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Related Concept Videos

Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
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.
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved in...
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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Merging multiconfigurational wavefunctions and correlation functionals to predict magnetic coupling constants.

Angel J Pérez-Jiménez1, José M Pérez-Jordá, Ibério de P R Moreira

  • 1Departamento de Química-Física, Universidad de Alicante, E-03080 Alicante, Spain. aj.perez@ua.us

Journal of Computational Chemistry
|May 9, 2007
PubMed
Summary

This study evaluates methods combining multiconfigurational wavefunctions and correlation functionals for magnetic coupling constants. A new approach using natural orbitals and occupation numbers yielded the best performance across various materials and molecules.

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Area of Science:

  • Quantum chemistry
  • Computational materials science
  • Solid-state physics

Background:

  • Accurate calculation of magnetic coupling constants is crucial for understanding magnetic materials and designing molecular magnets.
  • Combining multiconfigurational wavefunctions with correlation functionals offers a promising route to improve accuracy beyond traditional methods.
  • Evaluating diverse computational approaches is essential for identifying the most reliable techniques.

Purpose of the Study:

  • To assess the performance of various methods that integrate multiconfigurational wavefunctions with correlation functionals.
  • To determine the most effective approach for calculating magnetic coupling constants in a range of systems.
  • To validate a recently proposed method for estimating correlation energy using natural orbital properties.

Main Methods:

  • Application of multiconfigurational wavefunctions coupled with different correlation functionals.
  • Calculation of magnetic coupling constants for selected antiferromagnetic materials, biradicals, and molecular complexes.
  • Utilizing a novel method that derives correlation energy from natural orbitals and occupation numbers of multiconfigurational wavefunctions.

Main Results:

  • The recently proposed method [Phys. Rev. A 75, 012503 (2007)] demonstrated superior performance on average.
  • Accurate predictions of magnetic coupling constants were achieved for systems including NiO, KNiF(3), K(2)NiF(4), La(2)CuO(4), and various molecular systems.
  • The method effectively estimates correlation energy using natural orbital-derived spin densities.

Conclusions:

  • The integration of multiconfigurational wavefunctions with correlation functionals is a viable strategy for accurate magnetic coupling constant calculations.
  • The proposed method offers a significant advancement in computational chemistry for predicting magnetic properties.
  • This approach provides a robust tool for the study of magnetic phenomena in diverse chemical systems.