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Related Concept Videos

Reaction Mechanisms03:06

Reaction Mechanisms

Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates the reacting species and the product species, but it reveals no details about how the reaction occurs at the molecular level. The reaction mechanism (or reaction path) provides details regarding the precise, step-by-step process by which a reaction occurs.
For instance, the decomposition of ozone appears to follow a mechanism with two steps:
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.
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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
Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
Membrane Asymmetry Regulating Transporters01:19

Membrane Asymmetry Regulating Transporters

Enzymes like flippase, floppase, and scramblase transfer phospholipids from one layer to another in the membrane, thereby affecting membrane asymmetry.
Flippase
Eukaryotic flippases are type-IV P-type ATPases or P4-ATPases belonging to P-type ATPase family proteins that are membrane-bound pumps involved in the ATP-mediated transport of ions and molecules across the membrane. Flippases flip specific phospholipids from the outer to the inner leaflet of a membrane. All P4-ATPases have one...
Reversible or Opposing Reactions01:26

Reversible or Opposing Reactions

Reversible or opposing reactions play a crucial role in understanding the dynamic nature of chemical processes. While kinetics focuses on how reactions proceed, thermodynamics emphasizes that most reactions do not reach completion. Instead, a reverse reaction starts occurring over time, and when its rate equals that of the forward reaction, a dynamic equilibrium is established.For example, consider a simple chemical process where A forms B reversibly. The rate constants for the forward and...

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Synthesis of Antiviral Tetrahydrocarbazole Derivatives by Photochemical and Acid-catalyzed C-H Functionalization via Intermediate Peroxides (CHIPS)
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Barrierless electron transfer bond fragmentation reactions.

Edward D Lorance1, Wolfgang H Kramer, Ian R Gould

  • 1Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, USA.

Journal of the American Chemical Society
|October 28, 2004
PubMed
Summary

Ultrafast N-O bond fragmentation in N-methoxypyridyl radicals occurs via barrierless electron transfer. Molecular design principles for rapid reactivity were proposed and validated by femtosecond kinetic measurements.

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Area of Science:

  • Chemical kinetics
  • Photochemistry
  • Organic chemistry

Background:

  • N-methoxypyridyl radicals are formed by one-electron reduction of N-methoxypyridiniums.
  • Investigating ultrafast N-O bond fragmentation is crucial for understanding electron-transfer reactions.

Purpose of the Study:

  • To investigate the ultrafast N-O bond fragmentation in N-methoxypyridyl radicals.
  • To develop a model for predicting barrierless electron-transfer-initiated reactions.
  • To identify molecular structural features that promote ultrafast reactivity.

Main Methods:

  • Theoretical modeling of potential energy surfaces.
  • Femtosecond kinetic measurements.
  • Analysis of electronic and geometric factors influencing reaction dynamics.

Main Results:

  • A model was developed to describe the potential energy surface of the N-O bond fragmentation.
  • Molecular structural features for ultrafast reactivity were proposed.
  • Femtosecond data confirmed a kinetically barrierless reaction for p-methoxy-N-methoxypyridyl radical.

Conclusions:

  • The N-O bond fragmentation in these radicals can be an essentially barrierless process.
  • The developed model provides insights into designing molecules for ultrafast chemical transformations.
  • Femtosecond kinetics offer a powerful tool for characterizing rapid bond-breaking events.