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J-coupling
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.
==Vector model and manifestations for chemical structure assignments==
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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