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Related Concept Videos

Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

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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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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...
977
Valence Bond Theory02:42

Valence Bond Theory

8.5K
Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
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Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

892
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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Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

618
In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
618
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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

1.0K
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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Updated: Jun 10, 2025

Spin Saturation Transfer Difference NMR SSTD NMR: A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes
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UMRSF-TDDFT: Unrestricted Mixed-Reference Spin-Flip-TDDFT.

Konstantin Komarov1, Minseok Oh2, Hiroya Nakata3

  • 1Center for Quantum Dynamics, Pohang University of Science and Technology, Pohang 37673, South Korea.

The Journal of Physical Chemistry. A
|October 17, 2024
PubMed
Summary
This summary is machine-generated.

A new unrestricted method for Mixed-Reference Spin-Flip Time-Dependent Density Functional Theory (UMRSF-TDDFT) improves calculations for challenging quantum systems. This enhanced approach offers greater accuracy by incorporating unrestricted Kohn-Sham orbitals and an orbital reordering scheme.

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Area of Science:

  • Quantum Chemistry
  • Computational Physics
  • Theoretical Chemistry

Background:

  • Traditional Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) methods often fail for systems with degeneracies or bond dissociation.
  • Spin-Flip TDDFT (SF-TDDFT) variants have been developed but can suffer from issues like spin contamination.

Purpose of the Study:

  • To develop and validate an unrestricted version of Mixed-Reference Spin-Flip TDDFT (UMRSF-TDDFT).
  • To introduce a new molecular orbital reordering scheme and a method for estimating ⟨S²⟩ expectation values.
  • To assess the performance of UMRSF-TDDFT on systems where conventional methods falter.

Main Methods:

  • Development of Unrestricted Mixed-Reference Spin-Flip Time-Dependent Density Functional Theory (UMRSF-TDDFT).
  • Incorporation of unrestricted Kohn-Sham (UKS) orbitals and a novel molecular orbital reordering scheme.
  • Benchmarking against challenging chemical systems, including Be atom, HF bond breaking, and Jahn-Teller distortion in TMM.

Main Results:

  • UMRSF-TDDFT accurately recovers degenerate states for the Be atom, with energies slightly reduced due to UKS flexibility.
  • UMRSF-TDDFT correctly describes bond breaking in HF, unlike U-SF-TDDFT which misses a crucial configuration.
  • UMRSF-TDDFT shows a greater reduction in singlet-triplet energy difference for TMM compared to U-SF-TDDFT, attributed to orbital relaxations rather than spin contamination.

Conclusions:

  • The unrestricted nature of UKS orbitals in UMRSF-TDDFT allows for greater spatial orbital relaxation, leading to lower total energies.
  • UMRSF-TDDFT provides a practical and accurate quantum chemical theory, complementing existing restricted open-shell variants.
  • This new method enhances the ability to address complex quantum systems where standard theories are insufficient.