Clusters niobium

Alford J M, Weiss F D, Laaksonen R T and Smalley R E 1986 Dissociative chemisorption of H2 on niobium cluster ions. A supersonic cluster beam FT-ICR experiment J. Rhys. Chem. 90 4480... 

Knickelbein M B and Menezes W J C 1992 Optical response of small niobium clusters Rhys. Rev. Lett. 69 1046 Ceilings B A, Athanassenas K, Lacombe D, Rayner D M and Hackett P A 1994 Optical absorption spectra of AUy,... 

Small ey(Ib) that cobalt and niobium clusters react in a very selective fashion with deuterium was rapidly followed by similar studies on a variety of other metal systems. Three groups simultaneously reported similar dramatic behavior for iron c usters( 2b, 3b). Di hydrogen or di deuteri urn addition reactions have been reported for vanadium( 3e), i ron(lc,2a-d,3b), cobalt(lb),... 

The Exxon group has also reported reactivities of vanadi um(3e). These are shown in Fig. 3. Again the pattern is size selective but not identical to niobium clusters. Vanadi urn has specific inert clusters like Vg, which is similar to niobium s 8 and 10 but the pattern is better described by an even/odd alternation. This suggests possible analogies with the one electron metals like... 

Figure 9. Plots of the rate constants (X) of iron, vanadium and niobium clusters reacting with di hydrogen/di denteri urn, and their respective bare cluster ionization potentials (solid lines) scaled as described in the text.

Niobium and rhodium cluster anions have been prepared by laser vaporization and the reactions with benzene studied by FT-ICR/MS (58). The reactions of the anions and similar cations have been compared. With few exceptions the predominant reaction of the niobium cluster anions and cations was the total dehydrogenation of benzene to form the metal carbide cluster, [Nb C6]-. The Nb19 species, both anion and cation, reacted with benzene to form the coordinated species Nb 9C6I I6p as the predominant product ion. The Nb22 ions also formed some of the addition complex but the Nb2o Nb2i, and all the other higher clusters, formed the carbide ions, Nb C6. ... 

In contrast to the addition of only one benzene molecule to even large niobium cluster anions, small niobium cluster anions react by the addition of up to 5 N2 molecules and up to 15 CO molecules (229). The reactions of the niobium cluster anions Nb (n = 2-7) were qualitatively similar to the reactions of the cluster cations and in both cases CO was more reactive than N2. The rates of reaction generally increased with increasing size of the cluster but in many cases extensive fragmentation of the clusters was observed. 

R. Fournier, T. Pang, and C. Chen, Phys. Rev. A, 57,1 (1998). Structural Characterization of Niobium-Cluster Anions from Density Functional Calculations. 

Last but not least, mention should be made of the use of redox active organometallic clusters for the formation of CT complex. An early example was reported by Eisenberg, Miller and co-workers who exploited the trinuclear niobium cluster [Nb3(/i-Cl)6(>7 -C6Me6)3] and prepared the [TCNQ] salt, upon oxidation to the dicationic form. This material was found to be paramagnetic and to display a relatively modest conductivity (urt = 0.001 Scm , semiconductor) [75]. Finally, the pentanuclear trigonal bipyramidal vanadium cluster [(MeCp)5V5(/r -S)6] was shown to undergo CT complex formation with TCNQ affording [(MeCp)5V5(/i -S)6] [(TCNQ)2] [76]. This material, which was structurally characterized, is an n-type semiconductor with a band gap of 0.25 eV. 

L. Goodwin and D. R. Salahub, Density-functional study of niobium clusters, Phys. Rev. A 47, R774-R777 1993. 

The correlation implicit in Eq. (21) is quantitatively best for clusters of iron and vanadium consisting of more than eight atoms. For the case of niobium clusters, the correlation is more qualitative, but a distinct one-to-one correspondence of local minima in reactivity with local maxima in IP is indeed observed for clusters containing eight or more atoms. 

Figure 12. Relative rate constants (left-hand scale, data points) as a function of cluster size for Dj chemisorption onto iron, vanadium, and niobium clusters. Activation energies, derived from cluster photoionization thresholds [Eq. (21)], are indicated by solid line and right-hand scale. See text for definition of parameters.

FIG. 17. Low-energy structures for niobium clusters, from results of LCAO calculations [146,147]. 

Finally, the overall trend is toward antipathetic structure sensitivity iron atoms (n = 1), which have the highest IP, are not active (17). Of course, for supported small particles the size distribution would not ordinarily permit the observation of the detailed oscillations of Fig. 23. Other results for H2 chemisorption on bare cobalt and niobium clusters have been reported by Geusic et al. (16), and the reaction with 02 and H2S has been studied by Whetten et al. (355a). For the latter, the oscillations as the reactivity increases with n are much smaller than those of Fig. 23. 

Knickelbein M B and Menezes WJ C 1992 Optical response of small niobium clusters Phys. Rev. Lett. 69 1046... 

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