Fragmentation of clusters

L.S. Cederbaum, J. Zobeley, F. Tarantelli, Giant intermolecular decay and fragmentation of clusters, Phys. Rev. Lett. 79 (1997) 4778. 

R.G. COOKS and T.L. KRUGER propose the kinetic method for thermochemical determination based on measurement of the rates of competitive fragmentations of cluster ions [47]. 

Because of our previous success In applying Fourier-transform infrared spectroscopy to the study of the rhodium carbonyl clusters under high pressures of carbon monoxide and hydrogen 2. A, we have applied the same technique and equipment in this work. 3. The temperature has been kept In all these experiments below 200° with maximum pressures of 832.0 atm to maximize the trend towards fragmentation of clusters. The absence of bases, e.g., salts or amines, in the systems under evaluation in this work was desirable to eliminate the ambiguity that would result from the enhancement of the fragmentation of clusters by carbon monoxide In a basic medium. . ... 

FIGURE 21 Mass spectra of U+ (n= 1, 4, 8), UO+ (n=3, 5, 6, 7, 8), and 002" (n= 1, 3, 5) cartxmyl ions. The insets show representative fragmentation of clusters with second-shell coordination, as examined by photodissociation experiments. Reproduced with permission from Ricks et al. (2010). Copyright 2010 American Chemical Society. 

We first examined the isoprene polymerization on the fractions of the titanium catalyst particles which were isolated by sedimentation of the total mixture. The results obtained allow to consider large particle as clusters which are composed of smaller particles. In the formation of these clusters is modified ligands available titanium atoms. In the process of polymerization or catalyst preparation the most severe effects are large particles (clusters). This result in the developing process later on substantially smaller as compared to initial size particles. These particles are fragments of clusters which are located over the active centers of polymerization. Note that the stereospecificity is not dependent on the size of the catalyst particles. 

The size distribution of the clusters produced in the cluster source is quite smooth, containing no information about the clusters except their composition. To obtain information about, for example, the relative stability of clusters, it is often useful to heat the clusters. Hot clusters will evaporate atoms and molecules, preferably until a more stable cluster composition is reached that resists further evaporation. This causes an increase in abundance of the particularly stable species (i.e., enhancing the corresponding peak in the mass spectrum, then commonly termed fragmentation spectrum ). Using sufficiently high laser fluences (=50 /iJ/mm ), the clusters can be heated and ionized simultaneously with one laser pulse. 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

Mercury has a marked ability to bond to other metals. In addition to the amalgams aheady mentioned (p. 1206) it acts as a versatile structural building block by forming Hg-M bonds with cluster fragments of various types e.g. reduction... 

The use of direct electrochemical methods (cyclic voltammetry Pig. 17) has enabled us to measure the thermodynamic parameters of isolated water-soluble fragments of the Rieske proteins of various bci complexes (Table XII)). (55, 92). The values determined for the standard reaction entropy, AS°, for both the mitochondrial and the bacterial Rieske fragments are similar to values obtained for water-soluble cytochromes they are more negative than values measured for other electron transfer proteins (93). Large negative values of AS° have been correlated with a less exposed metal site (93). However, this is opposite to what is observed in Rieske proteins, since the cluster appears to be less exposed in Rieske-type ferredoxins that show less negative values of AS° (see Section V,B). 

From the two-dimensional, graphite-like clusters, the extension to three-dimensional structures is obvious. Symmetric structures developed in a similar fashion to the planar systems would grow in three dimensions with increasing N, and the number of atoms would increase faster. In this work clusters of T symmetry were studied, resembling a small fragment of a diamond structure. Only systems with saturated external bonds were considered. The number of carbon and hydrogen atoms in such a structure is given by... 

Fig. 6.4 (a) Fragment of the structure of Cs2Ti4Nb6Clis06 (light and dark) polyhedra represent (Ti(l)Cl402) and (Ti(2)Cl6) octahedra, respectively), (b) Cluster unit in... 

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