Heteronuclear double resonance: Individual overhauser effects in coupled AX 2 and AX 3 spectra

Abstract

We describe a new way of investigating relaxation in multispin systems. In a double-resonance experiment, if the decoupler is off while the spectrum is collected then the line positions are the same as those in the single-resonance spectrum but the intensities are perturbed. This is due to population changes induced by the rf irradiation. A theory of these effects is developed which is free of many of the usual restrictive assumptions. Individual Overhauser effects for each line in the A part of an AX 2 or AX 3 spectrum are defined and can be calculated. In the calculation, all the correlations in the dipolar relaxation are included. The Overhauser effects for the three lines in the carbon spectrum of a methylene group are calculated as a function of decoupler offset for a particular decoupler power. These results are found to be in good agreement with experimental work on the central methylene group in diethyl malonate. The experiment complements longitudinal relaxation measurements and provides a good way of selectively probing relaxation mechanisms.