear magnetic resonance spin echo method. A 40 Mc/sec. spin echo spectrometer, which enables measurements to be made of the transverse relaxation time (T₂) from a Carr Purcell sequence of radio-frequency pulses, is described and its performance illustrated. The precision of values obtained is estimated to be of the order of 5%. Equations, derived from 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, have been used to extract the kinetic data for the two molecules. The chemical shift, rate constant and natural relaxation time (T2₀) have been calculated from the data by curve fitting procedures using computer techniques, and the experimental verification of the theory has been noted. The accuracy of the rate constants and activation parameters derived is discussed in the light of previous steady state nuclear magnetic resonance studies. The relative merits of the spin echo and steady state methods are examined and the possibility of the occurrence of systematic errors in such studies is investigated. Steady state studies of the proton exchange between methanol and two phenols (meta-cresol and ortho-hydroxyaceto-phenone) and of the ring inversion of N,N-dimethylpiperazine are also presented and the relative merits of the kinetic parameters discussed.

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