Chromium clusters

Cheng H S and Wang L S 1996 Dimer growth, structure transition and antiferromagnetic ordering in small chromium clusters Phys. Rev. Lett. 77 51... 

Transition, and Antiferromagnetic Ordering of Small Chromium Clusters. 

Symposium on Cluster and Nanostructure Interfaces. P. Jena, S. N. Khanna, and B. K. Rao, Eds., World Scientific, Singapore, 2000, pp. 131-138. Magnetism and Magnetic Isomers in Chromium Clusters. 

A very recent and exciting example of a 3-D coordination polymers can found in the work of Ferey et al,25 on the use of trinuclear chromium clusters in combination with benzene-1,4-dicarboxylic acid. This reaction yields porous structures with unit cells volumes of up to 700 000 A3, i.e. similar to that of a small protein In the absence of single crystals for structure determination, the structures were solved by the ingenious use of Monte Carlo simulations with simulated annealing. 

A further less commonly employed but very classical method for mass-selection is the Wien-filter which combines an electrostatic and a magnetic field for mass-selective cluster deflection. This device is applied in particular with the purpose to obtain large cluster currents for cluster-surface interaction and deposition experiments at moderate resolution [75, 76]. Another approach for size-selection in combination with a sputter source ]47] or a laser vaporization source [61,77,78] has been chosen recently. In both cases, a magnetic dipole field has achieved the size-selection. Subsequent deceleration of the size-selected clusters to less than 1 eV per atom enables soft-landing under UHV conditions [47]. Monodispersed chromium cluster beam densities range from 0.1 to 5nA mm, depending on the cluster size ]47]. 

Some of the chelating ligands used to form chromium clusters are shown in Fig. 8. 

As an important industrial catalyst, the Ti-modified Phillips catalyst is widely used in ethylene polymerization. Recently, the mechanism of ethylene polymerizatiOTi by the Ti-modified Phillips catalyst has been studied theoretically and experimentally [71]. In the DPT calculations, six mononuclear chromium cluster models including three hexavalent chromate sites (16g, 17g, and 18g) and the corresponding divalent chromium sites (4g, 12g, and 15g) were employed to mimic various Ti-modification environments on the surface of the Phillips catalyst, as shown in Fig. 26. Among these cluster models, 16g/4g represented Cr(VI)/Cr(II) sites without Ti-modification. In 17g/12g and 18g/15g, the Ti/Cr atomic ratio was set to 1 1 and 2 1, respectively. 

Inorganic nodes composed by coordinatively unsaturated metal sites, such as accessible copper or chromium clusters in desolvated HKUST-1 or MIL-101, can act as Lewis and even Brmsted acids. Inorganic nodes made from noble transition metal, such as palladiiun, can also act as metal catalyst for carbon-carbon coupling reactions [60]. 

Hachimi, A., Muller, J.F. (1997) Chromium cluster formation under resonant and non-resonant laser ablation. Chemical Physics Letters, 268,485-492. 

Cr Cr ratio after reduction increase with increasing SiOj-content of the support, at least for low Cr-loadings. For higher Cr loadings, chromium clusters are detected (polychromates, Cr203) even on an alumina support. 

Payne, R W., Jiang, W, 8c Bloomfield, L. A. (2006). Magnetism and magnetic isomers in free chromium clusters. Physical Review Letters, 97,... 

M. Reif, L. Glaser, M. Martins, W. Wurth, Size-dependent properties of small deposited chromium clusters by X-ray absorption spectroscopy. Phys. Rev. B 72(15), 155405 (2005)... 

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