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. In NMR spectroscopy J-coupling contains information about relative bond distances and angles. Most importantly, J-coupling provides information on the connectivity of chemical bonds. It is responsible for the often complex splitting of resonance lines in the NMR spectra of fairly simple molecules. 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. In NMR spectroscopy J-coupling contains information about relative bond distances and angles. Most importantly, J-coupling provides information on the connectivity of chemical bonds. It is responsible for the often complex splitting of resonance lines 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 several important lighter spin 1/2 nuclei are either monoisotopic, e.g. 31P and 19F, or have very high natural abundance, e.g. 1H. 12C and 16O have no nuclear spin. For 1H–1H coupling, the magnitude of J provides information on the proximity of the coupling partners. Generally speaking 2-bond coupling (i.e. 1H–C–1H) is stronger than three-bond coupling (1H–C–C–1H). The magnitude of the coupling also provides information on the dihedral angles relating the coupling partners, as described by the Karplus relationship. For heteronuclear coupling, the magnitude of J is related to the nuclear magnetic moments of the coupling partners. 19F, with a high nuclear magnetic moment, gives rise to large coupling to protons. 103Rh, with a very small nuclear magnetic moment, gives only small couplings to 1H. To correct for the effect of the nuclear magnetic moment (or equivalently the gyromagnetic ratio γ), the 'reduced coupling constant' K is often discussed, where The value of J also has a sign, and couplings constants of comparable magnitude often have opposite signs. The Hamiltonian of a molecular system may be taken as: For a singlet molecular state and frequent molecular collisions, D1 and D3 are almost zero. The full form of the J-coupling interaction between spins 'Ij and Ik on the same molecule is: