Localized cationic clusters

Fig. 3.13. (Top) An electron micrograph of an artificial chromatin model composed of T4 DNA and cationic nanoparticles of diameter 15nm. (Bottom) Typical snapshots of a model DNA (semiflexible polyelectrolyte) complexed with cationic nanoparticles. At low salt concentration (Debye screening length m/a = 1), a beads-on-a-string nucleosome-like structure is observed (left), while locally segregated clusters are formed at higher salt concentrations (rn/a = 0.3) (right) (See [46] for more details)...

Polyhedral clusters in which the electron count is less than Sn are electron-poor in the sense that the number of skeleton electrons is insufficient to form localized, 2c,2e bonds. Instead, models involving 3c,2e bonds or full global electron delocalization ( three-dimensional aromaticity) must be invoked to describe the bonding conditions. Electron-poor naked clusters are, as would be expected, formed by the early post-transition elements in groups 13 and 14. In addition, the cationic clusters of group 15 are electron-poor. Examples, derived from the compilation in Table 5, include the iso-electronic Pbg and Big " clusters and the 22-electron M5 species of thallium, tin, lead, antimony, and bismuth. 

Identification and localization of cluster and hydroxylated forms of dvalent cation oxide in Y zeolite", J. Phys. Chem., Si, 397-402. 

The local cation structures and their differences originate with the clusters. The Zr-CI bonding appears to be the most structurally demanding, particularly as expressed in the many and diverse networks with intercluster bridging chlorine. In contrast, the cations, particularly the larger and lower field members, are repeatedly found in relatively irregular, asymmetric, or even poorly defined sites compared with those in simple salts. These circumstances give the impression that the structural... 

Of the family of nonmagnetic transition metals, niobium clusters are those for which most theoretical investigations have been made, although these were mainly restricted to the structural properties neutral clusters Nb , with n up to 7 (LDA [229]), with n = 8-10 (LDA [230, 231], LDA and B-LYP GC [223]), anionic clusters up to n = 8 (LDA [113]), anionic and cationic clusters with n = 8-10 (LDA and B-LYP GC [223]). Reference [223] (see Appendix C) presents an extensive structural study using CP molecular dynamics as well as local optimization procedures, and is corroborated by electronic and vibrational spectra as well as by an extension to cations and anions, thus allowing comparison... 

In the Re and Tc dichalcogenides there are three excess d electrons that must be localized in order to produce non-metallic properties. These compounds are diamagnetic semiconductors with energy gaps near 1 eV. Thus, the cation clustering is evidently sufficient to secure non-metallic properties. 

The second-order difference in total energy (A ) can be considered as a measure of relative stability of clusters. Accordingly, a high value of A indicates a higher stability of the size as compared to its two left and right neighbors. From Fig. 6b, a consistent odd-even oscillation is easily found for the curves of A . The closed-shell systems with an odd number of B-atoms for cation BlJ" (with even number of B-atoms for neutral B ) reveal local maximum peaks. Interestingly, the A plot of cationic clusters B shows the enhanced peaks at n = 5 and 13, while the closed-shell systems Bg" " and Big correspond to local minimum peaks. These observations are consistent with experimental results of mass spectroscopy previously... 

In the second cluster, the two Ru6 octahedra are linked through two palladium atoms. The third cluster contains two additional palladium atoms. The Pd4 skeleton adopts the form of a bent square. The two Ru6 octahedra have local structures similar to those in the second cluster, but their relative orientation is now twisted. Apparently, formation of these heterometallic cluster complexes does not result from a simple combination reaction between cationic and anionic complexes but is accompanied by partial redox reactions.900... 

The electrostatically favored cation (Li) and anion (RE) arrangement implies the presence of two different E-, Si- and Li sorts, which has been established by solution and solid-state NMR spectroscopy. The electronic structures of the mixed-valent pnictides 10 and 11 have been simply described as electron-deficient clusters with delocalized framework electrons. Formally the latter consist of two low-valent anediyl moieties RE and eight andiides (RE)2- (E = P, As). The relatively large E-E distances of >4 A exclude the occurrence of localized E-E bonds. However, delocalization of the cluster valence electrons is achieved without Li-Li bonds via Li-mediated multiple bonding. Evidence for this has been seen in the NMR spectra (31P, 7Li, 29Si), which are in accordance with the electron delocalization model (see later discussion). 

Photoinduced deposition of various noble metals onto semiconductor particles has been extensively reported [310-315]. Several factors are controlling this reaction. To control the morphology of metal clusters with desired size and distribution pattern on a given surface area of titania, the most relevant factors are the surfactant, pH, local concentration of cations, and the source of cation [316], In the case of the Ag clusters, the reaction steps proposed include the creation of electron (e )-hole (p+) pairs, the reaction of holes with OH surface species, and the reaction of electrons with adsorbed Ag+ ions ... 

The calculation shows that the 2 2 2 and the 4 3 2 Willis clusters are essentially interstitial dimers stabilized by a coupled interstitial-lattice relaxation mechanism Oxidized (U ) cations localized at cation sites adjacent the cluster are also needed ... 

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