Clusters DFT calculations

Fig. 7. Energy levels of various defects at the MgO surface from embedded cluster DFT calculations. Reproduced from ref. [50]. Copyright 2000 Elsevier.

Cluster DFT calculations were also used to identify a transition state for the formation of ethylene oxide. In this transition state, the Ag-0 bonds are elongated relative to the oxametallacycle. The product of this reaction is gaseous EO. The activation energy for this step had previously been determined by Linic and Barteau experimentally.62 The predicted activation barrier from DFT calculations, 16 kcal/mol, is in very good agreement with the experimental result of 17 kcal/mol. 

One important component of practical epoxidation catalysis for which many questions remain open is the role of promoters. Industrially, both Cs and Cl are used as promoters.76 Saravanan et al. used cluster DFT calculations to examine the interactions between adsorbed Cs and oxametallacycles on Ag(lll).77 These calculations suggested that both neutral Cs and Cs+ gave similar outcomes and that the promoter atom made the formation of a surface oxametallacycle less energetically favorable than on the bare Ag surface. More recently, Linic and Barteau used plane wave DFT calculations to probe the effect of adsorbed Cs on the transition states controlling the formation of EO and acetaldehyde from oxametallacycles on Ag(lll).78 The role of Cs was... 

Is 11 to be described as a closo cluster with 6 cluster binding electron pairs, and 12 with only 5 cluster binding electron pairs as a hypercloso cluster DFT calculations on various trigonal-bipyramidal cage compounds favor the closo description of 11 (Scheme 3). This is also supported by the results of a formal two electron reduction of 12, which results in a closo structure, too. 

In the study of Wu and co-workers [34], the authors used NMR experiments and molecular cluster DFT calculations to determine the NMR tensor parameters for the carbonyl oxygen of guanine in several [ Ojguanosine derivatives that form either G-ribbons or G-quartets. The most significant finding is that QC and CS tensors are remarkably sensitive to the presence of both hydrogen bonding and ion—carbonyl interactions. As illustrated in Fig. 3.16, the presences ofboth interactions cause a net decrease of and Cq. 

Witko M, Tokarz-Sobieraj R (2004) Surface oxygen in catalysts based on transition metal oxides what can we learn from cluster DFT calculations. Catal Today 91-92 171-176... 

Cartesian coordinates system for locating points in space based on three coordinates, which are usually given the symbols x, y, z or i, j, k CBS (complete basis set) an ah initio method CC (coupled cluster) a correlated ah initio method CFF (consistent force field) a class of molecular mechanics force fields CFMM (continuous fast multipole method) a method for fast DFT calculations on large molecules... 

The ultrahigh vacuum STM was used to investigate the addition of the 2,2,6,6-tetramethyI-l-piperidinyloxy (TEMPO) radical to the dangling bond of Si(l 0 0)-2 X 1 surface. ° ° The TEMPO can bond with a single dangling bond to form stable Si-O coupling products, in contrast to the thermal decomposition of TEMPO-silicon compounds. Semiempiiical and DFT calculations of TEMPO bound to a three-dimer silicon cluster model yielded... 

In conjunction with the experimental data, DFT calculations have shown that O2 binding to an Au(l 11) surface is promoted by the adsorption of Au clusters [178]. Because O2 does not bind to the flat faces of gold surfaces, this area has quite significant implication in the area of CO catalysis. A review discussing the role of DFT in understanding CO oxidation on gold particles has been recently published [179]. 

In molecular DFT calculations, it is natural to include all electrons in the calculations and hence no further subtleties than the ones described arise in the calculation of the isomer shift. However, there are situations where other approaches are advantageous. The most prominent situation is met in the case of solids. Here, it is difficult to capture the effects of an infinite system with a finite size cluster model and one should resort to dedicated solid state techniques. It appears that very efficient solid state DFT implementations are possible on the basis of plane wave basis sets. However, it is difficult to describe the core region with plane wave basis sets. Hence, the core electrons need to be replaced by pseudopotentials, which precludes a direct calculation of the electron density at the Mossbauer absorber atom. However, there are workarounds and the subtleties involved in this subject are discussed in a complementary chapter by Blaha (see CD-ROM, Part HI). 

Figure 6.10 STS differential conductance spectra taken in constant height (left, blue curves) and constant current (right, blackcurves) modes from the (a) ( j7 x 7)R19.1°, (b) (5 x /3)-rect, (c) (9 x 9) structures, and (d) from the star cluster. The DFT-calculated DOS for the oxygen (red) and vanadium (green) atoms in the star cluster are shown in (d) for comparison. (Reproduced with permission from Ref. [23].)...

DFT calculations have afforded a relatively large j3xxx value of 28.5 x 10 30 esu at 1,064 nm for the compound [NEt4][CoFe2(CO)8S(PPh3)] which contains a tetrahedral CoFe2S cluster.550... 

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