In the formalism that separates bonds into σ and π types, hyperconjugation is the interaction of σ-bonds (e.g. C–H, C–C, etc.) with a π @NT07562@. This interaction is customarily illustrated by @C01309@, e.g. for toluene (below), sometimes said to be an example of 'heterovalent' or 'sacrificial hyperconjugation', so named because the @C01309@ contains one two-electron bond less than the normal @L03513@ for toluene:
At present, there is no evidence for sacrificial hyperconjugation in neutral @H02889@. The concept of hyperconjugation is also applied to @C00812@ ions and @R05066@, where the interaction is now between σ-bonds and an unfilled or partially filled π- or p-orbital. A @C01309@ illustrating this for the tert-butyl @C00907@ is:
This latter example is sometimes called an example of 'isovalent hyper-@C01267@' (the @C01309@ containing the same number of two-electron bonds as the normal Lewis formula). Both structures shown on the right hand side are also examples of 'double bond-@N04173@'. The interaction between filled π- or p- orbitals and adjacent antibonding σ* orbitals is referred to as 'negative hyperconjugation', as for example in the fluoroethyl @A00358@:
See also:
sigma, pi
n-σ*, delocalization
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1123 [Terms] [Paper]