As ideal model system

In addition to dendrimers, hyperbranched polymers have been used by several groups as soluble supports for catalysts [7, 17]. These supports are polydisperse and randomly branched, and, since they are prepared in a single reaction step, are generally much cheaper materials. Nevertheless, it has been shown that catalysts immobilized on hyperbranched polymers may possess similar properties as dendritic systems [18]. Therefore, dendritic catalysts serve as ideal model systems for catalysts attached to hyperbranched polymers. We functionalized several hyperbranched polyethyleneimines (PEIs) employing the peptide coupling protocol in reactions with the pyrphos linker system. The pyrphos-rhodium complexes bound to the hyperbranched polymers were also found to be active catalysts for the hydrogenation ofZ-methyl-a-acetamidocinnamate [16]. As observed for the... 

The relative simplicity of CO oxidation makes this reaction an ideal model system of a heterogeneous catalytic reaction. Each of the mechanistic steps (adsorption and desorption of the reactants, surface reaction, and desorption of products) has been probed extensively with surface science techniques, as has the interaction between O2 and CO " . These studies have provided essential information necessary for understanding the elementary processes which occur in CO oxidation. 

In addition, these thin films have been important in studies of electron transfer, relevant for catalytic systems [64], molecular recognition [65], biomaterial interfaces [66], cell growth [67], crystallization [68], adhesion [69], and many other aspects [70]. SAMs provide ideal model systems, because fine control of surface functional group concentration is possible by preparing mixed SAM systems of two or more compounds, evenly distributed over the surface [71, 72], as two- or... 

Mixtures of fulleranes produced by hydrogenation of solid C60 films under atomic H flux have revealed spectral features that bear striking similarity to those observed in the diffuse interstellar medium, both in the far IR and in the UV spectral windows. Of course, one must be cautious not to overextend the interpretation of laboratory data, for a number of reasons firstly, because electron spectroscopy, the experimental technique used in these studies, differs in several important aspects from the spectroscopic methods employed in observational astronomy, and secondly, because of the specifics of specimen preparation and environmental conditions. In this regard, there is a need to explore the stability of fulleranes to energetic and corpuscular radiation (Cataldo et al. 2009). Nonetheless, our findings lend support to the suggestion of fulleranes as candidates for unidentified emission and absorption features of interstellar and circumstellar media. Whether or not they exist in sufficient abundance is still unclear however, their spectral features make them undoubtedly an ideal model system for laboratory studies of these fascinating astrophysical phenomena. 

Abstract Enantioselection in a stoichiometric or catalytic reaction is governed by small increments of free enthalpy of activation, and such transformations are thus in principle suited to assessing dendrimer effects which result from the immobilization of molecular catalysts. Chiral dendrimer catalysts, which possess a high level of structural regularity, molecular monodispersity and well-defined catalytic sites, have been generated either by attachment of achiral complexes to chiral dendrimer structures or by immobilization of chiral catalysts to non-chiral dendrimers. As monodispersed macromolecular supports they provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers, and have been successfully employed in continuous-flow membrane reactors. The combination of an efficient control over the environment of the active sites of multi-functional catalysts and their immobilization on an insoluble macromolecular support has resulted in the synthesis of catalytic dendronized polymers. In these, the catalysts are attached in a well-defined way to the dendritic sections, thus ensuring a well-defined microenvironment which is similar to that of the soluble molecular species or at least closely related to the dendrimer catalysts themselves. 

As has been emphasized at the beginning of this overview of asymmetric den-drimer catalysis, the kinetically controlled stereoselection depends on very small increments of free activation enthalpy. It is therefore an excellent sensitive probe for dendrimer effects and will continue to be studied in this fundamental context. As mono dispersed macromolecules, chiral dendrimer catalysts provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers. 

Systems, for which dimer diffusion plays a particularly important role, are the elementary semiconductor (001) surfaces. As mentioned above, especially the diffusion of silicon on silicon (0 0 1) is studied intensively [4-10]. Silicon (and also germanium) (00 1) surfaces have a relatively small and simple surface unit cell, which makes them ideal model systems to study [34]. Yet even these surfaces display a surprisingly complex behavior. 

In typical surface science experiments as presented previously, oxide-supported metal nanoparticles are deposited onto a clean oxide surface by physical vapor deposition. The precursor in this process is metal atoms in the gas phase, which impinge on the surface, diffuse until they eventually get trapped (either at surface defects or by dimer formation), and then act as nuclei for the growth of larger particles. These processes are well understood for ideal model systems under ultrahigh vacuum (UHV) conditions [56, 57]. In contrast, most realistic supported metal catalyst... 

Intercalated compounds offer a unique avenue for studying the static and dynamic properties of small molecules and macromolecules in a confined environment. More specifically, layered nanocomposites are ideal model systems to study small molecule and polymer dynamics in restrictive environments with conventional analytical techniques, such as thermal analysis, NMR, dielectric spectroscopy and inelastic neutron scattering. Understanding the changes in the dynamics due to this extreme confinement (layer spacing < Rg and comparable to the statistical segment length of the polymer) would provide complementary information to those obtained from traditional Surface-Force Apparatus (SFA) measurements on confined polymers (confinement distances comparable to Rp [36]. 

Einally, X-ray free electron lasers (XFELs) are expected to become operational by 2008 and there is much excitement at the potential they may have for imaging macromolecules with femtosecond X-ray pulses (H du, 2000 Miao et al, 2000, 2001). We consider that the published studies do not fully address the sources of error likely to arise in such analyses and that the case has been overstated (as has also been pointed out by Henderson, 2002). Nevertheless, viruses would be an ideal model system in the development of this method. 

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