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Related Concept Videos

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...
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Atomic Nuclei: Nuclear Spin State Population Distribution01:14

Atomic Nuclei: Nuclear Spin State Population Distribution

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Near absolute zero temperatures, in the presence of a magnetic field, the majority of nuclei prefer the lower energy spin-up state to the higher energy spin-down state. As temperatures increase, the energy from thermal collisions distributes the spins more equally between the two states. The Boltzmann distribution equation gives the ratio of the number of spins predicted in the spin −½ (N−) and spin +½ (N+) states.
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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...
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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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

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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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Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

950
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,...
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Atomic Nuclei: Nuclear Spin State Overview01:03

Atomic Nuclei: Nuclear Spin State Overview

902
NMR-active nuclei have energy levels called 'spin states' that are associated with the orientations of their nuclear magnetic moments. In the absence of a magnetic field, the nuclear magnetic moments are randomly oriented, and the spin states are degenerate. When an external magnetic field is applied, the spin states have only 2 + 1 orientations available to them. A proton with = ½ has two available orientations. Similarly, for a quadrupolar nucleus with a nuclear spin value of...
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Related Experiment Video

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Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser
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Spin parameter optimization for spin-polarized extended tight-binding methods.

Siyavash Moradi1, Rebecca Tomann2, Josie Hendrix2

  • 1Department of Chemistry, Technical University of Munich, TUM School of Natural Sciences and Catalysis Research Center, Garching, Germany.

Journal of Computational Chemistry
|August 23, 2024
PubMed
Summary
This summary is machine-generated.

We optimized atom-specific spin-polarization constants in GFN2-xTB simulations for improved accuracy. While effective for specific datasets like W4-11, parameter transferability across different molecular properties is limited, suggesting property-specific optimization.

Keywords:
benchmarkdensity functional tight‐bindingparameter optimizationsemi‐empirical methodssensitivity analysisspin‐polarization

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Materials Science

Background:

  • Accurate molecular simulations are crucial for predicting chemical properties.
  • The GFN2-xTB method offers a balance of speed and accuracy but can be improved.
  • Spin-polarization constants are key parameters influencing simulation outcomes.

Purpose of the Study:

  • To develop an optimization strategy for atom-specific spin-polarization constants in GFN2-xTB.
  • To assess the impact of sequential vs. global optimization on simulation accuracy.
  • To evaluate the transferability of optimized parameters across different molecular properties.

Main Methods:

  • Implemented an optimization strategy for spin-polarization constants.
  • Performed sequential and global optimization for H, C, N, O, and F atoms.
  • Utilized Sobol indices for sensitivity analysis to identify influential parameters.
  • Tested optimized parameters on the W4-11 dataset and singlet-triplet gaps in carbenes.

Main Results:

  • Achieved substantial error reduction on the W4-11 dataset.
  • Demonstrated limited transferability of optimized spin-polarization constants to other properties like singlet-triplet gaps.
  • Identified inherent limitations of extended tight-binding methods beyond parameter optimization.

Conclusions:

  • Re-optimization of spin-polarization constants significantly improves accuracy for specific datasets.
  • A property-specific optimization strategy is motivated due to limited transferability.
  • Further development of extended tight-binding methods is needed to address inherent limitations.