Knowledge Bank

The effect of nuclear relaxation on the symmetry properties of NMR spectra is examined in terms of the properties of the relaxation matrix elements $Rijkl$. For “extreme narrowing” conditions, the symmetry relation $Rijkl=R$ obtains for (a) only one dominant relaxation mechanism, or (b) two dominant relaxation mechanisms with the same commutation properties with $\lambda$, a spin inversion operator. This symmetry relation does not obtain for two dominant relaxation mechanisms with the same order of spatial dependence and different commutation properties with $\lambda$. The effect of nuclear relaxation can therefore yield unsymmetrical first-order spectra; the application of this theorem to the determination of the absolute sign of spin-spin coupling constants is proposed.