Nuclear magnetic resonance hyperfine shifts

Electron spin resonance, nuclear magnetic resonance, and neutron diffraction methods allow a quantitative determination of the degree of covalence. The reasonance methods utilize the hyperfine interaction between the spin of the transferred electrons and the nuclear spin of the ligands (Stevens, 1953), whereas the neutron diffraction methods use the reduction of spin of the metallic ion as well as the expansion of the form factor [Hubbard and Marshall, 1965). The Mossbauer isomer shift which depends on the total electron density of the nucleus (Walker et al., 1961 Danon, 1966) can be used in the case of Fe. It will be particularly influenced by transfer to the empty 4 s orbitals, but transfer to 3 d orbitals will indirectly influence the 1 s, 2 s, and 3 s electron density at the nucleus. 

Hyperfine field (HF), quadrupole splitting (QS) and isomer shifts (IS) relative to a-Fe at the six different iron sites 16k, 4c and at the metalloid site 4g occupied by carbon or boron determined from Mossbauer and nuclear magnetic resonance (NMR) spectra (Erdmann et al. 1988, 1989). All these data were determined... 

Whereas the paramagnetic shift of the nuclear magnetic resonance frequency for a given applied field is related to the strength of the local hyperfine field at the nuclear site, induced by the electronic moments, the nuclear spin-lattice relaxation rate yields information about the low-frequency spectrum of thermally induced spin fluctuations. The influence of pair-correlation effects on the NMR relaxation in paramagnets was analysed experimentally and theoretically by... 

Actually, our mixed discrete-continuum model is not limited to the study of UV-vis spectra, but it has been already successfully employed to model solvent effects on several different spectral properties, such as electron paramagnetic resonance (EPR) hyperfine coupling constants, nuclear magnetic resonance (NMR) chemical shifts, and so on [45, 121]. 

The nuclear hyperfine interaction splits the paramagnetic states of an electron when it is close to a nucleus with a magnetic moment. For a random orientation of spins and nuclei, the tensor quantities in Eq. (4.11) are replaced by scalar distributions, and the resonance magnetic field is shifted from the Zeeman field // by... 

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. 

In a macroscopically isotropic sample (all molecular orientations have the same probability), the spectrum consists of contributions from aU orientations when the rotational motion is frozen on the time scale of the experiment. As ESR lines are derivative absorption lines, negative and positive contributions from neighboring orientations cancel. Powder spectra are thus dominated by contributions at the minimum and maximum resonance fields, and by contributions at resonance fields that are common to many spins. The latter contribution provides the center line in the nitroxide powder spectrum (Fig. 3b). It corresponds mainly to molecules with nuclear magnetic quantum number rrii = 0 (center line of all triplets, only g-shift). The detailed shape of this powder spectrum can be simulated, but interpretation is not easy, mainly because hyperfine and g anisotropy are of similar magnitude. 

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