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Related Concept Videos

SN1 Reaction: Stereochemistry02:15

SN1 Reaction: Stereochemistry

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This lesson provides an in-depth discussion of the stereochemical outcomes in an SN1 reaction.
In the first step of an SN1 reaction, the bond between the electrophilic carbon and the leaving group ionizes to generate the carbocation intermediate. The second step of the mechanism is the nucleophilic attack.
In the formed carbocation, the positively charged carbon is sp2 hybridized with a trigonal planar geometry. As all the three substituents lie on the same plane, a plane of symmetry for the...
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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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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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Extraction: Advanced Methods00:56

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Metal ions can be separated from one another by complexation with organic ligands–the chelating agent– to form uncharged chelates. Here, the chelating agent must contain hydrophobic groups and behave as a weak acid, losing a proton to bind with the metal. Since most organic ligands used in this process are insoluble or undergo oxidation in the aqueous phase, the chelating agent is initially added to the organic phase and extracted into the aqueous phase. The metal-ligand complex is...
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Ion Exchange01:17

Ion Exchange

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Ion exchange chromatography separates charged molecules from a solution by reversibly exchanging them with mobile, or 'active', ions associated with the oppositely charged stationary phase. This method can be used to separate ions, soften and deionize water, and purify solutions. The polymers comprising the ion-exchange column are high-molecular-weight and chemically stable polymers, crosslinked to be porous and essentially insoluble. They are also functionalized with either acidic or...
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Ion-Exchange Chromatography01:09

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Ion-exchange chromatography, or IEC, is a technique for separating ions based on their affinity for the stationary phase. The stationary phase is a cross-linked polymer resin with covalently attached ionic functional groups. The functional groups can be either positively charged (cation exchangers) or negatively charged (anion exchangers). A cation exchanger consists of a polymeric anion and active cations, while an anion exchanger is a polymeric cation with active anions. The choice of...
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Merging Ion Concentration Polarization between Juxtaposed Ion Exchange Membranes to Block the Propagation of the Polarization Zone
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Reversible charge separation followed by exciplex formation.

M V Petrova1, A I Burshtein

  • 1International Tomography Center, and Novosibirsk State University, Novosibirsk, 630090, Russia.

The Journal of Physical Chemistry. A
|February 14, 2009
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Summary

This study explores fluorescence quenching via reversible ionization and exciplex formation using integral encounter theory. Results differ between pulse and stationary fluorescence detection, challenging conventional free-energy gap laws.

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Area of Science:

  • Photochemistry
  • Chemical Kinetics
  • Electron Transfer Theory

Background:

  • Fluorescence quenching mechanisms are crucial for understanding photochemical processes.
  • Reversible bimolecular ionization and exciplex formation are complex pathways.
  • Spin dynamics play a significant role in radical-ion recombination.

Purpose of the Study:

  • To investigate fluorescence quenching through reversible ionization and exciplex formation using integral encounter theory (IET).
  • To analyze the influence of spin-conversion and radical-ion recombination on fluorescence quenching.
  • To compare results obtained from pulse excitation and stationary fluorescence detection.

Main Methods:

  • Application of integral encounter theory (IET) to model the quenching process.
  • Incorporation of an incoherent (rate) model for spin-conversion.
  • Analysis of ion recombination to excited triplet products.
  • Comparison with experimental data and the free-energy gap (FEG) law.

Main Results:

  • Calculations demonstrate distinct outcomes for pulse versus stationary fluorescence detection.
  • The study quantifies the free-energy dependence of forward and backward electron transfer properties.
  • Integral encounter theory successfully models the complex quenching mechanism.
  • Deviations from the conventional free-energy gap law are observed.

Conclusions:

  • The proposed mechanism involving reversible ionization, exciplex formation, and spin-conversion accurately describes fluorescence quenching.
  • Integral encounter theory provides a robust framework for studying complex electron transfer processes.
  • The distinction between pulse and stationary detection methods is critical for accurate interpretation.
  • The findings offer insights into the limitations of the conventional free-energy gap law in certain scenarios.