hyperconjugation


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hyperconjugation

[‚hī·pər‚kän·jə′gā·shən]
(physical chemistry)
An arrangement of bonds in a molecule that is similar to conjugation in its formulation and manifestations, but the effects are weaker; it occurs when a CH2 or CH3 group (or in general, an AR2 or AR3 group where A may be any polyvalent atom and R any atom or radical) is adjacent to a multiple bond or to a group containing an atom with a lone π-electron, π-electron pair or quartet, or π-electron vacancy; it can be sacrificial (relatively weak) or isovalent (stronger).
References in periodicals archive ?
This led to a model called negative hyperconjugation that was based on p oxygen orbitals interacting with silicon sigma antibonding orbitals.
Hyperconjugation effects on RDX conformation were evaluated because of the lone pair of electrons on the atrazine ring nitrogens.
This would donate a charge through hyperconjugation and by the inductive effect, thus concentrating the charge density on nitrogen and sulfur atoms--thereby increasing their adsorption at the anodic sites of the metal.
It was found that the mesomeric and the hyperconjugation effects have a strong influence on the antibacterial activity.
This might come from the hyperconjugation effect which created by the methyl group of the 3-methylbenzene substituent Scheme 3.
Therefore, on the basis of the observed antibacterial activity values of these compounds, it can be concluded that (i) the compounds containing phenyl and H or Cl groups in combination with a pyridine ring, and phenyl or methyl or ethyl and H groups in combination with a thiophene ring as substituents on the [alpha], [beta]-unsaturated carbonyl compounds can act as very effective anti E-coli agents; (ii) the mesomeric and the hyperconjugation effects have a strong influence on the antibacterial activity.
This result is consistent with the concept that the stabilization of aliphatic and alicyclic carbanions is largely dependent on anionic hyperconjugation, involving the interaction between the electronic "lone pair" and antibonding C-H and C-C [[sigma].
This tends to suggest fluorine hyperconjugation with a transfer of electron density from the [sigma](C-C) orbitals to the [[sigma].
Then, Pophristic looked at the other known influence on ethane's twisting--a quantum mechanical effect known as hyperconjugation.
Hyperconjugation, not steric effects, makes staggered ethane stable, they conclude.
Discussing various electronic effects, particularly stereoelectronic effects, including hyperconjugation, negative hyperconjugation, the Bohlmann and anomeric effects, and how and why these cause changes in structures and properties of molecules
They have been using spin density calculations in free radicals to show hyperconjugation and resonance phenomena.