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 \(5\)–\(20\ \text{kcal/mol}\) (\(21\)–\(85\ \text{kJ}/\text{mol}\)).