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NMR Spectroscopy: Spin–Spin Coupling

NMR Spectroscopy: Spin–Spin Coupling

The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...

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,...

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...

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: 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...

Atomic Nuclei: Nuclear Spin

Atomic Nuclei: Nuclear Spin

All atomic particles possess an intrinsic angular momentum, or 'spin'. Electrons, protons, and neutrons each have a spin value of ½, although protons and neutrons in nuclei may have higher half-integer spins owing to energetic factors.
Atomic nuclei have a net nuclear spin, , which can have an integer or half-integer value. In atomic nuclei, the spins of protons are paired against each other but not with neutrons, and vice versa. Consequently, an even number of protons does not contribute to...

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Spin Excitations in a 4f-3d Heterodimer on MgO.

A Singha^1,2,3, F Donati^1,2,3, F D Natterer^2,4

¹Center for Quantum Nanoscience, Institute for Basic Science (IBS), Seoul 03760, Republic of Korea.

Physical Review Letters

|January 5, 2019

Summary

We studied holmium-cobalt (HoCo) dimers, the smallest lanthanide-transition metal compounds. We observed magnetic excitations and found ferromagnetic coupling, differing from bulk materials.

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Area of Science:

Condensed Matter Physics
Materials Science
Surface Science

Background:

Lanthanide-transition metal intermetallic compounds are crucial in magnetism.
Understanding small-scale magnetic systems is key to developing novel magnetic materials.
The magnetic properties of single dimers remain largely unexplored.

Purpose of the Study:

To investigate the magnetic properties of holmium-cobalt (HoCo) dimers.
To model the smallest intermetallic compound of a lanthanide and a transition metal.
To determine the exchange interaction and magnetic level distribution in HoCo dimers.

Main Methods:

Adsorption of HoCo dimers on ultrathin magnesium oxide (MgO) films on silver (Ag).
Inelastic electron tunneling spectroscopy (IETS) to detect magnetic excitations.
Application of an external magnetic field to confirm magnetic origin of excitations.
Density functional theory (DFT) and spin Hamiltonian analysis for theoretical modeling.

Main Results:

Detection of inelastic excitations in HoCo dimers using scanning tunneling spectroscopy (STS).
Confirmation of the magnetic origin of these excitations via external magnetic field application.
Determination of the magnetic level distribution and exchange interaction sign and magnitude.
Observation of ferromagnetic coupling between holmium and cobalt atoms in the dimer.

Conclusions:

HoCo dimers serve as a model system for studying magnetism in the smallest lanthanide-transition metal intermetallic compounds.
The study demonstrates the capability of STS to probe magnetic excitations in single 4f-element-containing dimers.
Ferromagnetic coupling in HoCo dimers contrasts with typical antiferromagnetic coupling found in bulk 4f-3d compounds, highlighting unique nanoscale magnetic behavior.