Molecules that undergo rapid intramolecular rearrangements among equivalent structures in which the component atoms are interchanged. The rearrangement process is usually detected by nuclear magnetic resonance (NMR) spectroscopy, which can measure rearrangement rates from 0.5 to 10,000 s^-1. With sufficiently rapid rates, a single resonance is observed in the NMR spectrum for a molecule that might be expected to have several nonequivalent nuclei on the basis of its instantaneous structure.