Ligands, inner-shell

We can exploit the new results for packing contributions to reconsider the outer shell contribution in Eq. (33). For ionic solutes, the outer shell term would represent the Born contribution because it describes a hard ion stripped of any inner shell ligands. A Born model based on a picture of a dielectric continuum solvent is reasonable (see Section III,B, and Fig. 9, color insert). With that motivation, we first separate the outer shell term into an initial packing contribution and an approximate electrostatic contribution as... 

Figure 4. Calculated HAB values as a function of Fe -Fe separation, based on the structural model given in Figure 1 and the diabatic wavefunctions I/a and f/B. Curves 1 and 2 are based on separate models in which the inner-shell ligands are represented, respectively, by a point charge crystal field model [Fe(H20)62 -Fe(HsO)63 ] and by explicit quantum mechanical inclusion of their valence electrons [Fe(HgO)s2 -Fe(H20)s3+] (as defined by the dashed rectangle in Figure 1). The corresponding values of Kei, the electronic transmission factor, are displayed for various Fe-Fe separations of interest.

The calculation of E] and X from computational methods is the focus here. Generally, the energetics of these quantities are separated into contributions from the inner and outer shells. For transfer between small molecules, the inner shell generally is defined as the entire solutes A and D, and the outer shell is generally defined as only the solvent. However, in a more practical approach for proteins, the inner shell is defined as only the redox site, which consists of the metal plus its ligands no further than atoms of the side chains that are directly coordinated to the metal, and the outer shell is defined as the rest of the protein plus the surrounding solvent. Thus... 

Fig. 1. The surface sites of the Au-core of Aujj are shown schematically, with the ligands removed. The light grey circles represent the uncoordinated surface atoms, the dark grey circles represent PPhj-coordinated surface atoms, and the black circles represent the Cl-coordinated surface atoms. The 13 atoms of the two inner shells are not visible...

It is not clear whether an increase in the effective nuclear charge is reasonable. In principle, such an increase could be explained by an increased covalency between the inner shell electrons and the ligands, leading to a charge transfer from inner shells to the ligands and thus to an increase of the effective nuclear charge Z for the f-electrons. This effect could be called anti-screening . 

An increase in speed can be achieved by using pseudopotential calculations122151 >. In these type of calculations the inner shell electrons are approximated with a potential and the problem is reduced to a valence electron problem. This technique is very powerful for heavy atoms but the time saving is not more than roughly 50 % for molecules containing first-row atoms only 152). Ion-ligand interactions have been studied with pseudopotential calculations in several cases 153 157). 

A single-particle effect that adds features in the X-ray absorption spectrum of molecules not present in that of atoms is the shape resonance (74, 75). (In the case of solids this effect, caused by a modification of the density of states due to the presence of the other atoms in the molecule, is automatically accounted for in band calculations.) Localization of the excited electron inside the molecule in states resulting from an effective potential barrier located near the electronegative atoms in the molecule causes strong absorption bands in free molecules and near the inner-shell ionization limits of positive ions in ionic crystals (74). Consequently, molecular inner-shell spectra depart markedly from the corresponding atomic spectra. The type of structure of an inner-shell photoabsorption spectrum depends on the geometry of the molecule, the nature of its ligands, etc., and can sometimes be used to determine the structure of the molecule. 

---