J-coupling

In nuclear chemistry and nuclear physics, Scalar or J-couplings (also called indirect dipole–dipole coupling) are mediated through chemical bonds connecting two spins. It is an indirect interaction between two nuclear spins which arises from hyperfine interactions between the nuclei and local electrons.J-coupling contains information about bond distance and angles. Most importantly, J-coupling provides information on the connectivity of molecules. In NMR spectroscopy, it is responsible for the appearance of many signals in the NMR spectra of fairly simple molecules.

The origin of J-coupling can be visualized by a vector model for a simple molecule such as hydrogen fluoride (HF). In HF, the two nuclei have spin 1/2. Four states are possible, depending on the relative alignment of the H and F nuclear spins with the external magnetic field. The selection rules of NMR spectroscopy dictate that ΔI = 1, which means that a given photon (in the radio frequency range) can affect ("flip") only one of the two nuclear spins.

J-coupling provides three parameters: the multiplicity (the "number of lines"), the magnitude of the coupling (strong, medium, weak), and the sign of the coupling.

The multiplicity provides information on the number of centers coupled to the signal of interest, and their nuclear spin. For simple systems, as in ¹H-¹H coupling in NMR spectroscopy, the multiplicity reflects the number of adjacent, magnetically nonequivalent protons. Nuclei with spins greater than 1/2, which are called quadrupolar, can give rise to greater splitting, although in many cases coupling to quadrupolar nuclei is not observed. Many elements consist of nuclei with nuclear spin and without. In these cases the observed spectrum is the sum of spectra for each isotopomer. One of the great conveniences of NMR spectroscopy for organic molecules is that the many lighter elements are nearly monoisotopic: ¹H, ¹⁹F, and ³¹P each have spin 1/2. ¹²C and ¹⁶O have no nuclear spin.

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