 Early spectra at 60 MHz were more prone to distortion than spectra from later machines typically operating at frequencies at 200 MHz or above.More subtle effects can occur if chemically equivalent spins (i.e., nuclei related by symmetry and so having the same NMR frequency) have different coupling relationships to external spins.Spins that are chemically equivalent but are not indistinguishable (based on their coupling relationships) are termed magnetically inequivalent.