The NOE factor in heteronuclear A{X-2}, A{XX '}, A{XY} spin-1/2 systems

The longitudinal magnetization modes pertaining to the spin-1/2 systems AX( 2), AXX', and AXY in isotropic medium are reviewed (where typically, A is a carbon-13 nucleus and X, X', or Y are protons weakly or tightly coupled, p ossibly equivalent or magnetically equivalent). It is established how these modes must be recast whenever X, X', or Y is strongly irradiated. This lea ds to the so-called nuclear Overhauser effect (NOE) factor (eta), which cor responds to the relative enhancement of the A longitudinal magnetization un der such irradiation conditions. It is shown that significant deviations fr om the conventional result (eta = (gamma(X)/gamma(A))(sigma(AX)/R-1(A)), wh ere gamma s are gyromagnetic ratios, and R-1(A) is the A longitudinal relax ation rate, and where sigma(AX) stands for the sum of all cross-relaxation rates between A and X, X', or Y) can occur only when the two nuclei X and X ' become magnetically equivalent (AX(2) spin system). This conclusion can b e extended to any spin system, indicating that some caution in interpreting the NOE factor is necessary only when magnetic equivalence among the proto n spin system is involved. (C) 1999 John Wiley & Sons, Inc.