The interpretation of quadrupole splitting

In systems other than closed shell ionic lattices, (I j)ia, can normally be ignored. The magnitude of depends on the nature and arrangement of the ligands, and is discussed in some detail below. When non-bonding electrons are present their effects are nearly always evident, regardless of the ligands. 

For (V Xb to be non-zero, the population of the valence shell p orbitals (of main group elements) or d orbitals (in the case of transition metals) must be non-equivalent. Thus for tin(II) or antimony(III), the valence shell contains a single lone pair, which normally contributes significantly to the EFG. Only when the metal atom occupies a site with rigorous cubic symmetry, as in CsSnBr3 or Cs3SbClg, is (F ) b zero. In these cases the lone pair has a high 5s character. 

When the symmetry is lower, the lone pair is often stereochemically active, and is usually directed along the major symmetry axis of the system. Since this axis is also normally taken as the z axis, this means that the 5p population is higher than that of 5p or 5py. Such concentration of negative charge on the z axis gives substantial quadrupole splitting, with (l j) b 0 (Table 2.16 for both Sn and Sb, Q 0 positive values for eQV therefore indicate that is negative). 

A similar situation arises when two lone pairs are present, as for tellurium(IV) and iodine(III). In these cases, the bonding seems to involve 

Cases in which there are three lone pairs, as in iodine(I) compounds, are better considered as being derived from closed shell configurations by the loss of electron density from one p orbital to the neighbouring atom. The quadrupole coupling constant then reflects the extent of electron transfer, and follows the electronegativity of the neighbour (Table 2.17). 

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