Strain is present in a molecular entity or transition structure if the energy is enhanced because of unfavourable bond lengths, bond angles or dihedral angles ('torsional strain') relative to a standard. It is quantitatively defined as the standard enthalpy of a structure relative to a strainless structure (real or hypothetical) made up from the same atoms with the same types of bonding. (The enthalpy of formation of cyclopropane is 53.6 kJ mol-1, whereas the enthalpy of formation based on three 'normal' methylene groups, from acyclic models, is -62 kJ mol-1. On this basis cyclopropane is destabilized by ca.
115 kJ mol-1 of strain energy.)
See: molecular entity, molecular mechanics calculation, transition structure
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1168 (https://doi.org/10.1351/pac199466051077)