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Related Concept Videos

Carbocations02:10

Carbocations

Carbocations are one of the reaction intermediates formed during several nucleophilic substitutions or elimination reactions. A carbocation is an electron-deficient species with the central carbon atom having six electrons and three bonded atoms. The central carbon in a carbocation is sp2 hybridized with trigonal planar geometry. It has an empty p orbital perpendicular to the plane of the structure that can accept electrons. Thus, carbocations act as strong electrophiles and may react with any...
Hydrogen Bonds01:04

Hydrogen Bonds

A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another polar molecule, such as water (H2O), hydrogen fluoride (HF), or ammonia (NH3). The huge electronegativity difference between the H atom (2.1) and the atom to which it is bonded (4.0 for an F atom, 3.5 for an O atom, or 3.0 for an N atom), combined with the very small size of an H atom...
Hydrogen Bonds00:26

Hydrogen Bonds

Hydrogen bonds are weak attractions between atoms that have formed other chemical bonds. One of these atoms is electronegative, like oxygen, and has a partial negative charge. The other is a hydrogen atom that has bonded with another electronegative atom and has a partial positive charge.
Hydrogen Bonds Control the World!
Because hydrogen has very weak electronegativity when it binds with a strongly electronegative atom, such as oxygen or nitrogen, electrons in the bond are unequally shared.
π Molecular Orbitals of 1,3-Butadiene01:24

π Molecular Orbitals of 1,3-Butadiene

Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of conjugated systems can be understood from their π molecular orbitals.
The simplest conjugated diene is 1,3-butadiene: a four-carbon system where each carbon is sp2-hybridized and has an unhybridized p orbital that contains an unpaired electron. According to molecular orbital theory, atomic orbitals combine to form molecular orbitals such that the number...
π Molecular Orbitals of the Allyl Cation and Anion01:18

π Molecular Orbitals of the Allyl Cation and Anion

An allyl group is a three-carbon conjugated system where the sp³-hybridized allylic carbon is bonded to a CH=CH2 group via a single bond. Allyl anions can be obtained by treating propene with a strong base that can deprotonate methyl groups. Allyl cations are formed as intermediates during substitution reactions involving allylic halides. In both cases, the hybridization of the allylic carbon changes from sp3 to sp2, giving rise to a carbon chain with three sp2-hybridized carbons, each with an...
Hybridization of Atomic Orbitals II03:35

Hybridization of Atomic Orbitals II

sp3d and sp3d 2 Hybridization

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Updated: May 11, 2026

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
10:44

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

Published on: April 19, 2019

Hydrogen bonding versus hyperconjugation in condensed-phase carbocations.

Christopher A Reed1, Evgenii S Stoyanov, Fook S Tham

  • 1Department of Chemistry, University of California, Riverside, California 92651, USA. chris.reed@ucr.edu

Organic & Biomolecular Chemistry
|May 2, 2013
PubMed
Summary

Hyperconjugation is the textbook explanation for carbocation stability. However, new research suggests that C-H···anion hydrogen bonding, not hyperconjugation, is primarily responsible for low-frequency IR bands in t-butyl cations.

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Construction and Systematical Symmetric Studies of a Series of Supramolecular Clusters with Binary or Ternary Ammonium Triphenylacetates
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From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding
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From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding

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Published on: March 24, 2018

Area of Science:

  • Physical Chemistry
  • Spectroscopy
  • Computational Chemistry

Background:

  • Tertiary carbocations are traditionally explained by hyperconjugative stabilization.
  • Low-frequency infrared (IR) spectra (νCH bands) have been cited as evidence for hyperconjugation.
  • The role of hyperconjugation versus other interactions in condensed phases remains unclear.

Purpose of the Study:

  • To investigate the relative importance of hyperconjugation and hydrogen bonding in stabilizing carbocations.
  • To re-evaluate the interpretation of low-frequency νCH bands in IR spectra of t-butyl cation.
  • To explore the influence of counterions on carbocation stability and spectral features.

Main Methods:

  • Infrared (IR) spectroscopy of t-butyl cation in superacid media.
  • Comparison of gas-phase and condensed-phase IR spectra.
  • Analysis of secondary alkyl carborane spectra (e.g., i-Pr(CHB11Cl11)).

Main Results:

  • The congruence of gas-phase and superacid νCH(max) for t-butyl cation is coincidental.
  • νCH(max) in condensed phases varies significantly with the counterion, indicating C-H···anion hydrogen bonding.
  • Specific C-H bonds in t-butyl cation were assigned to low-frequency νCH bands, predominantly attributed to hydrogen bonding.
  • Similar low-frequency νCH bands in alkyl carboranes further support the dominance of hydrogen bonding.

Conclusions:

  • Low-frequency νCH bands in t-butyl cation IR spectra are mainly due to C-H···anion hydrogen bonding, not hyperconjugation.
  • Hydrogen bonding plays a more significant role than hyperconjugation in stabilizing carbocations in condensed phases.
  • Re-evaluation of spectral evidence is needed to understand carbocation stability.