Mixed-metal clusters, molecular dynamics

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

CP dynamics has also been very useful technique for estimation of the connectivities in the porous solids. Classic example in this case would be a clear distinction between Q2, Q3 and Q4 silicate sites in the molecular sieves on the basis of not only the chemical shift but also Tcp times which, as a result of no protons connected directly are much longer for Q4 sites (Si(OSi)4). Introduction of guest species in the pores has a dramatic effect on the CP-dynamics of the different silica sites. Thus, in the MCM-41 impregnated with the mixed metal clusters/counter-cation species, presence of embedded organic counter-cation lead to a significant reduction of Tcp and Tiph due to the presence of a different source of protons at the porous silica surface.21... 

Molecular Dynamics of H FeR CCO)-] and Related Mixed-Metal Clusters. Metal clusters have been shown to undergo a wide variety of fluxional processes in which carbonyls, hydrides, and even the metals themselves undergo rearrangement (25). Mixed-metal clusters are ideally suited for studies of fluxional processes because of the low symmetry which is inherent within their metal framework. In such clusters, the majority of... 

For metal clusters, it is now possible, through first principle theoretical (calculational) approaches, to predict and better understand vibrational spectra, optical band gaps, polarizability, quantum confinement, and stmctural predictions. One modern approach is to use pseudopotential density functional methods (PDFM), in particular to predict optical and dielectric properties. Similarly, using molecular dynamics simulations, it is possible to create models for cluster structures. This has been especially valuable for predicting a three-dimensional image for mixed metal clusters. Figure 6 illustrates computed stmctures for Cu-Ru bimetallic clusters. Note that in this case the dynamics simuiation predicted an enrichment of Cu at the edges and corners of the polyhedral structure. Indeed, this prediction was supported by later experimental catalysis data. 

However, the importance of temperature for interpretation of the experimental data should not be underestimated. That is, the presence of energetically close-lying isomeric forms corresponding to local minima on the shallow ground state energy surfaces is typical of small pure and mixed metallic clusters. Therefore, if the spectrum has not been recorded at very low temperature, it is not obvious whether the line broadening arises from thermal vibrations or from the contribution of different isomers. Advances in ah initio molecular dynamics techniques allow one to determine the temperature... 

In this paper we will show that quantum chemistry provides suitable approaches to extracting the specific properties of small metallic and mixed nonstochiometric clusters which cannot be obtained by more approximate methods. This accuracy is needed for controlling the properties through size, shape and composition of the cluster. For this purpose the methodological aspects will be briefly sketched in Section 2.2. We will first address the methods used for the determination of cluster structures at zero temperature and outline the ab initio molecular dynamics method which we developed for the determination of temperature-dependent ground-state properties. Then, ab initio methods for calculation of excited states valid at T = 0 will be described. 

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