Metal chemistry, theoretical approaches

This survey of theoretical methods for a qualitative description of homogeneous catalysis would not be complete without a mention to the Hartree-Fock-Slater, or Xot, method [36]. This approach, which can be formulated as a variation of the LDA DFT, was well known before the formal development of density functional theory, and was used as the more accurate alternative to extended Hiickel in the early days of computational transition metal chemistry. 

These attempts may be called thermodynamic semi-theoretical approaches . They concern mostly the simplest kind of bonding, namely the metallic bond. The underlying hypothesis is that the contributions of different outer orbitals (7 s, 6 d, 5 f) in some chosen thermodynamic or structural property can be linearly combined, the coefficients of this linear combination being related to the degree of participation of the different orbitals in the bonding an approach clearly related to the molecular orbital approach of quantum chemistry and to the hybridization concept, and which had been previously employed in other transition metals and to the rare-earth metallic systems " (for a criticism of this approach, see Ref. 6). The chosen thermodynamic and structural properties are, therefore, bonding indicators , since they will reflect contributions introduced by the fact that the wavefunctions of bonding electrons have mixed orbital characters. 

We have validated our theoretical approach by simulating MCD spectra of a large range of species. In this section we will survey the results of some of these studies. The emphasis will be on metal complexes with a few examples from organic chemistry. 

Quantum chemistry has so far had little impact on the field of photoelectrochemistry. This is largely due to the molecular complexity of the experimental systems, which has prevented reliable computational methods to be used on realistic model systems, although some theoretical approaches to various aspects of the performance of nanostructured metal oxide photoelectrochemical systems have appeared in the last 10 years, see e.g. [139, 140, 141]. Here we have focussed on quantum-chemical cluster and surface calculations of a number of relevant problems including adsorbates and intercalation. These calculations illustrate the emerging possibilities of using quantum chemical calculations to model complicated dye-sensitized photoelectrochemical systems. 

O. Eisenstein, Theoretical Approach to Transition Metal Chemistry From Molecules to Clusters and Solids, in New J. Chem., 15 (10-11), Gauthier-Villars, Gossin, France, 1991. 

A common theme throughout this volume involves the adsorption and interfacial, especially biointerfacial, behaviour of all of the above mentioned nanomaterials. For environmental and human protection, the adsorption of heavy metal ions, toxins, pollutants, drugs, chemical warfare agents, narcotics, etc. is often desirable. A healthy mix of experimental and theoretical approaches to address these problems is described in various contributions. In other cases the application of materials, particularly for biomedical applications, requires a surface rendered inactive to adsorption for long term biocompatibility. Adsorption, surface chemistry, and particle size also plays an important role in the toxicological behaviour of nanoparticles, a cause for concern in the application of nanomaterials. Each one of these issues is addressed in one or more contributions in this volume. 

As observed in other systems, the obvious difficulty in elucidating reaction mechanisms based on static structural snapshots subsequently initiated structural-dynamic theoretical studies of metalloproteinases. The active site chemistry of zinc-dependent enzymes has been studied using a variety of theoretical approaches. For example, mixed quantum mechaiucal/molecular calculations and classical molecular dynamic simulations have been employed, especially studies using density functional methods on redox-active metal centers (42). 

Shustorovich, E., Sellers, H. The UBI-QEP method A practical theoretical approach to understanding chemistry on transition metal surfaces. Surf. Sci. Rep. 1998, 31, 5-119. 

A major advantage of DFT theory is its relatively low computational intensity compared with post-afc initio electron correlation calculations.21 Because DFT calculations can be augmented with empirical data relevant to the transition metals, they are now the preferred approach to theoretical studies of organotransi-tion metal chemistry. We must point out, however, that theoreticians still cannot know ahead of time which DFT method will work best for a particular situation ... 

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