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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)

994
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...
994
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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

1.1K
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...
1.1K
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

1.7K
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...
1.7K
Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

905
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...
905
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

956
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,...
956
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.0K
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...
1.0K

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Achieving Efficient Fragment Screening at XChem Facility at Diamond Light Source
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Functionality optimization for effective singlet fission coupling screening in the full-dimensional molecular and

Johannes E Greiner1,2, Anurag Singh1,2, Merle I S Röhr1,2

  • 1Julius-Maximilians-Universität Würzburg, Center for Nanosystems Chemistry, Theodor-Boveri Weg, 97074 Würzburg, Germany. merle.roehr@uni-wuerzburg.de.

Physical Chemistry Chemical Physics : PCCP
|July 3, 2024
PubMed
Summary

Computational chemists can now efficiently screen molecular structures for desired properties. This method optimizes functionality by using algorithmic differentiation to find perylene bisimide (PBI) dimers with enhanced light-induced electronic couplings.

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Area of Science:

  • Computational Chemistry
  • Materials Science
  • Photochemistry

Background:

  • Predicting molecular configurations with specific properties is a key challenge in computational chemistry.
  • Light-induced functionality in molecular aggregates strongly depends on structure-dependent electronic couplings.

Purpose of the Study:

  • To develop an efficient strategy for targeted screening of molecular structural space.
  • To identify perylene bisimide (PBI) dimer motifs with enhanced effective spin-ல்-singlet (SF) coupling.

Main Methods:

  • Employed a "functionality optimization" technique to optimize a chosen descriptor constrained by ground state energy.
  • Utilized an algorithmic differentiation (AD) framework for automatic gradient computation.
  • Screened structural space for perylene bisimide (PBI) dimer motifs.

Main Results:

  • Successfully identified PBI dimer motifs with significantly enhanced effective SF coupling.
  • Demonstrated that specific structural modifications, including helical twisting, bending, and slipped-rotated packing, boost SF coupling.

Conclusions:

  • The proposed functionality optimization strategy combined with AD is effective for targeted structural screening.
  • Structural modifications of PBI monomers can be precisely tuned to enhance light-induced electronic couplings for improved functionality.