Semi-Empirical Band Structure Model

In this model, proposed by Griessen and Driessen [71], each metal is characterized by the difference between the Fermi level ( p) and the energy of the lowest conduction band of the host metal (E ). For a binary hydride, the heat of formation is then given by  

Before closing this section it should be pointed out that the formation of most hydrides of intermetallic compounds is metastable vith respect to disproportionation [35]. Again, taking LaNis as an example, the disproportionation reaction  

Nevertheless, because the disproportionation reaction requires rearrangement of metal atoms vhile in the hydride reaction the motion of atoms is minimal, the ternary hydride formation is kinetically favored at lo v temperature [35[. Ho vever, at high temperature (around 573 K [73]) the disproportionation reaction is more likely to 

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We will briefly discuss here the two principal semi-empirical models Miedema s model and the semi-empirical band structure model. 

A direct application of the semi-empirical band structure model of Griessen and Driessen (eq. 34) is to predict standard enthalpies of hydride formation of metals which absorb only negligible amounts of hydrogen under normal conditions. Driessen et al. and Hemmes et al. ) predicted on the basis of this model that formation of hydrides of Be, Cu, Ru, Ag, Cd, In,... 

The application of the semi-empirical band structure model to ternary metal-hydrogen systems Ai-y By H has been described in detail by Griessen... 

Fig. 3.10 Semiconductor band structure modeled by hand and genetic algorithm. A semi-empirical model is constructed that matches the available target energies and interpolates the complete band structure. The solution produced with the genetic algorithm outperforms the manual fit, both, in quality and a processing speed of an hour versus about a week (from [38]).

Photosynthetic water oxidation to dioxygen takes place in a Mn-containing protein in PSH via Kok s intermediates, labeled Sq-S [1]. These S-states probably reflect different redox states of some or all of four Mn ions involved in the enz3rme-substrate(H20) complex. The molecular structure and oxidation states of this Mn-cluster in each S-state has been the subject of considerable recent spectroscopic studies involving EPR[2-5]and X-ray absorption[6,7]. In this paper, we present high-quality Mn K-edge XANES spectra measured for spinach PSH membranes in Si and S2 states, which exhibit informative pre-edge feature due to Is to 3d transition as well as four fine structures superimposed on the principal absorption band. These features are semi-empirically analized in comparison with those of authentic Mn complexes. Based on the results obtained, we propose a hypothetical model for the Mn-cluster which is compatible with EPR data[2-5] in S2-state. 

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