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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
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UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions. The light-absorbing part of the molecule is called the chromophore, and the substituents directly attached to the chromophore are called auxochromes. A strong correlation exists between the absorption maxima, λmax, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λmax for a given...
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The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.
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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.
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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.
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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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4a,4b-Dihydrophenanthrene → cis-stilbene photoconversion: TD-DFT/DFT study.

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Dihydroartemisinin (DHP) to carbocation species (CS) photoconversion reveals why the C4a-C4b bond doesn't reform. Increased Coulomb repulsion and insufficient electron bonding prevent bond recovery during CS relaxation.

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Area of Science:

  • Computational chemistry
  • Photochemistry
  • Organic reaction mechanisms

Background:

  • First-time analysis of DHP → CS photoconversion focusing on electron density redistribution.
  • Investigates the non-recovery of the C4a-C4b bond after CS relaxation.
  • Examines structural changes in the linker connecting benzene rings during CS relaxation.

Purpose of the Study:

  • To elucidate the mechanism behind the non-recovery of the C4a-C4b bond in DHP → CS photoconversion.
  • To analyze electron density redistribution and its role in bond dynamics.
  • To understand the structural and electronic factors influencing the C4a-C4b bond stability.

Main Methods:

  • Utilized Gaussian16 software with B3LYP/6-311+G(d,p)/IEFPCM level of theory.
  • Employed Natural Population Analysis (NPA) for quantitative electron density analysis.
  • Used cyclohexane as an implicit solvent and Gaussview6 for visualization.

Main Results:

  • Coulomb repulsion energy between C4a-C4b atoms increases significantly during CS relaxation.
  • Bonding by a single electron at the Lowest Unoccupied Molecular Orbital (LUMO) is insufficient to restore the C4a-C4b bond.
  • The C4a-C4b bond distance increases from 3.00 Å to 3.28 Å upon CS relaxation.
  • Calculated IR spectra show very low intensity for C4a-C4b vibrations, indicating minimal contribution from thermal motion.

Conclusions:

  • The C4a-C4b bond cleavage in DHP is primarily driven by electronic factors during CS relaxation, not thermal effects.
  • Increased Coulombic repulsion and inadequate electron density at the LUMO prevent bond reformation.
  • Significant structural rearrangement of the linker moiety accompanies CS relaxation.