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Related Concept Videos

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

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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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¹H NMR: Complex Splitting01:13

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

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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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¹H NMR: Long-Range Coupling01:27

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2.9K
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...
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Singularity Functions for Bending Moment01:18

Singularity Functions for Bending Moment

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Singularity functions simplify the representation of bending moments in beams subjected to discontinuous loading, allowing the use of a single mathematical expression. For a supported beam AB, with uniform loading from its midpoint M to the right side end B, the approach involves conceptual 'cuts' at specific points to determine the bending moment in each segment. By cutting the beam at a point between A and M, the bending moment for the segment before reaching midpoint M is represented using a...
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Setting Limits on Supersymmetry Using Simplified Models
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First-Principle Characterization for Singlet Fission Couplings.

Chou-Hsun Yang1, Chao-Ping Hsu1

  • 1Institute of Chemistry, Academia Sinica, 128 Section 2 Academia Road, Nankang, Taipei 115, Taiwan.

The Journal of Physical Chemistry Letters
|August 12, 2015
PubMed
Summary
This summary is machine-generated.

The fragment spin difference (FSD) scheme accurately calculates singlet fission coupling, a key factor in solar energy conversion. This method, without explicit charge-transfer components, yields results consistent with experimental data.

Keywords:
charge transferelectronic couplingfragment spin differencesinglet fissiontriplet−triplet annihilation

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Area of Science:

  • Photochemistry
  • Materials Science
  • Quantum Chemistry

Background:

  • Singlet fission (SF) is a crucial photophysical process for enhancing solar cell efficiency.
  • Accurate calculation of electronic coupling is vital for predicting SF rates.
  • Previous methods often required explicit inclusion of charge-transfer (CT) components, complicating calculations.

Purpose of the Study:

  • To generalize the fragment spin difference (FSD) scheme for calculating singlet fission coupling.
  • To assess the FSD scheme's effectiveness without relying on explicit CT components.
  • To provide a reliable first-principle method for SF coupling relevant to organic electronics.

Main Methods:

  • Generalization of the fragment spin difference (FSD) scheme.
  • Calculation of singlet fission coupling for pentacene dimers in crystal and transition-state structures.
  • Correlation analysis between charge on fragments and FSD coupling.

Main Results:

  • The FSD scheme yielded significant coupling strengths (up to 33.7 meV).
  • Calculated singlet fission lifetimes (37-239 fs) align with experimental observations (80 fs).
  • Charge distribution in the S1 diabatic state strongly correlates with FSD coupling, confirming the importance of CT character.

Conclusions:

  • The generalized FSD scheme is a robust first-principle method for determining singlet fission coupling.
  • This approach effectively captures the necessary charge-transfer character without explicit parameterization.
  • The FSD method offers a simplified yet accurate pathway for designing efficient SF materials.