Cluster-Specific Mechanisms

2 Cluster-Specific Mechanisms Electronic Size Effects 

Analogous effects have also been discussed for the reactivity of different bulk metal surfaces, where the center of gravity of the d-band determines the bonding [350]. In this case, the effects on the reactivity are considerably smaller than in the case of clusters in the nonscalable size regime and are size independent. Additional factors that change the d-band structure for a particular element are strain effects and the crystallographic orientation of the respective surface plane (see Chap. 3). 

Coadsorption phenomena in heterogeneous catalysis and surface chemistry quite commonly consider competitive effects between two reactants on a metal surface [240,344]. Also cooperative mutual interaction in the adsorption behavior of two molecules has been reported [240]. Recently, this latter phenomenon was found to be very pronounced on small gas-phase metal cluster ions too [351-354]. This is mainly due to the fact that the metal cluster reactivity is often strongly charge state dependent and that an adsorbed molecule can effectively influence the metal cluster electronic structure by, e.g., charge transfer effects. This changed electronic complex structure in turn might foster (or also inhibit) adsorption and reaction of further reactant molecules that would otherwise not be possible. An example of cooperative adsorption effects on small free silver cluster ions identified in an ion trap experiment will be presented in the following. 

In this example, the observed adsorption of multiple oxygen molecules onto negatively charged odd size Ag clusters will be related to cooperative adsorbate effects. The reaction behavior of free Ag with O2 is in marked 

Adsorption of two O2 molecules as observed in this experiment for Ag had been predicted before for the gold cluster anions [356] however, it was never observed experimentally which lead to further discussion in the literature [357,358]. In combination with a systematic theoretical study performed by the group of Bonacic-Koutecky [351] an understanding of the measured rate constant evolution with cluster size depicted in Fig. 1.58a emerges and reasons for the distinct behavior of silver cluster anions can be given [351]. 

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Model of a supramolecular structure of polymolecular ensembles or clusters, determined by interaction and mutual arrangement of the forming molecules. At this level, the specific mechanisms of supramolecular chemistry, including molecular recognition, self-assembly, etc. [4] can be allocated. In most cases, it is possible to limit this area to objects with the sizes under 1 to 2 nm, since further increase in the sizes admits application of statistical concepts like phase and interphase surface. 

The biological and physicochemical data relevant to a certain project may be represented as two tables and may be analysed in various ways (see Fig. 22.3). Taking biological or physicochemical data either separately or combined, pattern recognition or classification studies may be useful to detect redundancy in the test systems or classify the compounds in a particular way which may be related to their specific mechanism of action. Clustering and classification of compounds based on their properties is central to molecular similarity smdies. Regression or correlation studies between the biological and chemical data are of... 

Future development of SAM-based analytical technology requires expansion of the size and shape selectivity of template stmctures, as well as introduction of advanced chemical and optical gating mechanisms. An important contribution of SAMs is in miniaturization of analytical instmmentation. This use may in turn have considerable importance in the biomedical analytical area, where miniature analytical probes will be introduced into the body and target-specific organs or even cell clusters. Advances in high resolution spatial patterning of SAMs open the way for such technologies (268,352). 

Quantum mechanics is essential for studying enzymatic processes [1-3]. Depending on the specific problem of interest, there are different requirements on the level of theory used and the scale of treatment involved. This ranges from the simplest cluster representation of the active site, modeled by the most accurate quantum chemical methods, to a hybrid description of the biomacromolecular catalyst by quantum mechanics and molecular mechanics (QM/MM) [1], to the full treatment of the entire enzyme-solvent system by a fully quantum-mechanical force field [4-8], In addition, the time-evolution of the macromolecular system can be modeled purely by classical mechanics in molecular dynamicssimulations, whereas the explicit incorporation... 

It should be noted that the above classification system of technetium cluster compounds is not the only possible one. In section 4 another classification is described, which is based on thermal stability and the mechanism of thermal decomposition. Section 2.2 is concerned with the classification based on methods of synthesizing cluster compounds. The classifications based on specific properties of clusters do not at all belittle the advantages of the basic structural classification they broaden the field of application of the latter, because for a better understanding and explanation of any chemical, physico-chemical and physical properties it is necessary to deal directly or indirectly with the molecular and/or electronic structures of the clusters. 

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