Jcouplings

J-couplings, or scalar couplings, are through-bond interactions between nuclear spins that cause splitting of NMR resonances. The coupling constant, J, is measured in hertz (Hz) and reflects an indirect interaction transmitted by bonding electrons rather than by through-space magnetic dipole interactions.

Couplings arise between nuclei connected by chemical bonds, and their magnitude generally decreases as the number of intervening bonds increases. One-bond (1J) couplings are the largest, while two-bond (2J) and three-bond (3J) couplings are smaller, and four-bond (4J) or longer-range couplings are typically small. Typical values span from tens to hundreds of hertz for strong, direct bonds, down to single digits or fractions of a hertz for long-range couplings, depending on the nuclei involved and the molecular environment. In practice, heteronuclear 1J couplings (for example 1H–13C or 1H–15N) are large, whereas homonuclear long-range couplings can be very small and often require specialized experiments to observe.

J-couplings can be categorized as homonuclear (between identical nuclei, such as 1H–1H or 13C–13C) or heteronuclear (between different nuclei, such as 1H–13C or 1H–15N). In natural-abundance samples, 13C–13C couplings are usually small, but they become observable in enriched materials. The values of J also depend on electronic structure, bond types, hybridization, electronegativity, and, for vicinal couplings, molecular geometry.

Practical use of J-couplings includes determining connectivities, estimating dihedral angles via the Karplus relationship for 3J_HH, and aiding in structure and stereochemistry assignments. They are measured directly from multiplet patterns in 1D spectra and refined with 2D experiments such as J-resolved, HSQC, and HMBC, often under decoupling or selective editing.