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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

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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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If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
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Photochemical Electrocyclic Reactions: Stereochemistry01:26

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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.
Selection Rules: Photochemical Activation
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Thermal Electrocyclic Reactions: Stereochemistry01:17

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2.1K
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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In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...
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π Electron Effects on Chemical Shift: Overview01:27

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An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0,...
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Quantum Tunneling in Reactions Modulated by External Electric Fields: Reactivity and Selectivity.

Zhifeng Ma1, Zeyin Yan1, Xin Li1

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External electric fields (EEFs) significantly impact reactions involving quantum tunneling by lowering energy barriers. This study reveals EEFs can control reaction rates and selectivity, even switching reactions on/off under cryogenic conditions.

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Area of Science:

  • Quantum chemistry
  • Chemical kinetics
  • Theoretical chemistry

Background:

  • Quantum tunneling and external electric fields (EEFs) are known to influence chemical reactions.
  • The combined effect of EEFs on tunneling-involved reactions and their temperature dependence requires further investigation.

Purpose of the Study:

  • To investigate the synergetic effect of EEFs on reactions involving hydrogen- or carbon-tunneling.
  • To understand the temperature-dependence of EEF effects on these reactions.
  • To explore EEF modulation of tunneling-driven reactions and selectivity.

Main Methods:

  • Density Functional Theory (DFT)
  • Dual-level spin-scaled Complete Active Space Self-Consistent Field (DLPNO-CCSD(T1))
  • Variational Transition-State Theory (VTST)

Main Results:

  • Oriented EEFs reduce reaction barriers and widths via electrostatic stabilization, enhancing reaction rates.
  • EEFs decrease crossover temperatures and quantum tunneling contributions.
  • EEFs can modulate and switch tunneling-driven reactions, like hydroxycarbene 1,2-H migration, under cryogenic conditions.
  • EEF/tunneling synergy can control chemo- or site-selectivity in molecules with multiple reactive sites.

Conclusions:

  • EEFs play a crucial role in modulating quantum tunneling effects in chemical reactions.
  • EEFs offer a powerful tool for controlling reaction rates, temperature dependence, and selectivity.
  • This research opens new avenues for designing reactions with precise control under various conditions, including cryogenic environments.