Cluster-size selectivity

Many factors and considerations are germane to future research in this area. On the technical side, achieving cluster size selection stands as one of the most important and sought-after goals. It would be most desirable to achieve this while maintaining sufficiently high densities for studies of photoinitiated reactions to be carried out with product state resolution and/or ultrafast time resolution. The two methods that, in our opinion, are most viable are molecular beam deflection, as pioneered by Buck (1994) and coworkers, and laser-based double-resonance methods. Less direct approaches are deemed inferior. 

Nickel reacts readily with hydrogen, but without significant cluster-size selectivity.Robata et al. found that hydrogen chemisorption on Ni(lll)... 

Iron clusters exhibit facile chemisorption toward methanol, the reaction proceeding with little or no cluster-size selectivity. An interesting feature of this system is that the chemisorption rate constants are nearly identical toward various isotopic sjjecies (CH3OH, CH30D,CD3 0H). If dissociation of a C—H or O—H bond was the initial step, then this should be manifested in an observable kinetic isotope effect. Thus the initial chemisorption step most likely involves the lone-pair orbital localized on the oxygen atom. More extensive studies of the chemistry of the Fe methanol system have been explored using infrared multiple-photon dissociation spectroscopy. These results are discussed in detail in Section Vlll. 

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

Buck U, Gu X, Lauenstein C and Rudolph A 1988 Infrared photodissociation spectra of size-selected (CHjGH) clusters from / = 2 to 8 J. Phys. Chem. 92 5561... 

Wang L S, Cheng H S and Fan J 1995 Photoeieotron spectroscopy of size-selected transition metal clusters Kc , n = 3-24 J. Chem. Phys. 102 9480... 

Repeat the studies, selecting intermediate temperature values using the relationships between Pq and J values for water shown in Table 3.2. For example, use Pb(WW) = 0.50 and J(WW) = 0.71 in Example 3.2. Compare the fx values with the Studies in Examples 3. land 3.2. A plot of each value from Studies in 3.1 and 3.2 will reveal the influence on these attributes with and without a density consideration. Also compare the average cluster sizes between these two groups of studies. How much difference is found in the fx values when water density is accounted for ... 

Buch, U. and Huisken, F. (2000) Infrared spectroscopy of size-selected water and methanol clusters. Chem. Rev., 100, 3863-3890. 

Over single crystal surfaces with defect sites, vacuum deposition of gold vapor or size-selected gold anion clusters at low temperatures can lead to relatively homogeneous... 

This section summarizes the results of size-selective synthesis of the Au SR clusters formed in reactions (l)-(3) of Figure 1. 

Figure 9.8 Relative reactivities (a) and selectivities (b) of palladium cluster size in the formation of C6H6, C4H8 and C4H6 in the polymerisation of acetylene. (Reproduced from Ref. 29).

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... 

Platinum clusters, n = 2-11 react with di oxygen at a rate that is within an order of magnitude of gas kinetic. There is no distinct size selective behavior. Products of these gas phase reactions observed with 7.87 eV ionization laser, are PtpO where for m=l,... 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

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