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Related Concept Videos

π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

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, resulting in...
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.
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

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 annulenes. In...
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.
E1 Reaction: Kinetics and Mechanism02:46

E1 Reaction: Kinetics and Mechanism

Here, in contrast to the E2 reaction mechanism, we delve into the aspects of the E1 reaction mechanism, which has two steps: rate-limiting loss of the leaving group and abstraction of the beta hydrogen by a weak base. Typically, the experimental proof for the E1 mechanism is via kinetic studies or isotope studies. While the former demonstrates the first-order kinetics—the dependence of the reaction solely on substrate concentration—the latter proves the abstraction of hydrogen only in the...
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

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Spin Saturation Transfer Difference NMR (SSTD NMR): A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes
11:44

Spin Saturation Transfer Difference NMR (SSTD NMR): A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes

Published on: November 12, 2016

Electronic structural effects in self-exchange reactions.

John C Goeltz1, Eric E Benson, Clifford P Kubiak

  • 1Department of Chemistry and Biochemistry, University of California San Diego, 9500 Gilman Drive, M/C 0358, La Jolla, California 92093-0358, USA.

The Journal of Physical Chemistry. B
|May 14, 2010
PubMed
Summary
This summary is machine-generated.

Electron transfer rates in ruthenium clusters depend on ligand properties. Electron-withdrawing ligands and larger aromatic systems accelerate electron self-exchange reactions by enhancing orbital overlap and reducing reorganization barriers.

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Area of Science:

  • Inorganic Chemistry
  • Electron Transfer Reactions
  • Coordination Chemistry

Background:

  • Ruthenium clusters are crucial in catalysis and materials science.
  • Understanding electron self-exchange kinetics is vital for designing functional molecules.
  • Ligand substitution significantly influences the electronic and structural properties of metal clusters.

Purpose of the Study:

  • To investigate the rate constants of electron self-exchange reactions in trinuclear ruthenium clusters.
  • To determine the influence of pyridine ligand substitution on electron transfer dynamics.
  • To elucidate the factors governing reorganization energy and orbital overlap in these systems.

Main Methods:

  • Synthesis and characterization of ruthenium clusters with varying pyridine ligands.
  • Electrochemical studies to determine rate constants for electron self-exchange.
  • Spectroscopic and structural analysis to probe electronic structure and molecular geometry.

Main Results:

  • For [Ru(3)O(OAc)(6)(CO)(L)(2)](0/-), electron-withdrawing pyridine ligands and larger aromatic systems increase electron transfer rates by enhancing donor-acceptor orbital overlap and reducing reorganization barriers (λ ≈ 10,000 cm(-1)).
  • For [Ru(3)O(OAc)(6)(L)(3)](+/0), ligand electron-withdrawing ability does not correlate with rate constants, suggesting peripheral charge density is not a key factor.
  • Reorganization energies for the +/0 couple (λ(tot) ≈ 3320–5120 cm(-1)) are significantly lower than for the 0/- couple, yet rate constants are similar, attributed to orbital overlap in the 0/- pair.

Conclusions:

  • Orbital overlap, rather than solely reorganization energy, dictates electron transfer rates in certain ruthenium cluster systems.
  • Ligand design, specifically through electronic and steric effects of pyridine substituents, offers a pathway to tune electron transfer kinetics.
  • The findings provide insights into the fundamental mechanisms of electron transfer in polynuclear metal complexes.