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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...

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: 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...

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...

Chemical Shift: Internal References and Solvent Effects

Chemical Shift: Internal References and Solvent Effects

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...

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Structural Significance and Reference Value of High-Precision Coupling Constants and Raw Data Sharing.

Daniela Rebollar-Ramos¹, Aysegul Caskurlu Oz^1,2,3, Guy H Harris¹

¹Pharmacognosy Institute and Department of Pharmaceutical Sciences, Retzky College of Pharmacy, University of Illinois Chicago, Chicago, Illinois 60612, United States.

Analytical Chemistry

|June 2, 2026

Summary

This summary is machine-generated.

High-precision proton NMR spin analysis (HifSA) accurately determines J-couplings, enabling precise structural identification of complex molecules and facilitating the creation of digital reference materials for metabolomics.

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Area of Science:

Analytical Chemistry
Organic Chemistry
Structural Biology

Background:

Proton NMR (¹H NMR) spectroscopy is crucial for structural and metabolomic analysis.
Determining J-couplings, essential for NMR analysis, presents a significant challenge due to complex peak patterns.
Existing methods often result in imprecise J-value measurements, limiting detailed structural elucidation.

Purpose of the Study:

To demonstrate the capability of ¹H iterative functionalized Spin Analysis (HifSA) in precisely determining J-couplings.
To apply high-precision J-value determination for differentiating structurally similar compounds.
To establish the utility of J-couplings as reliable indicators of conserved molecular geometries for digital reference material creation.

Main Methods:

Utilized ¹H iterative functionalized Spin Analysis (HifSA) to achieve J-coupling precision of 0.01 Hz or better.
Acquired high-precision NMR spin parameters for diastereomeric methyl angelate and methyl tiglate across various solvents and magnetic field strengths (60 vs 600 MHz).
Applied HifSA to analyze raw NMR data from 12 naturally occurring angelates and tiglates, and other small organic acids.

Main Results:

Achieved highly specific differentiation of near-identical structural motifs (methyl angelate and methyl tiglate) using high-precision J-value data.
Demonstrated that J-couplings of geometrically conserved fragments exhibit minimal experimental variation (0.02 Hz), acting as constants.
Successfully developed fragment digital reference materials (dRMs) suitable for FAIR data dissemination.

Conclusions:

High-precision J-value determination via HifSA significantly enhances structural specificity in NMR analysis.
J-couplings serve as reliable "seismographic sensors" for conserved molecular geometries, enabling fragment-based structural analysis.
Accessible raw NMR data combined with advanced spin analysis promotes reproducible, high-resolution metabolomic studies and supports open data practices.