Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/J-coupling

J-coupling (also called indirect dipole dipole coupling) is the coupling between two nuclear spins due to the influence of bonding electrons on the magnetic field running between the two nuclei. J-coupling contains information about dihedral angles, which can be estimated using the Karplus equation. It is an important observable effect in 1D nuclear magnetic resonance spectroscopy. Discovery In October 1951, E. L. 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 spin ensemble at the nuclear resonance condition and are separated by time interval of τ. The echo appears with a given maximum amplitude at time 2τ. For each setting of τ, the maximum of the echo signal is measured and plotted as a function of τ. If the spin ensemble consists of 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. (J/2π = 0.7 cycle per second) Such interaction came as a great surprise. The direct interaction between two magnetic dipole is dependent on the relative position of two nuclei in such a way that when averaged on all various orientation 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 1990s, direct evidence has been found for the presence of J-couplings between magnetically active nuclei on both sides of the hydrogen bond. P. Blake, B. Lee, M. Summers, M. Adams, J.-B. Park, Z. Zhou and A. Bax Quantitative measurement of small through-hydrogen-bond and 'through-space' 1H-113Cd and 1H-199Hg J couplings in metal-substituted rubredoxin from Pyrococcus furiosus 1992 Journal of Biomolecular NMR 2 5 527–533 10.1007/BF02192814 P. R. Blake, J. B. Park, M. W. W. Adams and M. F. Summers Novel observation of NH--S(Cys) hydrogen-bond-mediated scalar coupling in cadmium-113 substituted rubredoxin from Pyrococcus furiosus 1992 J. Am. Chem. Soc. 114 12 4931–4933 10.1021/ja00038a084 Initially, it was surprising to observe such couplings across hydrogen bonds since J-couplings are usually associated with the presence of purely covalent bonds. However, it is now well established that the H-bond J-couplings follow the same electron-mediated polarization mechanism as their covalent counterparts. An introduction to hydrogen bond scalar couplings Andrew J. Dingley, Florence Cordier and Stephan Grzesiek Concepts in Magnetic Resonance 13 2 103–127 2001 10.1002/1099-0534(2001)13:2<103::AID-CMR1001>3.0.CO;2-M J-coupling Hamiltonian The Hamiltonian of a molecular system may be taken as: H = D1 +D2 +D3. D1 = electron orbital-orbital, spin-orbital, spin-spin and electron spin-external field interactions D2 = magnetic interactions between nuclear spin and electron spin D3 = direct interaction of nuclei with each other for a singlet molecular state and frequent molecular collisions, D1 and D3 are almost zero. The full form of J-coupling interaction between spins Ij and Ik on the same molecule is: H = 2π Ij. Jjk. Ik where Jjk is the j-coupling tensor, a 3x3 real matrix. It depends on molecular orientation. In isotropic liquid it reduces to a number, so called scalar coupling. In 1D NMR, scalar coupling leads to oscillations in FID as well as splitting of lines in the spectrum. Measurement of J-coupling The Quantitative J correlation developed by Ad Bax et al. in 1994 is commonly the method of choice for accurate measurements of J couplings. E. de Alba and N. Tjandra Interference between Cross-correlated Relaxation and the Measurement of Scalar and Dipolar Couplings by Quantitative J 2006 Journal of Biomolecular NMR 35 1 1–16 10.1007/s10858-006-0028-4 G. W. Vuister and A. Bax Quantitative J correlation: a new approach for measuring homonuclear three-bond J(HNHα) coupling constants in 15N-enriched proteins 1993 J. Am. Chem. Soc. 115 17 7772–7777 10.1021/ja00070a024 References Further reading H. S. Gutowsky, D. W. McCall, C. P. Slichter Coupling among Nuclear Magnetic Dipoles in Molecules 1951 Physical Review 84 589–90 10.1103/PhysRev.84.589.2 E. L. Hahn and D. E. Maxwell Chemical Shift and Field Independent Frequency Modulation of the Spin Echo Envelope 1951 Physical Review 84 6 1246 10.1103/PhysRev.84.1246 N. F. Ramsey and E. M. Purcell Interactions between Nuclear Spins in Molecules 1952 Physical Review 85 1 143 10.1103/PhysRev.85.143