Electrons nuclear shielding

As well as being attracted to the nucleus, each electron in a many-electron atom is repelled by the other electrons present. As a result, it is less tightly bound to the nucleus than it would be if those other electrons were absent. We say that each electron is shielded from the full attraction of the nucleus by the other electrons in the atom. The shielding effectively reduces the pull of the nucleus on an electron. The effective nuclear charge, Z lle, experienced by the electron is always less than the actual nuclear charge, Ze, because the electron-electron repulsions work against the pull of the nucleus. A very approximate form of the energy of an electron in a many-electron atom is a version of Eq. 14b in which the true atomic number is replaced by the effective atomic number ... 

Electrons that are in filled sets of orbitals between the nucleus and outer shell electrons shield the outer shell electrons partially from the effect of the protons in the nucleus this effect is called nuclear shielding. 

As we move from left to right along a period, the outer shell electrons do experience a progressively stronger force of attraction to the nucleus due to the combination of an increase in the number of protons and a constant nuclear shielding by inner electrons. As a result the atomic radii decrease. 

When comparing the magnetic shielding tensors in Cgo and it is observed that the six extra electrons in the latter yield an important increase in the anisotropy and a strong shielding effect in the isotropic part of the nuclear shielding. 

The treatment of atoms with more than one electron (polyelectronic atoms) requires consideration of the effects of interelectronic repulsion, orbital penetration towards the nucleus, nuclear shielding, and an extra quantum number (the spin quantum number) which specifies the intrinsic energy of the electron in any orbital. The restriction on numbers of atomic orbitals and the number of electrons that they can contain leads to a discussion of the Pauli exclusion principle, Hund s rules and the aufbau principle. All these considerations are necessary to allow the construction of the modern form of the periodic classification of the elements. 

M FIGURE 5.16 The origin of electron shielding and Zeff. Outer electrons are attracted toward the nucleus by the nuclear charge but are pushed away by the repulsion of inner electrons. As a result, the nuclear attraction actually felt by outer electrons is diminished, and we say that the outer electrons are shielded from the full charge of the nucleus by the inner electrons. 

The relevant question regarding secondary IEs on acidity is the extent to which IEs affect the electronic distribution. How can an inductive effect be reconciled with the Born-Oppenheimer approximation Although the potential-energy function and the electronic wave function are independent of nuclear mass, an anharmonic potential leads to different vibrational wave functions for different masses. Averaging over the ground-state wave function leads to different positions for the nuclei and thus averaged electron densities that vary with isotope. This certainly leads to NMR isotope shifts (IEs on chemical shifts), because nuclear shielding is sensitive to electron density.16... 

More reactive atoms especially those with lone pair electrons, such as, 4N, l5N,, 70 and 19F, are very likely to have their nuclear shieldings influenced by interactions with solvent molecules. Such interactions rnay be specific, e.g. hydrogen bonding, or nonspecific, e.g. polarisability/polarity, or perhaps a combination of specific and nonspecific solute-solvent interactions. An empirical procedure has been developed for quantitatively unraveling the contributions made to the shielding of solute nuclei by specific and nonspecific interactions. 

The TB MO calculation on the 15N chemical shift of polypyrrole in the solid state allows useful information to be extracted from the observed spectra, namely that the two peaks obtained are correctly assigned to the quinoid and aromatic structures.(l 1,38) ( The quinoid structure is closely to the electric conductivity.) A decrease in the band gap leads to a downfleld shift. These results on conducting polymers demonstrate that the chemical shift behavior provides information about the band gap which, in turn, is a measure of the electric conductivity. It can be said that TB MO calculations offer useful perspectives in interpreting the results of NMR nuclear shieldings in polymers, both in terms of the structure in the solid state and in understanding the effect of intermolecular interactions on nuclear shieldings. The latter are shown to operate through the electronic structures of the polymers considered. 

Solid-state high-resolution NMR spectroscopy, combined with quantum chemistry, is able to provide detailed information on the electronic and stereochemical structures of molecules[l]. Quantum-chemical calculations produce three principal values of the nuclear shielding tensor. These principal values have more detailed information about structure of molecules as compared with isotropic chemical shifts. Comparison of the observed chemical shift and chemical shift tensor with the calculated shieldings, produced by quantum-chemical calculations, permits deep insight into the structures of the molecules under investigation. 

Table 1 Nonrelativistic one-electron magnetic terms in the Hamiltonian. Their derivatives with respect to /m and/or or [ip enter the expressions for the nuclear shielding and spin-spin coupling tensors via the perturbation operators ) ancj g is the spin-operator for an electron, va a distance vector with respect to nucleus A etc. ...

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