method of isotopic perturbation
NMR shift difference measurement of the isotope effect
on a fast (degenerate) equilibrium between two species which are equivalent except
for isotopic substitution. This can be used to distinguish a rapidly equilibrating
mixture with time-averaged symmetry from a single structure with higher symmetry.
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1132