Structures of Clusters

In this section, we will investigate the structure of clusters produced when the metal oven is filled with one of the alkaline earth metals Ca, Sr, or Ba. 

The structure of cluster compounds of transition metals and the limits of applicability of the electron counting rules forpolyhedral molecules. Y. L. Slovokhotov and Y. T. Struchkov, Russ. Chem. Rev. (Engl. Transl), 1985, 54, 323 (150). 

It appears that Cluster C catalyzes the chemistry of CO oxidation and transfers electrons to Cluster B, which donates electrons to external acceptors such as ferredoxin. Since a crystal structure of this protein does not exist, the proposed structure of Cluster C is based on spectroscopic measurements. In some cases, the EPR spectrum of a metal center is diagnostic of the type of center. However, the EPR spectra of Cluster C are unusual. The paramagnetic states of Cluster C (Credi and Cred2) have g-values that are atypical of standard [4Fe-4S] clusters (Table III) and are similar to those in a variety of structurally unrelated systems including a t-oxo bridged ion dimer), a [Fe4S4] ... 

UV-visible spectra of the cluster 1 and molybdenum/zeolite catalysts are shown in Figure 1. The cluster 1 showed bands at 300, 390 and ca. 650 nm. Similar bands were observed for the spectrum of each molybdenum/zeolite catalyst, suggesting that the structure of cluster 1 was practically unchanged after ion exchange. 

In order to obtain more structural information about the molybdenum species in Mo/NaY, EXAFS measurements of the cluster 1 and Mo/NaY were carried out. The Fourier transforms of the EXAFS data are shown in Figure 2. Structural parameters (Table 3) showed no change of the Mo-0, Mo-S and Mo-Mo distances, suggesting that there is no significant structural difference between the cluster 1 and the molybdenum compound in the Mo/NaY. From these EXAFS parameters and the UV-visible spectra, it is considered the structure of cluster 1 remained vinually intact after ion exchange. 

A useful introduction to the structures of cluster molecules and the electron counting rules proposed by Wade and others. 

While the structure of clusters responds to directionally specific electrostatic interactions, their stabilization energy reflects also the intervention of less specific dispersion interactions. This is, e.g., the case for stacked DNA base pairs. Stability of these pairs stems from dispersion energy while their structure is determined by dipole-dipole electrostatic interactions. Dispersion energy plays an important role in stabilizing clusters of biomacromolecules, where it may be the dominant attractive term. 

The postsynaptic nerve ending, which is usually the tip of an axonal dendrite, has its own set of proteins, which varies to some extent with the nature of the neurotransmitter. In excitatory cells the plasma membrane of the postsynaptic neuron is thickened to — 30—40 ran to form the "postsynaptic density," a disc-like structure of clustered receptors of two types, which extends 30 ran into the cytosol.593 594 Only single receptor channels are indicated in Fig. 30-20, but many receptors are present in the clusters594 595 as are other specialized proteins. One of these, designated... 

The structure of cluster 914 was proved by x-ray diffraction. The copper complexes with the same ratio M L [Cu(PySe)] and [Cu(6-Me3SiPySe)] were isolated according to a similar scheme, although their structure is unclear [270]. 

Although gas phase clusters can be generated by a number of techniques, adiabatic expansions are the most widely utilized method for the generation of vdW clusters. A wealth of information regarding the energetics, dynamics, and structures of clusters has been recently obtained due to the availability of many new and improved experimental techniques. Although a number of spectroscopic techniques have been utilized in investigating vdW clusters, mass spectrometry (MS) is extensively employed for the study of clusters, as it enables size selective... 

In order to derive positive indications of the importance of many-body contributions to structures of clusters we compare some very general results with experimental data from systems which have structures sufficiently simple to allow a straightforward interpretation. Some crystals will be well suited for this purpose. 

Performance of the Diffusion Equation Method in Searches for Optimum Structures of Clusters of Lennard-Jones Atoms. 

In the following, structures of clusters Mm with m > 13 shall be considered. Although this row, up to m = 38, is rather incomplete, it may be assumed that the structure of larger clusters also follows the principle of closest packings. 

Electron counting rules have been developed to explain the observed geometries of many cluster compounds (see Electronic Structure of Clusters). Interesting, heavy main group element-transition metal complexes tend to violate... 

The polyhedral structures adopted by most metallacarbab-orane compounds can usually be rationalized using polyhedral skeletal electron pair theory (PSEPT) (see Electronic Structure of Clusters). This approach is summarized as follows ... 

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