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Magnetic inequivalence
In the context of nuclear magnetic resonance (NMR), the term magnetic inequivalence refers to the distinction between magnetically active nuclear spins by their NMR signals, owing to a difference in either chemical shift (''magnetic inequivalence by the chemical shift criterion'') or spin-spin coupling (J-coupling) (''magnetic inequivalence by the coupling criterion''). Since chemically inequivalent spins (''i.e.'' nuclei not related by symmetry) are expected to also be magnetically distinct (barring accidental overlap of signals), and since an observed difference in chemical shift makes their inequivalence clear, the term magnetic inequivalence most commonly refers solely to the latter type, ''i.e.'' to situations of chemically equivalent spins differing in their coupling relationships.
This situation can arise in a number of ways and can give rise to complexities in the corresponding NMR signals (beyond what a first-order analysis would handle) that range from the unnoticeable to the dramatic.
==Occurrence==
Two (or more) chemically equivalent (symmetry-related) spins will have the same chemical shift, but those that have a different coupling relationship to the same coupling partner are magnetically inequivalent by the coupling criterion. This occurs in molecules bearing two (or more) chemically distinct groups of symmetry-related nuclei, with just one element of symmetry relating them.〔If there were no symmetry relation among them, the nuclei would all be chemically inequivalent and therefore automatically magnetically inequivalent by the chemical shift criterion, showing as many signals as there are nuclei and with no additional complexity to the signal splitting. If there is more than one element of symmetry among them, the nuclei would all be chemically equivalent and therefore automatically magnetically equivalent by both the chemical shift and coupling criteria. A molecule may, of course, possess more than one set of magnetically inequivalent nuclei, related or not by additional symmetry elements that do not affect each.〕 Most commonly, two chemically inequivalent pairs of hydrogen nuclei (protons) are involved, although other magnetically active nuclei will also show this phenomenon, and the spin system is often labelled an AA′BB′ system. Additional coupling partners may also be present, but it is the two A/A′ and B/B′ signals (at different chemical shifts) that are said to show magnetic inequivalence between the symmetry-related A and A′ (or B and B′) pairs at the same chemical shift. If the chemical shift difference (νA−νB) is large compared to the largest coupling constant, the spin system may be designated AA′XX′.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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