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Related Concept Videos

Stability of Substituted Cyclohexanes02:30

Stability of Substituted Cyclohexanes

This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in...
Disubstituted Cyclohexanes: cis-trans Isomerism02:37

Disubstituted Cyclohexanes: cis-trans Isomerism

Depending upon the different spatial orientation of the substituents, the disubstituted cycloalkanes exhibit two types of stereoisomers. The cis isomers have the substituents on the same side of the ring, whereas the trans isomers have the substituents on the opposite sides. These stereoisomers exhibit different physical properties and cannot be interconverted without breaking the carbon-carbon bonds.
In cyclohexane, the substituents can occupy different positions generating distinct isomers.
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group with both...
Chair Conformation of Cyclohexane02:02

Chair Conformation of Cyclohexane

The chair conformation is the most stable form of cyclohexane due to the absence of angle and torsional strain. The absence of angle strain is a result of cyclohexane’s bond angle being very close to the ideal tetrahedral bond angle of 109.5° in its chair conformer. Similarly, the torsional strain is also absent owing to the perfectly staggered arrangement of bonds.
The hydrogen atoms linked to carbons are arranged in two different axial and equatorial orientations to achieve this staggered...
Stereoisomerism of Cyclic Compounds02:33

Stereoisomerism of Cyclic Compounds

In this lesson, we delve into the role of ring conformation and its stability, which determines the spatial arrangement and, consequently, the molecular symmetry and stereoisomerism of cyclic compounds. 1,2-Dimethylcyclohexane is used as a case study to evaluate the possible number of stereoisomers. Here, given the multiple (n = 2) chiral centers, there are 2n = 4 possible configurations that lack a plane of symmetry, as the ring skeleton exists in a non-planar chair conformation. In addition,...
Conformations of Cyclohexane02:11

Conformations of Cyclohexane

Cyclohexane does not exist in a planar form due to the high angle and torsional strain it would experience in the planar structure. Instead, it adopts non-planar chair and boat conformations.
The chair form is the most stable and derives its name from its resemblance to the “easy chair.” In the chair conformation, two carbon atoms are arranged out-of-plane — one above and one below, minimizing the torsional strain. In the chair form, the bond angle is very close to the ideal tetrahedral value,...

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Preparation of 6-aminocyclohepta-2,4-dien-1-one Derivatives via Tricarbonyl(tropone)iron
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Substituent effects on dynamics at conical intersections: cycloheptatrienes.

Oliver Schalk1, Andrey E Boguslavskiy, Michael S Schuurman

  • 1AlbaNova University Centre, Stockholm University , Roslagstullsbacken 21, 106 91 Stockholm, Sweden.

The Journal of Physical Chemistry. A
|September 10, 2013
PubMed
Summary

Methyl substitution in cycloheptatriene (CHT) reveals early excited-state dynamics. Vibrational motion at conical intersections dictates nonadiabatic transitions, occurring before full planarization in unsaturated hydrocarbons.

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Published on: February 7, 2019

Area of Science:

  • Physical Chemistry
  • Chemical Dynamics
  • Spectroscopy

Background:

  • Understanding excited-state dynamics in unsaturated hydrocarbons is crucial for photochemistry.
  • Conical intersections play a key role in ultrafast nonadiabatic transitions.
  • Vibrational dynamics influence the pathways and efficiency of these transitions.

Purpose of the Study:

  • To investigate the excited-state nonadiabatic dynamics of cycloheptatriene (CHT).
  • To explore the role of vibrational dynamics at conical intersections.
  • To compare CHT dynamics with substituted and deuterated analogs to understand substituent effects.

Main Methods:

  • Time-resolved photoelectron spectroscopy (TRPES).
  • Photoelectron anisotropy measurements.
  • Selective methyl substitution on CHT (7-methyl CHT, 7-ethyl CHT) and perdeuterated CHT.

Main Results:

  • Observed early intersection between the bright 2A" and dark 2A' states near the Franck-Condon region upon ππ*-excitation.
  • Evidence of wavepacket bifurcation, evolving on both states along a planarization coordinate.
  • Majority of the wavepacket undergoes nonadiabatic transition via conical intersections within 100 fs.

Conclusions:

  • Substitutions at the 7-position of CHT did not significantly alter the initial nonadiabatic step, suggesting it occurs before planarization.
  • Nonadiabatic transitions likely happen away from local minima on the potential energy surfaces.
  • Vibrational dynamics at conical intersections are critical for excited-state evolution in polyenes.