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Related Concept Videos

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...
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
Thermal Sigmatropic Reactions: Overview01:16

Thermal Sigmatropic Reactions: Overview

Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in 1,5-hexadiene, referred to as...
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...

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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
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Perturbative triples corrections in state-specific multireference coupled cluster theory.

Francesco A Evangelista1, Eric Prochnow, Jürgen Gauss

  • 1Institut für Physikalische Chemie, Universität Mainz, D-55099 Mainz, Germany. frank@ccqc.uga.edu

The Journal of Chemical Physics
|February 23, 2010
PubMed
Summary
This summary is machine-generated.

We developed a new computational method, Mk-MRCCSD(T), to improve the accuracy of multireference coupled cluster calculations. This approach enhances the study of complex chemical systems requiring multireference wave functions.

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Construction and Systematical Symmetric Studies of a Series of Supramolecular Clusters with Binary or Ternary Ammonium Triphenylacetates

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Multireference coupled cluster (MRCC) methods are essential for describing systems with strong electron correlation.
  • The state-specific Mukherjee multireference coupled cluster with singles and doubles (Mk-MRCCSD) approach provides a robust framework.
  • Accurate treatment of electron correlation, especially triples, is crucial for quantitative predictions.

Purpose of the Study:

  • To formulate and implement a perturbative triples correction for the Mk-MRCCSD method.
  • To develop the Mk-MRCCSD(T) and Lambda-Mk-MRCCSD(T) approaches for improved accuracy.
  • To assess the performance of the new method for challenging chemical problems.

Main Methods:

  • Derivation of the Mk-MRCCSD(T) energy correction using a constrained search and perturbative expansion.
  • Development of the Lambda-Mk-MRCCSD(T) approach with corrections to effective Hamiltonian matrix elements.
  • Application to model systems like BeH(2) and F(2) potential energy curves, and ozone geometry and frequencies.

Main Results:

  • The Mk-MRCCSD(T) method was successfully formulated and implemented.
  • The Lambda-Mk-MRCCSD(T) approach includes unique corrections to the effective Hamiltonian.
  • The method demonstrated its capability in studying the potential energy curves and molecular properties.

Conclusions:

  • The developed Mk-MRCCSD(T) method offers a significant improvement for multireference coupled cluster calculations.
  • It provides a valuable tool for investigating complex chemical systems, such as diradicals, that necessitate multireference wave functions.
  • The approach enhances the accuracy of theoretical predictions in quantum chemistry.