Hyperconjugation

Abstract

This review outlines the ubiquitous nature of hyperconjugative interactions and their role in the structure and reactivity of organic molecules. After defining the common hyperconjugative patterns, we discuss the main factors controlling the magnitude of hyperconjugative effects, including orbital symmetry, energy gap, electronegativity, and polarizibility. The danger of underestimating the magnitude of hyperconjugative interactions are illustrated by a number of spectroscopic, conformational, and structural effects. Through the use of advanced computational techniques, the true role of hyperconjugative effects, as it pertains to their influence on stereoelectronics, conformational equilibria, and reactivities relative to other electronic effects, continue to be uncovered.

Intuitive Understanding of σ Delocalization in Loose and σ Localization in Tight Helical Conformations of an Oligosilane Chain

“…In at raditional usage of the terms [25] (which is not accepted generally), [30,31] the system is s-conjugated if the deviation from strict bond localization is due to geminal interactions (those between hybrid orbitals located on the same atom), shyperconjugated if it is due to vicinal interactions (those between hybrid orbitals located on neighboring atoms with neither pointing at the other atom, often referred to as "throughbond coupled"), and s-homoconjugatedi fi ti sd ue to interactions between hybrid orbitals located on non-neighboring atoms (sometimes called "through-space coupled"). Interactions between hybrids on neighboring atoms chosen such that one points at the other atom, and the other does not, are considered lessi mportanta nd do not seem to have an ame.I n real systems, the various types of conjugation act simultaneously and coherentlyt od elocalize the s electrons, but one of them often predominates.…”

Anaerobic Activation of p -Cymene in Denitrifying Betaproteobacteria: Methyl Group Hydroxylation versus Addition to Fumarate

“…Values for formation of a carbenium ion and a hydride have been reported to decrease in the following order: toluene (992 kJ mol Ϫ1 [72] [73]). The lower values for p-xylene and ethylbenzene than for toluene can be ascribed to hyperconjugative stabilization of the carbenium ion center by the p-and ␣-methyl groups, respectively (74,75). Values for 4-ethyltoluene and p-cymene are not available in the literature, but the known similarity of the hyperconjugative effect of methyl (Me), ethyl (Et), and isopropyl ( i Pr) groups (albeit i Pr Ͻ Et Ͻ Me) (74,76) suggests comparable stabilization.…”

Unusual hydrogen bond patterns contributing to supramolecular assembly: conformational study, Hirshfeld surface analysis and density functional calculations of a new steroid derivative

“…NBO analysis of π CC -π* CO and π CO -π* CC interactions agrees well with the general trend that vicinal hyperconjugation produced among CC and CO is increased for antiperiplanar conformations, as is the case of the 0°conformer. 33 The other stabilizing contribution observed in the global minimum energy structure is the formation of an intramolecular C-H⋯Cl interaction, which in addition to the stabilization given by the π CC -π* CO and π CO -π* CC interactions could be the cause of its highest stability. As expected, the n (π) Cl -σ* C18-H interaction is observed only in the global minimum energy geometry (0°).…”