Cluster reaction studies

In two studies we have investigated the transformation of the spiked triangular cluster Ru3Pt(//-H)(//4-CCBu )(CO)9(dppe) into the butterfly cluster Ru3Pt( 4-// -C=CHBu )(CO)9(dppe) (Eq. 1) and the formation of the tri-nuclear cluster complex WCo2(CO)8(//-CC6H4Me-4)Cp, from W(CO)2(C6H4Me-4)Cp and Co2(CO)8 in THE(Eq. 2). 

These reactions have desirable properties for such studies in being quantitative (as measured by NMR spectroscopy) and having a unique metal atom from which to view the progress of the reaction (Pt or W respectively). Therefore Pt and W Lm- 

The extraction of individual component spectra from the spectrum of mixtures is less straightforward. Thus the computed spectrum of the product in Eq. (1) was well reproduced (see Fig. 12) in general although the features in the region used for fit- 

Predieri, A. Tiripicchio, A. Vignali, J. Organomet. Chem. 1984, 378, 109. 

Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro P. R. Raithby Copyright WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999 

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Among the first cluster reactions studied were reactions of readily formed dimers. These are usually dimers of open-shell molecules, such as NO2. Wren and Menzinger studied the chemiluminescence resulting from the reaction of Ba( S) with NOj monomers and dimers. They concentrated on the effect of the collision energy on the reaction s cross section. The monomers in this case react with appreciable cross sections even at low collision energy, and their excitation function is bimodal. The cross section for the clusters, in contrast, rises monotonically from a threshold near zero collision energy. 

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]. 

Silicon cluster reactions are an example of a newly emerging field of research which is very amenable to study with electronic structure methods. This exercise will examine the potential surface for silicon cation reacting with silane (SiH4). Such reactions are central to the growth of large silicon clusters, which occurs by sequential additions of -SiHj ... 

Recently, similar findings133 have been made in studies of the photoionization of NO containing water clusters134 which established that the cluster reaction analogous to the last step in reaction 36 occurs for n > 4. 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

The presence of permanent [Fe2S2] clusters in holo-NifU impeded spectroscopic characterization of transient clusters assembled in a NifS-mediated process. However, unambiguous evidence for NifS-directed assembly of oxidatively and reductively labile [Fe2S2] " clusters on NifU-1 was obtained using the combination of UV-visible absorption and resonance Raman spectroscopies (Yuvaniyama et al. 2000). The anaerobic reaction mixture involved NifU ferric ammonium citrate P-mercaptoethanol l-cysteine NifS in a 50 100 5000 1000 1 ratio. The presence of catalytic amounts of NifS facilitated meaningful UV-visible absorption results and monitoring the time course of cluster assembly. Although ferric ammonium citrate was used for these cluster assembly studies, control experiments... 

A number of other groups have investigated the clustering reactions of small cation silicon species with silanes and other small molecules. The ion-molecule reactions occurring between SiHv+ (x = 0-3) cations and neutral ammonia, as well as the reactions between NHt+ cations and SiH4 were studied by FIMS113,114. The main channel for the reaction between SiHv+ ions and NH3 was found to correspond to the elimination-addition reaction, well-known for silanes (equation 20), which formally corresponds to the transfer of a nitrene-unit (NH)113. 

K. Raghavachari, J. Chem. Phys., 92, 452 (1990). Theoretical Studies of Clustering Reactions. Sequential Reactions of SiH3 with Silane. 

Ray Kapral came to Toronto from the United States in 1969. His research interests center on theories of rate processes both in systems close to equilibrium, where the goal is the development of a microscopic theory of condensed phase reaction rates,89 and in systems far from chemical equilibrium, where descriptions of the complex spatial and temporal reactive dynamics that these systems exhibit have been developed.90 He and his collaborators have carried out research on the dynamics of phase transitions and critical phenomena, the dynamics of colloidal suspensions, the kinetic theory of chemical reactions in liquids, nonequilibrium statistical mechanics of liquids and mode coupling theory, mechanisms for the onset of chaos in nonlinear dynamical systems, the stochastic theory of chemical rate processes, studies of pattern formation in chemically reacting systems, and the development of molecular dynamics simulation methods for activated chemical rate processes. His recent research activities center on the theory of quantum and classical rate processes in the condensed phase91 and in clusters, and studies of chemical waves and patterns in reacting systems at both the macroscopic and mesoscopic levels. 

So far the discussion on synthetic clusters illustrates the accumulating information that a variety of nitrogenase substrates can be transformed by simple hydronation reactions at reduced synthetic Fe-S-based clusters. The next level of detail must address the mechanisms of these transformations. Already we have indicated several cases where kinetic studies have been performed. The major problem with the approaches taken so far, looking directly at substrate transformation, is that they can lead to erroneous conclusions. This is because in this approach the experimenter relies on the kinetics to define the number of species essential to accomplish the transformation. For example, the order with respect to hydrons has been established in several of the catalyic systems discussed and invariably found to be one. It is tempting to jump to the conclusion that only one hydron is necessary to activate the cluster. However, studies on the hydronation of Fe-S clusters show that the kinetics of simple hydronation reactions is much more complicated. 

Thus, reactions in metal-containing films are of interest from the standpoint of producing small metal clusters and studying the specific features of their formation and interaction with organic components. In the following sections, these questions will be surveyed as applied to the classical reaction of magnesium with organic halides in films of their co-condensates. 

This survey of higher Os clusters covers their direct formation from Os3 clusters only. Studies of the structures, properties, and reactions of high nuclearity osmium clusters encompass a large area, too big to be considered in this article. 

This combination of experiment and theory has enabled us to elucidate the BR/C2H4 cluster reaction and has encouraged us with further studies of small radical cluster chemistry. Clearly, the synergy between theory and experiment within our own laboratory has afforded this progress and enabled this new work to be conceived and planned. 

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