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Related Concept Videos

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

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

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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...
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Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

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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...
1.6K
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

2.8K
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...
2.8K
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

1.6K
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,...
1.6K
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.7K
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
1.7K
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

2.1K
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...
2.1K

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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
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Hyperfine Coupling Constants from Internally Contracted Multireference Perturbation Theory.

Toru Shiozaki1, Takeshi Yanai2

  • 1Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.

Journal of Chemical Theory and Computation
|August 2, 2016
PubMed
Summary
This summary is machine-generated.

We developed a new method using complete active space second-order perturbation theory (CASPT2) to accurately calculate hyperfine coupling constants (HFCCs). This approach accounts for electron correlation effects, showing excellent agreement with experimental and other high-level computational methods.

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Area of Science:

  • Quantum chemistry
  • Computational physics
  • Spectroscopy

Background:

  • Accurate calculation of hyperfine coupling constants (HFCCs) is crucial for understanding molecular electronic structure and properties.
  • Existing methods may struggle to fully account for electron correlation effects influencing spin density.

Purpose of the Study:

  • To present a novel and accurate method for computing HFCCs using CASPT2 theory.
  • To incorporate orbital and configurational relaxation effects for improved accuracy.

Main Methods:

  • Utilizing complete active space second-order perturbation theory (CASPT2) with full internal contraction.
  • Calculating HFCCs as a first-order property from a relaxed CASPT2 spin-density matrix.
  • Employing one- and two-body spin-free counterparts and Z-vectors from CASPT2 nuclear gradient programs.

Main Results:

  • The developed CASPT2 method provides accurate HFCCs for CN and AlO radicals.
  • Results demonstrate comparability or superiority to coupled-cluster and density matrix renormalization group methods.
  • HFCCs for hexaaqua complexes of V(II), Cr(III), and Mn(II) were successfully computed.

Conclusions:

  • The new CASPT2-based method is accurate and efficient for calculating HFCCs.
  • The approach effectively captures electron correlation and relaxation effects.
  • The code is validated for various molecular systems, including radicals and transition metal complexes.