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¹H NMR: Long-Range Coupling

¹H NMR: Long-Range Coupling

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 π orbitals.

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

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

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...

Spin–Spin Coupling: One-Bond Coupling

Spin–Spin Coupling: One-Bond Coupling

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,...

Double Resonance Techniques: Overview

Double Resonance Techniques: Overview

Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
Spin decoupling is usually achieved by...

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

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

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...

Spin–Spin Coupling Constant: Overview

Spin–Spin Coupling Constant: Overview

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 have a...

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Single-Molecule Förster Resonance Energy Transfer Methods for Real-Time Investigation of the Holliday Junction Resolution by GEN1

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Electron transfer through a single barrier inside a molecule: from strong to weak coupling.

Robert Stadler¹, Jérôme Cornil, Victor Geskin

¹Department of Physical Chemistry, University of Vienna, Sensengasse 8/7, A-1090 Vienna, Austria.

The Journal of Chemical Physics

|August 28, 2012

Summary

Investigating electron transport through single molecules reveals a third scenario with a single internal barrier. Accurate modeling requires advanced quantum-chemical methods beyond standard density functional theory functionals.

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Area of Science:

Quantum Chemistry
Molecular Electronics
Condensed Matter Physics

Background:

Electron transport through single molecules is crucial for molecular electronics.
Theoretical models distinguish between coherent transport (strong coupling) and Coulomb blockade (weak coupling).
A less-explored scenario involves a single barrier localized within the molecule.

Purpose of the Study:

To investigate electron transport through a single internal molecular barrier.
To study the effects of electric fields on charge distribution in biphenyl radical ions.
To evaluate the accuracy of different quantum-chemical methods for describing this transport regime.

Main Methods:

Utilized various quantum-chemical methods to study partial charge shifts in biphenyl radical ions.
Applied electric fields at different angles to modulate molecular coupling and barrier properties.
Analyzed charge versus field curves to identify transport characteristics.

Main Results:

Observed rounded steps in charge-field curves for weak and intermediate coupling.
Demonstrated that standard density functional theory (DFT) functionals fail to reproduce these features.
Found that long-range corrected hybrid functionals provide a better description.

Conclusions:

Accurate modeling of single-barrier electron transport necessitates including exchange and dynamical correlation effects.
Standard DFT functionals are insufficient for describing this scenario.
Long-range corrected hybrid functionals offer a promising approach for DFT-based transport calculations in single-barrier systems.