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,
Cetc).
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