Carbon clusters electronic structure calculations

There is another series of cluster-type ternary carbides that contain not only metal and carbon atoms but also the group VIIA elements, Cl, Br and I (Simon et al. 1981, Warkentin et al. 1982, Simon and Warkentin 1983, Schwanitz-Schuller and Simon 1985, see also chapter 100, this volume). Electronic structure calculations have been... 

To study the possible reasons and elementary mechanisms of the catalytic activity of CPs, we have modeled the electronic structure of some molecular CPs clusters and its adsorption complexes with oxygen. A MOPAC computer complex and, in particular, the PM3 quantum-chemical program of this complex was used for calculations. The results of calculations have shown that both oxygen atoms form bonds with two more active carbon atoms of CP molecular cluster (so-called bridge model of adsorption). The total energy of system after chemical adsorption at such active atoms is minimal (Fig. 2). 

There has been little attempt to go beyond calculations on the cluster and to model the full carbonate structure, although such work has been quite successfully undertaken for nitrates. Tossell (1985a) has performed a modified electron-gas calculation on MgCOj using R(C-0) = 1.27 A as a constant value and varying the distance. The... 

Early calculations for copper, palladium, and silver clusters were carried out by various investigators using the EH and CNDO methods, and among these is an attempt by Baetzold to take into account the effect of a carbon support on the electronic structure of a palladium cluster.In 1976, Messmer etal. compared the efficacies of the three methods of calcu-... 

Carbon atoms crystallize in several forms. Graphite and diamond are well known carbon polymorphs. Fullerenes, which were discovered in the 1980 s, have also been well characterized. Carbon materials show a variety of different physical and chemical properties. Because of this the electronic structure of carbon materials has been investigated using a number of different experimental techniques, for example, XPS, UPS and XANES. Theoretical studies of carbon materials have been also performed. However, experimentally observed spectra are not always consistent with theoretical predictions. Recently, in order to understand the various kinds of observed electronic spectra, DV-Xa calculations have been performed on a small cluster model. [1] In the present paper, we report results of DV-Xa calculations performed on the carbon materials graphite, alkali graphite intercalation compounds (GIC), fullerene, and fluorinated fullerenes. 

We used the C24 model cluster, which consists of seven carbon hexagons, to represent the graphite plane (Fig. 1 (a)). To perform the ground state calculations, we classified the carbon atoms into three types, and performed the calculations under symmetry. To avoid model cluster termination effects, we treated the electronic structure of the inner carbon atoms as that of graphite. When we performed Slater s transition state calculations, however, we took four types of carbon atom into account. This was because one of the inner carbon atoms, in which the Is electron transits to an unoccupied level, should be distinguished from the other inner carbon atoms. Further we didn t use any symmetry orbitals for the transition state calculations. 

One of the major applications of the CURES-EC procedure has been in calculating the electron affinities of the carbon clusters C [46]. Carbon clusters are common species in nature, since they are a product of combustion and contribute to environmental problems. They have been observed in the interstellar medium. The fuller-enes represent one of the most recent discoveries of a new form of carbon. Carbon clusters have many forms—linear chains, cyclic compounds and the fullerenes— because carbon can form covalent bonds. The NIST tables give 54 electron affinities values for C 14 for Si 17 for Ge 45 for Sn and 55 for Pb . Only the structures for the carbon compounds from n = 3 to 30 are identified [12]. 

There have been numerous photodetachment studies of small cluster anions, and we now give some examples. Noble metal clusters (Cu, Ag,7, Au , n = 1-10) have been studied by Ho et al. [23], who resolved vibrations in all three dimers. Studies of alkali metal cluster anions have included those of Na ( = 2-5), K (n = 2-19), RbJ 3, and CS2-3 [24,25]. Carbon cluster anions C,T have photoelectron spectra that are consistent with linear chains for n = 2-9 and monocyclic rings for n = 10-29 [26]. Photoelectron spectra of Sb and Bi to n = 4 [27] show rich vibrational structure for the dimers, and the spectra of the larger clusters could be interpreted in terms of ab initio calculations. The threshold photodetachment (zero electron kinetic energy, ZEKE) spectrum of Si4 [28] shows a progression of well-resolved transitions between the ground state of the rhombic anion (Dzh, and vibrational levels of the first excited... 

In this section DFT treatments of the iron-molybdenum cofactor and the activation, reduction and protonation of N2 proceeding at this cluster are presented. The earliest of these calculations appeared after publication of the first crystal structures of nitrogenase, and this was well before the discovery of the central atom X. After the discovery of X and its identification as carbon these treatments were, in part, updated. Here we will focus both on the basic theoretical methods to treat the electronic structure of the FeMoco and the reduction of N2 mediated by this cluster. 

Theory has been used predominantly to probe the nature of the sites on vanadium clusters and model vanadium oxide surfaces. Cluster and p>eriodic DFT calculations [68,69] have been carried out in order to imderstand the electronic and structural properties of the exposed (100) surface of (VO)2P207. Both cluster and slab calculations reveal that surface vanadium sites can act as both local acid and base sites, thus enhancing the selective activation of n-butane as well as the adsorption of 1-butene. Vanadium accepts electron density from methylene carbon atoms and, thus aids in the subsequent activation of other C-H bonds. Calculations reveal that that the terminal P=0 bonds lie close to the Fermi level and thus present the most nucleophihc oxygen species present at the surface for both the stoichiometric as well as phosphate-terminated surfaces. These sites may be involved in the nucleophilic activation of subsequent CCH bonds necessary in the selective oxidative conversion of butane into maleic anhydride. Full relaxation of the surface, however, tends to lead to a contraction of the terminal P=0 bonds and a lengthening of the P V bonds. This pushes the P V states, initially centered on the oxygen atoms, higher in energy and thus increases their tendency to be involved in nucleophilic attack . 

Accurate calculations for excitations with DMC are possible for systems as large as free-base porphyrin and models of the green fluorescent protein chromophore. Drummond et al. investigated the electron emission from diamondoids with DMC. Using DFT orbitals in DMC, they calculated the excitation energy for the HOMO-LUMO transition (optical gap), the electron affinity, and the ionization potential for carbon clusters with diamond structure up to CgvHvs. ... 

An AMI comparison of the geometry and electronic structure of Dsh Siyo (118, Figure 18) with that of C70, Si o and Ceo was reported by Piqueras and coworkers. The calculated bond lengths and bond orders, which are given in Table 13, suggest that the silicon clusters have a more localized structure than the carbon clusters, and that Siyo... 

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