Reactivity of Molecular Clusters

A recent review by Deeming [4] has considered in detail and in a very comprehensive way, the large field of the reactivity of molecular clusters. One can classify reactions of molecular clusters in two areas depending on 

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Jouvet, C. Lardeux-Dedonder, C. Richard-Viard, M. SoLGADi, D. Tramer, A. Reactivity of molecular clusters in the gas phase. Proton-transfer reactions in Neutral phenol-(NH3) and phenol-(C2H5NH2) . J. Phys. Chem. 1990, 94, 5041-5048. 

If one considers the van der Waals complexes as a way to study binary collisions, the possibility of the formation of clusters of given size is a way to probe the role of the environment of other molecules on the reactivity. It is well known that solvent effects play an important role, not only in the kinetics but also in the results of chemical reaction. The study of molecular clusters in supersonic jet experiments allows step-by-step solvation of reactants as will be shown in this chapter, most of the reactions which have been studied occur when a finite number of molecules is reached—this number being often small (less than ten molecules). 

This concept requires the preparation of molecular clusters covered by reactive organic groups that are capable of binding to organic polymers. The discussion in this article is restricted to metal oxo clusters however, the concept is rather general. 

The reactivity of species with open structures is certainly relatively much higher than that of the equivalent closed derivatives since open polyhedron faces make the interaction of other chemical agents with cluster HOMOs and LUMOs much more probable. That is directly related to the formation of Lewis acid-base mWo-carborane complexes described above. Furthermore the reactivity of open-cluster species is also enhanced by the presence of three-center B-H-B bonds which originate occupied and unoccupied molecular orbitals with relatively high and low energy respectively. This class of compounds is, in general, therefore an excellent intermediate for synthesis. 

Finally, we would like to show two examples where this approximation was used to predict the interaction of molecular oxygen with metallic copper clusters. It has been shown that one can make useful predictions of the binding sites, based on the knowledge of the donor local reactivity of the cluster, by using the condensed Fukui function,. In this way, it was reported that CU3, CU5, and Cus have the highest reactivity toward molecular oxygen. In Fig. 8.5, the results for CU3 (panel a) and for CuJ (panel b) are shown. These results are similar to those reported in Ref. [57]. 

ZEKE spectroscopy has been applied to a wide variety of molecular ions, clusters, van der Waals molecules, free radicals, reactive intermediates, and even to elusive transition states of chemical reactions. Examples of such typical applications of high-resolution ZEKE spectroscopy to molecules and clusters are given here. Compared to conventional photoelectron spectroscopy, ZEKE spectroscopy offers greatly increased spectral resolution, allowing the rotational structure of large molecular cations such as the benzene cation and the intermolecular vibrations of molecular clusters like phenol-water to be obtained. 

ABSTRACT. Recent studies of supported and unsupported naked metal clusters of low-nuclearity are examined with the aim of comparing their structure and reactivity to those of molecular clusters. It is shown that naked clusters can be prepared with a nuclearity comparable to that of molecular clusters. Moreover, their structure is flexible in response to adsorbates which act as the ligands in coordination compounds. Transformations of molecular clusters into naked clusters and vice versa are examined. The reactivities of unsupported clusters (cluster beams) and of low-nuclearity bimetallic clusters involving two complementary active sites are also discussed. 

Anyhow, the proton-induced conversion of carbide atoms to give hydrocarbons, and their corresponding reaction with molecular hydrogen under mild conditions, are indicative of a close analogy in the reactivity of molecular and bulk carbides. Analogous chemical behaviour is indeed shown by bulk metal carbides [113]. The above stoichiometric reactions, in conjunction with the remarkable proton-induced reduction of CO discovered by Whitmire and Shriver [110], allow one to mimic Fischer-Tropsch products with clusters. 

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

Synthesis, molecular dynamics and reactivity of mixed-metal clusters. G. L. Geoffrey, Acc. Chem. Res., 1980,13,469-476 (29). 

The similarity of the reactivity patterns for niobium and cobalt and the non-reacti vi ty of iron with nitrogen suggests that dissociative chemisorption is taking place. Dissociation of molecularly chemisorbed nitrogen is an activated process on all metals(35) and is most exothermic for the early metals in the periodic tab e(36). The limited observations on clusters seems to be consistent with these trends. 

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