## stereochemical non-rigidity

https://doi.org/10.1351/goldbook.ST07107
The capability of a molecule to undergo fast and reversible @I03130@ @I03295@, the energy barrier to which is lower than that allowing for the preparative isolation of the individual isomers at room temperature. It is conventional to assign to the sterochemically non-rigid systems those compounds whose molecules rearrange rapidly enough to influence NMR line shapes at temperatures within the practical range (from $$-100\ ^{\,\unicode{x26ac}}\text{C}$$ to $$200\ ^{\,\unicode{x26ac}}\text{C}$$) of experimentation. The energy barriers to thus defined rearrangements fall into the range of $$5$$–$$20\ \text{kcal/mol}$$ ($$21$$–$$85\ \text{kJ}/\text{mol}$$).
Source:
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1964 [Terms] [Paper]