Fluxional (or non-rigid) molecules are molecules that undergo dynamics such that some or all of their nuclei interchange, or tunnel, between symmetrically equivalent positions. Because virtually all molecules are fluxional at some time scale, the term fluxional depends on the method used to assess the dynamics. A molecule is considered to be fluxional if its spectroscopic signature exhibits line-splitting, or line-broadening beyond that dictated by the Heisenberg uncertainty principle. When such is not observed to occur, the molecule is said to be semi-rigid. Longuet-Higgins introduced the use of permutation-inversion groups for the symmetry classification of the states of fluxional (or non-rigid) molecules.

A well-studied fluxional molecule is the methanium ion (protonated methane) CH⁺
₅. In this unusual species, whose IR spectrum has been experimentally observed and theoretically studied, the barriers to proton exchange are extremely low. On the other hand, the parent molecule CH₄, methane, is semi-rigid.

Many organometallic compounds exhibit fluxionality. Fluxionality is, however, pervasive.

Temperature dependent changes in the NMR spectra result from dynamics associated with the fluxional molecules when those dynamics proceed at rates comparable to the frequency differences observed by NMR. The experiment is called DNMR and typically involves recording spectra at various temperatures. In the ideal case, low temperature spectra can be assigned to the "slow exchange limit", whereas spectra recorded at higher temperatures correspond to molecules at "fast exchange limit". Typically, high temperature spectra are simpler than those recorded at low temperatures, since at high temperatures, equivalent sites are averaged out. Prior to the advent of DNMR, kinetics of reactions were measured on non-equilibrium mixtures, monitoring the approach to equilibrium.

Many molecular processes exhibit fluxionality that can be probed on the NMR time scale. Beyond the examples highlighted below, other classic examples include the Cope rearrangement in bullvalene and the chair inversion in cyclohexane.

For processes that are too slow for traditional DNMR analysis, the technique spin saturation transfer (SST, also called EXSY for exchange spectroscopy) is applicable. This magnetization transfer technique gives rate information, provided that the rates exceed 1/T₁.

Although less common, some dynamics are also observable on the time-scale of IR spectroscopy. One example is electron transfer in a mixed-valence dimer of metal clusters. Application of the equation for coalescence of two signals separated by 10 cm⁻¹ gives the following result:

Clearly, processes that induce line-broadening on the IR time-scale must be much more rapid than the cases that exchange on the NMR time scale.