Term: strain
https://doi.org/10.1351/goldbook.S06037

Definition:
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.)

Related Terms:
1) molecular entity (http://doi.org/10.1351/goldbook.M03986).
2) transition structure (http://doi.org/10.1351/goldbook.T06471).
3) torsional strain (http://doi.org/10.1351/goldbook.E01886).
4) enthalpy (http://doi.org/10.1351/goldbook.E02141).
5) methylene (http://doi.org/10.1351/goldbook.M03881).
6) strain energy (http://doi.org/10.1351/goldbook.ST07108).
7) molecular mechanics calculation (http://doi.org/10.1351/goldbook.M03993).

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1168 (https://doi.org/10.1351/pac199466051077)

Citation: 'strain' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.S06037

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