Carbon clusters optimization

Naturally, group 4 elements are again the focus of interest. Structures of hydrocarbons (which maybe thought of as partly or completely passivated carbon clusters) have been optimized by Hobday and Smith [65]. Hydrogen-passivated silicon clusters have been studied a few times, for example by Chakraborti and coworkers [97,98] at the TB level, as well as by Ge and Head [99] at the semiem-pirical Austin method 1 (AMI) level, with DFT and MP2 refinement calculations they noted a marked influence of the passivation layer on cluster structures, with Si10H16 and Si14H20 already exhibiting bulk structure, in stark contrast to bare silicon clusters. 

The mechanism of spontaneous fullerene formation has been the subject of much debate and speculation [90]. One of the major advances in this area comes from gas-phase ion chromatography, which allows to separate charged carbon clusters of specific mass into families of isomers based on their mobilities through helium-filled drift tubes [91-93]. These families (e.g., monocyclic rings, bi- and tricyclic compounds, fullerenes) are identified by comparing the observed mobilities with those calculated from optimized semiempirical equilibrium geometries [92], which are aceurate enough for this purpose. Such studies lead to product distributions of the cations C as a function of n [90-93] and allow the tentative formulation of a detailed mechanism for fullerene synthesis [93]. 

Fig. 2.33 Optimized configurations of CO2 adsorption on carbon clusters with different polar groups cyan C white H blue N red O light green K) and the corresponding contour plots of the differential charge density. The contour value is 0.001. The purple and lime regions represent the charge accumulation and charge depletion regions, respectively. Reprinted with the permission from Ref. [166]. Copyright 2012 American Chemical Society...

The discovery of buckminsterftillerene, for example, shows several of the components of the way research in chemistry proceeds. The researchers at Rice University who discovered C o, led by Richard Smalley and Robert Curl, had already established the ability to study other elemental clusters, or small clumps of atoms. In doing so, they constructed sophisticated and unique instruments to generate small, gas-phase clusters as well as to detect their existence and measure the number of atoms in each cluster. When Harry Kroto visited the Smalley laboratories and suggested that they study carbon, there was no expectation that they would soon discover an entirely new field. Then they noticed that the experimental peak associated with a 60-atom carbon cluster, or C o, was unusually large. They decided to optimize the production of this species and attempted to explain its apparent unusual stability. Once they annormced the discovery, other scientists repeated and extended the studies, and 5 years later, physicists at the University of Arizona discovered a means to make macroscopic quantities of C o and other... 

Fig. 4.22. Mass spectrum of carbon clusters produced by laser vaporization of graphite and detected by Fourier transform ion cyclotron resonance mass spectrometer. The experiment has been optimized to trap positive clusters in the C300 to C350 mass range. Reproduced with permission from Maruyama S, Anderson LR, Smalley RE (1990) Rev. Sci. Instrum. 61 3686...

Finally, several special MNDO parametrizations are available for certain classes of compounds or for specific properties. These treatments retain the original MNDO approach (a)-(c), but use parameters (d) that have been optimized for the intended applications. It is obvious that such specialized methods ought to be more accurate in their area of applicability than the original general-purpose MNDO method. Samples of this kind include the special MNDO variants for small carbon clusters, fullerenes, and hydrogen-bonded systems as well as special parametrizations for electrostatic potentials. ... 

As it follows from Table 1, a jr-charge of over le is concentrated on the carbon atoms. Therefore, each carbon atom can be in principle an adsorption site. The optimization of the position of oxygen atoms relative to the surface was performed at frozen geometric parameters of the PANI cluster. Calculations of the adsorption of molecular oxygen on PANI showed that the carbon atoms of the phenyl ring, which are shown by Figure 4, are the most stable surface adsorption sites (AS). 

Deactivation is due primarily to two mechanisms formation of carbon-containing deposits and sulfur poisoning. Carbon deposition may be minimized by the addition of alkali metals, optimization of metal cluster size, and use of oxygen ion-conducting supports. Sulfur poisoning is usually irreversible and there are few reports of catalysts that are tolerant of sulfur levels typical of commercial fuels. 

The syntheses of many Co3(CO)9X compounds from dicobalt octacarbonyl and XCCI3 have been optimized 347), and further reactions starting from 0(60)4 and XCCI3 360), or RCF —Co(CO)4 46) have been investigated in order to determine the mechanism of formation of the clusters. Methinyltricobalt enneacarbonyls are also formed from Co2(CO)g and such apical carbon precursors as acetylenes 140), dimethyl ketene 408), or carbyne chromium complexes (7 73). In several cases (7 72,... 

Table 2 Results of Variational Localized-site Cluster expansions from either a Neel-state based ansatze or a Resonating Valence Bond ansatz. We notice that the lower level NSBA is unable of showing the dimerization of polyacetylene. rc is the critical bifurcation mean bond length, r and r2 are the optimized short and long bond distances (in A). E is the energy per carbon atom (in eV), taking the energy of the Neel state with 1.40 A equal bond lengths as zero of energy. NSB forth order perturbative and Dimer-covering second order perturbative (see Ref. 34), CEPA ab-initio estimate of Kpnig and Stollhoff [52], and the experimental results [46,47] for rx and r2 have been added for comparison.

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