Nuclear magnetic resonance electron-nucleus coupling

A nucleus under study by nuclear magnetic resonance techniques is affected by other nuclei in the same molecule. This phenomenon is known as spin-spin coupling. The effect arises (in adjacent nuclei) from the two electrons joining the nuclei in a covalent bond. Suppose the energy of states in which the electrons in the bond have opposing spins is lower than the state in which the electron spins are parallel. Then the AE between the two states (in this case a negative number) is called the coupling constant, J, expressed in frequency units, Hz. Internuclear... 

ENDOR = electron nuclear double resonance EPR = electron-paramagnetic resonance ESR = electron-spin resonance NMR = nuclear magnetic resonance MA = modulation amplitude SOFT = second-order perturbation theory s-o = spin-orbit zfs = zero-field splitting (for S > 1 /2) D = uniaxial zfs E = rhombic zfs g = g-factor with principal components gy, and g ge = free electron g-factor a = hyperfrne splitting constant A = hyperftne coupling constant for a given nucleus N (nuclear spin 7>0). 

The magnetic character of some nuclei also plays an important role in mass-independent fractionation effects. Nuclides characterized by an odd number of protons or odd number of neutrons are characterized by a non-zero nuclear spin. This is what makes these nuclides amenable to investigation by nuclear magnetic resonance (NMR) spectroscopy. A non-zero nuclear spin, however, also affects the interaction between the nucleus and the surrounding electron cloud via hyperfine nuclear spin-electron spin coupling, and thus also the behavior of these nuclides in chemical reactions [49, 50]. 

Ideally we would like to derive from investigations of proteins in solution the same structural details which X-ray diffraction methods have yielded for the crystalline state. Of the spectroscopic methods perhaps nuclear magnetic resonance has the greatest potential since, in principle, and C nuclear magnetic resonance spectra are dependent on the properties of all amino acid components. The parameters which describe NMR spectra provide a wealth of information on each magnetic nucleus. The chemical shift is sensitive to the electronic and molecular environment the area of each resonance is proportional to the number of nuclei involved the fine structure, characterised by coupling... 

To a physical organic chemist, dipole moment and molecular refraction are electronic properties par excellence—so is optical activity, which is determined, as it were, by the topology of the motion of charge through the molecule under the influence of the electric component of a radiation field so also are the chemical shift of the frequency of nuclear magnetic resonance and the nuclear qvadrupoU coupling constant, both of which serve as sensitive probes into the electronic environment of the nucleus. 

In principle, there are several contributions to nuclear Tm" however, the dipolar coupling term often dominates (10,22). The dipolai contribution depends on the reciprocal of the sixth power of the distance between the resonating nucleus and the relaxing electron, on the square of the magnetic moments associated with the unpaired electrons (ge2 B S(S+l)) and with the nucleus on the magnetic field as expressed by the Larmor... 

The coupling of the unpaired electrons with the nucleus being observed generally results in a shift in resonance frequency that is referred to as a hyperfine isotropic or simply isotropic shift. This shift is usually dissected into two principal components. One, the hyperfine contact, Fermi contact or contact shift derives from a transfer of spin density from the unpaired electrons to the nucleus being observed. The other, the dipolar or pseudocontact shift, derives from a classical dipole-dipole interaction between the electron magnetic moment and the nuclear magnetic moment and is geometry dependent. 

There are also pulse EPR methods that probe the chemical or rather magnetic environment. These are pulse electron nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopy, which allow measuring hyperfine couplings from the unpaired electron spin to surrounding magnetically active nuclei ([20] in Fig. 3 this is a P nucleus). As these experiments are performed in frozen solution (e.g., in all examples of this chapter) or in solids, from the anisotropy and orientation dependence of the hyperfine coupling one can obtain valuable information on the structure up to 1 nm. 

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