Multiple-pulse FT-NMR experiments for kinetic applications

Abstract

NMR multiple-pulse techniques using either the two-pulse spin-echo sequence (Hahn-echo) or the Carr-Purcell-Meiboom-Gill (CPMG) sequence are combined with Fourier transformation of the resulting echos. For dynamic NMR spectra the observed spin—spin relaxation times T 2 of individual lines depend on the pulse spacing of the sequences and allow the evaluation of kinetic information (1, 2). In order to demonstrate the practical potential of these techniques the CPMG method was applied to study the intramolecular exchange of methyl groups in N,N-dimethylbenzamide. Further computer-generated examples illustrate that (1) simple two-pulse spin-echo sequences may be applied for qualitative identification of kinetically influenced lines, (2) CPMG sequences easily provide quantitative kinetic data, and (3) multiline spectra with extensive overlap can be simplified considerably if the spectra obtained for different pulse spacings are combined to give “kinetic difference spectra.” The methods presented here are applicable to the whole dynamic range from slow to fast exchange. Assuming a chemical shift difference of 250 Hz, natural spin—spin relaxation times of 1 sec, and a minimum pulse spacing of 10 −5 sec, the time resolution of the technique reaches 2 × 10 −5 sec with a dynamic range of five orders of magnitude. Since no temperature variations are needed for the evaluation of kinetic parameters, multiple-pulse experiments are especially suited for samples with low tolerance to temperature changes, in particular for biological systems.