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Related Concept Videos

Structure of Conjugated Dienes01:16

Structure of Conjugated Dienes

Introduction
Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like 1,3-butadiene, the unhybridized 2p orbital on each carbon overlaps continuously, allowing the π electrons to be delocalized across the entire molecule. In contrast, this type of overlap does not occur in cumulated and isolated dienes, such as 2,3-pentadiene and 1,4-pentadiene, respectively. Instead, the π electrons remain localized between the double...
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...
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,...
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.

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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Conical intersection seams in polyenes derived from their chemical composition.

Artur Nenov1, Regina de Vivie-Riedle

  • 1Department Chemie, Ludwig-Maximilians-Univerisität, München Butenandtstr. 11, 81377 München, Germany.

The Journal of Chemical Physics
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Summary

Predicting conical intersection seams is crucial for understanding ultrafast reactions in photochemistry. This study presents a systematic method to locate these seams in complex molecules, aiding in reaction outcome prediction.

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Area of Science:

  • Photochemistry and Photobiology
  • Theoretical Chemistry
  • Computational Chemistry

Background:

  • Conical intersections are critical for ultrafast non-radiative transitions in photochemical and photobiological reactions.
  • Locating conical intersection seams in complex molecules is challenging.
  • These seams dictate reaction pathways and energy dissipation.

Purpose of the Study:

  • To develop a systematic approach for predicting conical intersection seams in multifunctionalized polyenes.
  • To investigate the sensitivity of these seams to substituent effects.
  • To identify seams relevant to desired photoreactions and competing loss channels.

Main Methods:

  • Utilizing the extended two-electron two-orbital method.
  • Extracting low-lying regions for non-radiative transitions.
  • Employing linear interpolation based on support points for initial seam estimation.
  • Performing quantum chemical optimization for precise seam localization.

Main Results:

  • A systematic method for predicting conical intersection seams in multifunctionalized polyenes has been established.
  • The influence of substituents on seam location and photoreaction pathways is quantified.
  • The method successfully identified relevant seams for the electrocyclic isomerization of trifluoromethyl-pyrrolylfulgide.

Conclusions:

  • The presented method provides an efficient strategy for locating conical intersection seams in complex molecules.
  • This approach aids in understanding and controlling ultrafast photochemical reactions.
  • It offers insights into both productive reaction channels and unwanted decay pathways.