Strain is present in a @[email protected] or @[email protected] if the energy is enhanced because of unfavourable bond lengths, bond angles or dihedral angles ('@[email protected]') relative to a standard. It is quantitatively defined as the standard @[email protected] of a structure relative to a strainless structure (real or hypothetical) made up from the same atoms with the same types of bonding. (The @[email protected] of formation of cyclopropane is \(53.6\ \text{kJ mol}^{-1}\), whereas the @[email protected] of formation based on three 'normal' @[email protected] groups, from acyclic models, is \(-62\ \text{kJ mol}^{-1}\). On this basis cyclopropane is destabilized by ca. \(115\ \text{kJ mol}^{-1}\) of @[email protected])