Electric field gradient valence contribution

For transition metal atoms, the dominant contribution to the electric field gradient at a nucleus originates in the valence shell centered on that nucleus. The application of Eq. (8.41) to transition metal complexes will be discussed in chapter... 

The quadrupole coupling constant is proportional to the electric-field gradient, eq, which measures the asymmetry of the electron density surrounding the nucleus. Since the core electrons and the valence-shell s electrons are spherically distributed, they contribute... 

The quadrupolar splittings depend mainly on the electric field gradient at the nucleus, which is caused by the spatial distribution of the electrons around it, via electric Coulomb forces. Both electrons (valence contribution f/yai) and neighboring anions and cations (lattice contribution U x) in the vicinity of the nucleus contribute to this electric field gradient U, which can be expressed as follows ... 

The electric field gradient is the negative second derivative of the potential at the nucleus of all surrounding electric charge. It therefore embraces contributions from both the valence electrons of the atom and from surrounding ions. In ionic complexes it is customary to consider these separately, and to write q as... 

The total electric field gradient from the valency electrons of the ion can be obtained by summing the appropriate wavefunction contributions. If the orbital populations alter with temperature because of excitation to low-lying higher states, then the electric field gradient will probably show a strong temperature dependence. This point is developed more fully on p. 99. 

Generally the valence term is the major contribution to the electric field gradient unless the ion has the high intrinsic symmetry of an iS-state ion such as high-spin Fe ( i ). In the latter case the lattice term will be dominant. 

In the original theory of Townes and Daily, in order to simplify Eq. (10), several reasonable assumptions are made which also stress the essential terms of the problem. The electric charges of a molecule are divided into three groups (a) the electrons of the closed shells of the atom bearing the quadmpolar nucleus, which are supposed to be unperturbed (b) the valence electrons of the same atom (c) the electrons and nuclei of all the other atoms of the molecule. The contribution of the electrons of group (a) to the field gradient is assumed to be zero because of the symmetry of the closed shells. The contribution of any... 

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