fluxional molecules, molecules that exhibit rapid intramolecular rearrangements among their component atoms. As in structural isomerism and tautomerism (see isomer; tautomer), fluxional compounds maintain the same number of component atoms. At equilibrium, fluxional molecules may manifest many different isomers and fluctuate rapidly among them (for example, bullvalene, which has over 1,200,000 equivalent structures). The rearrangement process is usually detected by nuclear magnetic resonance (NMR) spectroscopy, which can measure rearrangement rates occurring between 10^-1 and 10³ Hz.