Local electronic structure

Hamers R J and Kohler U K 1989 Determination of the local electronic structure of atomic-sized defects on Si(OOI) by tunnelling spectroscopy J. Vac. Sc/. Technol. A 7 2854... 

Parameters measured Surface topography (SFM and STM) local electronic structure (STM)... 

Other artifacts that have been mentioned arise from the sensitivity of STM to local electronic structure, and the sensitivity of SFM to the rigidity of the sample s surface. Regions of variable conductivity will be convolved with topographic features in STM, and soft surfaces can deform under the pressure of the SFM tip. The latter can be addressed by operating SFM in the attractive mode, at some sacrifice in the lateral resolution. A limitation of both techniques is their inability to distinguish among atomic species, except in a limited number of circumstances with STM microscopy. 

According to its IR spectrum, phospharinane 141 exists in the solid state exclusively as the tautomer 141b possessing the localized electronic structure (Scheme 43) (83UK1761). However, by means of NMR and IR spectroscopy, it was shown that in CDCI3 solution the delocalized structure 141a is also present, although at a low relative concentration. 

Homogeneous alloys of metals with atoms of similar radius are substitutional alloys. For example, in brass, zinc atoms readily replace copper atoms in the crystalline lattice, because they are nearly the same size (Fig. 16.41). However, the presence of the substituted atoms changes the lattice parameters and distorts the local electronic structure. This distortion lowers the electrical and thermal conductivity of the host metal, but it also increases hardness and strength. Coinage alloys are usually substitutional alloys. They are selected for durability—a coin must last for at least 3 years—and electrical resistance so that genuine coins can be identified by vending machines. 

Figure 6.28. Schematic illustration of the change in local electronic structure of an oxygen atom adsorbing on the late transition metal rhodium, the DOS of which is shown on the right-hand side. The interaction of the oxygen 2p orbital with the sp band of the transition metal is illustrated through interaction with the idealized free-electron...

Figure 9.11 Thiophene adsorbed at 500 K on an H-atom pretreated MoS2 cluster (50 x 54 A2). Beam-like features at the metallic edge [scan line (i)] and the shifted intensity of the outermost edge protrusions relative to the clean edge (triangles refer to the clean edge). These shifts in intensity [line scan (ii)] are associated with changes in the local electronic structure after adsorption of thiophene observed with STM. All the images were taken at room temperature subsequent to thiophene adsorption at 500 K. (Reproduced from Ref. 34).

When normal sites in a crystal structure are replaced by impurity atoms, or vacancies, or interstitial atoms, the local electronic structure is disturbed and local electronic states are introduced. Now when a dislocation kink moves into such a site, its energy changes, not by a minute amount but by some significant amount. The resistance to further motion is best described as an increase in the local viscosity coefficient, remembering that plastic deformation is time dependent. A viscosity coefficient, q relates a rate d8/dt with a stress, x ... 

Soon after the quantum revolution of the mid 1920s, Linus Pauling and John C. Slater expanded Lewis s localized electronic-structural concepts with the introduction of directed covalency in which bond directionality was achieved by the hybridization of atomic orbitals.1 For normal and hypovalent molecules, Pauling and Slater proposed that sp" hybrid orbitals are involved in forming shared-electron-pair bonds. Time has proven this proposal to be remarkably robust, as has been demonstrated by many examples in Chapter 3. 

Table 4.52. A synopsis of localized electronic structures for simple metallocenes in terms of MLX formulation, spin-state multiplicity, nonbonding d electrons (d count) and orbitals (nd), ordinary (2c/2e) and tu (3c/4e) bonds, and nominal sd" hybridization...

Among the recently published works, the one which showed that the cyclic structures of water clusters open up to form a linear structure above a certain threshold electric field value a was a systematic ab initio study on the effect of electric field on structure, energetics, and transition states of trimer, tetramer, and pentamer water clusters (both cyclic and acyclic) [36], Considering c/.v-butadiene as a model system, the strength and the direction of a static electric field has been used to examine the delocalization energy, the probabilities of some local electronic structures, the behavior of electron pairs, and the electronic fluctuations [37]. Another recent work performed by Rai et al. focused on the studies using the DFT and its time-dependent counterpart of effects of uniform static electric field on aromatic and aliphatic hydrocarbons [38],... 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

Calculation of the electronic structure of freshly cleaved hematite surfaces suggest that the local electronic structure of the surface may be very different from the bulk... 

It is essential to have tools that allow studies of the electronic structure of adsorbates in a molecular orbital picture. In the following, we will demonstrate how we can use X-ray and electron spectroscopies together with Density Functional Theory (DFT) calculations to obtain an understanding of the local electronic structure and chemical bonding of adsorbates on metal surfaces. The goal is to use molecular orbital theory and relate the chemical bond formation to perturbations of the orbital structure of the free molecule. This chapter is complementary to Chapter 4, which... 

The NMR chemical shift, the most prevalent parameter in NMR spectroscopy, carries a wealth of information regarding the environment and the local electronic structure in the vicinity of the nucleus under study.(i). For example, one normally finds a different chemical shift for the Ca nucleus of each alanine residue in a protein. Ideally, a thorough analysis of the NMR chemical shift can yield information regarding the structure and interactions in the vicinity of the nucleus concerned. To achieve this, a detailed understanding of how geometrical factors and intermolecular interactions influence the chemical shift is crucial. The development and validation of the methods towards this end have combined powerful and efficient ab initio quantum mechanical techniques, which have been... 

The specificity of an oxide catalyst depends on its local electronic structure, both the electron density and the spin density. Making and breaking of bonds affects the spin of the active metal - spin-allowed reactions are kine-tically faster and thus have an advantage over spin-forbidden reactions. 

Thus, the change in oxidation state that would alter the spin is either essential to cater for spin-forbidden transitions or should not occur, to let a spin-allowed reaction to occur rapidly. This principle of specificity was identified in a large array of metal-oxo cluster reactions with small molecules [53, 54] and should be transferable to real catalyst under the concept of local electronic structures at active sites. 

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