Nuclear magnetic moments, chemically induced

Paramagnetic lanthanides (except for Gd ) induce large chemical shifts in the NMR absorptions of ligands in their vicinity. One of the main origins of these shifts is the dipolar interaction between the nuclear magnetic moment and the anisotropic magnetic moment of the unpaired 4f electrons. For complexes of axial symmetry the magnitude of the shift may be expressed as... 

A different class of phenomena can be related to the interaction of the electronic polarization density and induced current with the nuclei. Thus the chemical shifts in nuclear magnetic resonance (NMR) spectroscopy are interpreted in terms of magnetic shielding of the electrons, perturbed by a static magnetic field, at those nuclei possessing an intrinsic magnetic moment [7]. 

There are many experimental techniques for the determination of the Spin-Hamiltonian parameters g, Ux, J. D, E. Often applied are Electron Paramagnetic or Spin Resonance (EPR, ESR), Electron Nuclear Double Resonance (ENDOR) or Triple Resonance, Electron-Electron Double Resonance (ELDOR), Nuclear Magnetic Resonance (NMR), occasionally utilizing effects of Chemically Induced Dynamic Nuclear Polarization (CIDNP), Optical Detections of Magnetic Resonance (ODMR) or Microwave Optical Double Resonance (MODR), Laser Magnetic Resonance (LMR), Atomic Beam Spectroscopy, and Muon Spin Rotation (/iSR). The extraction of data from the spectra varies with the methods, the system studied and the physical state of the sample (gas, liquid, unordered or ordered solid). For these procedures the reader is referred to the monographs (D). Further, effective magnetic moments of free radicals are often obtained from static... 

Another technique for the study of reactions that is highly specific for radical processes is known as CIDNP, an abbreviation for chemically induced dynamic nuclear polarization." The instrumentation required for such studies is a normal NMR spectrometer. CIDNP is observed as a strong perturbation of the intensity of NMR signals in products formed in certain types of free radical reactions. CIDNP is observed when the normal population of nuclear spin states dictated by the Boltzmann distribution is disturbed by the presence of an unpaired electron. The intense magnetic moment associated with an electron causes a polarization of nuclear spin states, which is manifested by enhanced absorption or emission, or both, in the NMR spectrum of the diamagnetic product of a free radical reaction. The technique is less general than EPR spectroscopy because not all free radicals can be expected to exhibit the phenomenon. 

The most common interactions are dipolar coupling between two spins, chemical shielding and quadrupolar interaction for spins I> 1. The chemical-shielding interaction arises from the motion of electrons around a nucleus induced by the external applied static magnetic field. This motion generates local magnetic fields that modify the total field experienced by the nucleus, and are characteristic of the local chemical environment of the nuclei. All nuclei with a spin /> 1 possess a quadrupolar moment Q that interacts with the EFG Vrs — d2 V/dXfdx at the nuclear site. The EFG tensor Vrs is a symmetric tensor with zero trace Tr( E) = A E = 0 (from Laplace equation). 

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