Nuclear overhauser effect in coupled-spin systems using selective perturbation

Abstract

Interest in the nuclear Overhauser effect (NOE) has increased enormously in recent years because of its wide application to the structure determination of molecules in the liquid state, particularly large molecules, such as biomolecules. Most NOE studies are carried out in spin systems that exhibit splittings because of indirect spin-spin (J) couplings, but the most common NMR analyses ignore I splitting. We focus attention on the presence of J coupling and particularly on selective perturbation of a transition of coupled spins. Both steady-state and transient NOE have been calculated in closed form. There is a large multiplet effect that results from the direct pumping effect that is observed in the intensities, careful measurement of which requires the use of small-angle pulses. The use of a 90° pulse obliterates the multiplet effect. As an extension, cross-correlation effects in the three-spin system and the three-spin effect in NOE spectra also are discussed. A brief analysis of spin diffusion in a multiple-spin system is also included.