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In nuclear chemistry and nuclear physics, J-couplings (also called spin-spin coupling or indirect dipole–dipole coupling) are mediated through connecting two spins. It is an indirect interaction between two nuclear spins that 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.
J-coupling is a frequency difference that is not affected by the strength of the magnetic field, so is always stated in Hz.
Nuclei with spins greater than , 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 nuclei are either monoisotopic, e.g. 31P and 19F, or have very high natural abundance, e.g. 1H. An additional convenience is that 12C and 16O have no nuclear spin so these nuclei, which are common in organic molecules, do not cause splitting patterns in NMR.
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
For coupling of a 13C nucleus and a directly bonded proton, the dominant term in the coupling constant JC–H is the Fermi contact interaction, which is a measure of the s-character of the bond at the two nuclei. (1977). 9780721631844, W.B.Saunders Company. ISBN 9780721631844
Where the external magnetic field is very low, e.g. as Earth's field NMR, J-coupling signals of the order of hertz usually dominate which are of the order of millihertz and are not normally resolvable.
However for some molecules with two distinct J-coupling constants, the relative signs of the two constants can be experimentally determined by a double resonance experiment.
The first experimental method to determine the absolute sign of a J-coupling constant was proposed in 1962 by Buckingham and Lovering, who suggested the use of a strong electric field to align the molecules of a polar liquid. The field produces a direct dipolar coupling of the two spins, which adds to the observed J-coupling if their signs are parallel and subtracts from the observed J-coupling if their signs are opposed.Buckingham A.D. and Lovering E.G., Effects of a strong electric field on NMR spectra. The absolute sign of the spin coupling constant
Another way to align molecules for NMR spectroscopy is to dissolve them in a nematic liquid crystal solvent. This method has also been used to determine the absolute sign of J-coupling constants.
For a Singlet state molecular state and frequent molecular collisions, D1 and D3 are almost zero. The full form of the J-coupling interaction between spins 'I j and Ik on the same molecule is:
where J jk is the J-coupling tensor, a real 3 × 3 matrix. It depends on molecular orientation, but in an isotropic liquid it reduces to a number, the so-called scalar coupling. In 1D NMR, the scalar coupling leads to oscillations in the free induction decay as well as splittings of lines in the spectrum.
Independently, in October 1951, Erwin Hahn and D. E. Maxwell reported a spin echo experiment which indicates the existence of an interaction between two protons in dichloroacetaldehyde. In the echo experiment, two short, intense pulses of radiofrequency magnetic field are applied to the spin ensemble at the nuclear resonance condition and are separated by a time interval of τ. The echo appears with a given amplitude at time 2 τ. For each setting of τ, the maximum value of the echo signal is measured and plotted as a function of τ. If the spin ensemble consists of a magnetic moment, a monotonic decay in the echo envelope is obtained. In the Hahn–Maxwell experiment, the decay was modulated by two frequencies: one frequency corresponded with the difference in chemical shift between the two non-equivalent spins and a second frequency, J, that was smaller and independent of magnetic field strength ( = 0.7 Hz).
Such interaction came as a great surprise. The direct interaction between two magnetic dipoles depends on the relative position of two nuclei in such a way that when averaged over all possible orientations of the molecule it equals to zero.
In November 1951, N. F. Ramsey and E. M. Purcell proposed a mechanism that explained the observation and gave rise to an interaction of the form I1· I2. The mechanism is the magnetic interaction between each nucleus and the electron spin of its own atom together with the exchange coupling of the electron spins with each other.
In the 1990s, direct evidence was found for the presence of J-couplings between magnetically active nuclei on both sides of the hydrogen bond. Initially, it was surprising to observe such couplings across hydrogen bonds since J-couplings are usually associated with the presence of purely . However, it is now well established that the H-bond J-couplings follow the same electron-mediated polarization mechanism as their covalent counterparts.
The spin–spin coupling between nonbonded atoms in close proximity has sometimes been observed between fluorine, nitrogen, carbon, silicon and phosphorus atoms.
(1977). 9780721631844, W.B.Saunders Company. ISBN 9780721631844 For example in the diethylthallium ion (C2H5)2Tl+, this method showed that the methyl-thallium (CH3-Tl) and methylene-thallium (CH2-Tl) coupling constants have opposite signs.
/ref> This method was first applied to 4-nitrotoluene, for which the J-coupling constant between two adjacent (or ortho) ring protons was shown to be positive because the splitting of the two peaks for each proton decreases with the applied electric field.
J-coupling Hamiltonian
The Hamiltonian of a molecular system may be taken as:
Decoupling
By selective radio frequency irradiation, NMR spectra can be fully or partially decoupled, eliminating or selectively reducing the coupling effect. Carbon-13 NMR spectra are often recorded with proton decoupling.
History
In September 1951, H. S. Gutowsky, D. W. McCall, and C. P. Slichter reported experiments on
See also
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