Title: negative hyper-conjugation
Long Title: IUPAC Gold Book - negative hyper-conjugation
DOI: 10.1351/goldbook.NT07081
Status: current

Definition
The effect of donation of electron density from filled $\ce{\uppi\!}$ or $\ce{n_{\uppi\!}\mbox{-}orbitals}$ to the symmetry adapted $\ce{\upsigma\!^{\ast}\mbox{-}orbital(s)}$ of $\ce{\upsigma\!^{\ast}\mbox{-}bond(s)}$ of a molecular entity resulting in building $\ce{\uppi\!\mbox{-}character}$ into bonds that nominally possess only $\ce{\upsigma\!\mbox{-}character}$. The consequences of the effect are, for example, in elongation of the $\ce{C–F}$ bond in the $\ce{\upbeta\!\mbox{-}fluoroethyl}$ anion and stabilization of the anti-periplanar conformation of fluoromethylamine. 

Related Terms
- anion: https://dev.goldbook.iupac.org/terms/view/A00358
- anti: https://dev.goldbook.iupac.org/terms/view/A00381
- conformation: https://dev.goldbook.iupac.org/terms/view/C01258
- electron density: https://dev.goldbook.iupac.org/terms/view/E01986
- periplanar: https://dev.goldbook.iupac.org/terms/view/T06406

Source
- PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1954 (https://doi.org/10.1351/pac199971101919)

Other Outputs
- html: https://dev.goldbook.iupac.org/terms/view/NT07081/html
- json: https://dev.goldbook.iupac.org/terms/view/NT07081/json
- xml: https://dev.goldbook.iupac.org/terms/view/NT07081/xml

Citation: Citation: 'negative hyper-conjugation' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.NT07081
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Accessed: 2025-12-13T23:22:48+00:00