Scalar coupling magnetic dipoles

Experimental approaches to direct characterization of the conformational exchange motions in proteins have been suggested earlier [67-69]. The most recent methods [66, 70-73] are based on a relaxation-compensated version of CPMG that alleviates the previous restriction on the duration of the refocusing delay due to evolution of magnetization from scalar couplings and dipole-dipole cross-correlations. 

One of the two or both nuclei of a diatomic molecule may interact with rotation via their electric quadrupole moments, or their magnetic dipole moments may interact with the rotational magnetic field. The two nuclei may be coupled by the direct (tensorial) or indirect (electron-coupled scalar) magnetic dipole interaction which also influences rotation. Furthermore, in a state other than E the nuclei cause magnetic perturbations when their dipole moments interact with those of the unpaired - electron spins or with the orbital magnetic field. The energetic effects of these so-called hyperfine interactions can be quantified with the aid of interaction constants which in favorable cases can be determined from high-resolution spectra. 

Nuclear relaxation is caused by interaction between the nuclear magnetization M and small incoherent magnetic fields which arise from random Brownian motions of molecules (Abragam, 1961 Farrar and Becker, 1971 Becker, 1974). In the case of quadrupolar nuclei, electrical fields interact with the electric quadrupole moment of the nucleus. The fluctuating fields can arise from a number of processes, including (1) magnetic dipole-dipole interaction, (2) electric quadrupole interaction, (3) scalar coupling, (4) spin-rotation interaction, and (5) chemical shift anisotropy. 

Information on the structure of molecules can be obtained from nuclear pair interactions magnetic dipole-dipole interactions provide distance information, while scalar J couplings allow one to determine dihedral angles. 

NMR spectra cannot be measured in solids in the same way in which they are routinely obtained in solutions because NMR lines from solids are too broad. In solution all interactions apart from chemical shift and indirect coupling are averaged to zero by thermal motions of molecules. Magnetic interactions in the solid state are described by a Hamiltonian H [1], which is a sum of several contributions Zeeman interaction (the same as in solution), direct dipole-dipole interaction, magnetic shielding (giving chemical shifts), scalar spin-spin coupling to other nucleus, and for nuclei with / >1/2 also quadrupolar interactions. 

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