Term: strain energy
https://doi.org/10.1351/goldbook.ST07108

Definition:
The excess energy due to steric strain of a molecular entity or transition state structure, i.e. distortions relative to a reference (real or hypothetical) 'strainless' structure with the standard bond lengths, bond angles and dihedral angles. The strain energy components involve the following destabilizing terms: non-bonded repulsions, bond-angle distortions, bond stretch or compression, rotation around or twisting of double bonds, and electrostatic strain. In general, the contributions of these components are inseparable and interdependent. A quantitative assessment of strain and strain energies can be made by taking the difference between the heat of formation of the substance under consideration and that of a hypothetical strain-free model. Several approaches to the assessment of strain energies have been developed based on the use of energies of isodesmic and homodesmotic reactions and on the so-called 'strainless increments', i.e. heats of formation of certain groups (CH3, CH2, CH, C etc).

Related Terms:
1) steric strain (http://doi.org/10.1351/goldbook.S06002).
2) transition state (http://doi.org/10.1351/goldbook.T06468).
3) strain (http://doi.org/10.1351/goldbook.S06037).
4) angle (http://doi.org/10.1351/goldbook.A00346).

Source: PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1964 (https://doi.org/10.1351/pac199971101919)

Citation: 'strain energy' 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.ST07108

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