Whereas a four-spin AA′BB′ (or AA′XX′) system may have the requisite symmetry and coupling properties, its signals may show more or less complexity and, as with other coupling phenomena, the appearance of a signal from magnetically inequivalent nuclei will also depend on the instrumental field strength. A large number of such systems show less complexity, with fewer lines than is possible, particularly when the instrumental resolution is low, whence nearby peaks appear to coalesce, when J ≈ J, when J ≈ −J, when J ≈ J or when J ≈ 0. The apparent complexity is also diminished in AA′XX′ systems when ν−ν >> J. This kind of simplication is enhanced as the instrumental magnetic field is increased, since the field-independent differences between coupling constants or between a coupling constant and zero appear proportionately smaller on the δ (ppm) scale, and since the field-dependent quantity (ν−ν)/J is magnified.
In molecules of uncertain or otherwise unproven structure, the definitive appearance of complexity in a pair of signals, beyond what can be explained by first-order analysis of HC-CH pairs or HC-CH fragments, can be taken to signify the presence of magnetic inequivalence and, therefore, of an element of symmetry aggregating them. Thus, the appearance of such complexity in the aromatic region of the H-NMR spectrum of the bis-(acetylacetonato)ruthenium complex of o-benzoquinonediimine served to prove its C-symmetrical nature.