Both formal reasoning and numerical calculations of spectral is operative 5). This is ultimately due to the fact that for lineshape functions confirm that magnetic equivalence between a pair of like nuclei of spin-1 2 the J-coupling Hamiltonian nuclei of spin greater than 1 2 is generally broken in presence of J jk I j I k can be expressed in terms of permutation operator relaxation. This results in a dependence of the lineshape functions interchanging nuclei j and k (6). The J-coupling between on the values of J-couplings between the nuclei that would be nuclei j and k is totally irrelevant when interchange of j and equivalent in absence of relaxation. Effects of this sort may be of k is a valid symmetry operation (macroscopic symmetry practical importance for systems A n X, where the A nuclei are operation, see (5, 7)) on the molecular skeleton (3, 5). relaxed by quadrupolar interactions which are highly cross-corre-However, for nuclei of spin greater than 1 2 the J-coupling lated at different A sites. ᭧ 1997 Academic Press
Hamiltonian cannot be described by a permutation operator and, accordingly, the above property does not apply. In the present contribution we briefly address the problem of mag-The concept of magnetic equivalence, formulated in the netic equivalence between such nuclei in presence of relaxearly days of high-resolution NMR (1), has further given ation. We consider spin systems A n and A n X, where I A ú 1 2 rise to a common view that J-couplings between equivalent and I X Å 1 2 , undergoing relaxation in the extreme narrowing nuclei are totally irrelevant. However, in the framework of regime. the standard NMR theory (2) a rigorous discussion of magnetic equivalence comprises only the so called ''static 199