Chemical cyclic cluster

A series of papers by Beran et al. (102-112) was devoted to quantum-chemical studies of the cationic forms of zeolites using cluster models and the CNDO/2 technique. As a rule, cyclic clusters consisting of four or six T04 (T = Si, Al) tetrahedra were considered as models for four-membered (S, sites) and six-membered (S S, and S,r sites) oxygen windows in the zeolite structures. 

Tables 21 and 22 (and Figure 8) review Pt chemical shifts and metal-metal coupling constants of several chain clusters containing platinum, tungsten, nickel and molybdenum [119-121]. Likewise, Table 23 presents similar data for cyclic clusters containing eight metal atoms [120,122]. All these results are used to provide evidence of the structures, the symmetry and the isomerism of the clusters in solution.

Chemical Bonding in Cyclic-cluster Model Local Properties of Composite Crystalline Oxides... 

Janetzko, F., Koster, A. M., 8c Salahub, D. R. (2008). Development of the cyclic cluster model formalism for Kohn-Sham auxiliary density functional theory methods. Journal of Chemical Physics, 128, 024102. 

Fig. 6.—One of the Molecular-ion Clusters Obtained from a Sample of Deuteropermethy-lated. Cyclic /3(l- 2)-Glucans (see Section VI,S). [All high-mass samples give unresolved clusters of this type if the mass spectrometer is operated at low resolution. The peak is 6 mass units wide at half height. The mass is assigned by using the mass marker, which gives marks every 4 mass units, as shown. The center of the peak corresponds to the chemical molecular weight of an [M + NKJ species.]...

Data reduction. We used the log-transformed data in all analyses presented here. The PCA resulted in eight principal components with eigenvalues > 1 and they explained 93.5% of the variation in the original data (Table II). The first three principal components all convey generalized information on chemical structure size (PC 1), degree of branchness (PC 2), and number of cycles (PC 3). PC 1 was positively correlated with all 90 variables (X 32), except for the cyclic variables in which r was as low as. 07 for the 3rd order cyclic variables. PC 2 was positively correlated ( r >. 26) with all cluster variables, but negatively correlated with all path and cyclic variables. PC 3... 

FIGURE 3. Cyclic voltammetric curves for the tetracobaltcarbonyl cluster in thf containing 0.1 M tetrabutylammonium perchlorate. Sweep rate, 200 mVs-1. The inset shows the scans of the first (0/—1) redox couple at 1000,500,200 and 100mVs-1. Reprinted with permission from Reference 27. Copyright 1986, American Chemical Society... 

The first reduction of [Fe3(CO)i2] is now established as a diffusion-controlled, fully reversible process (137-139) giving [Fe3(CO)i2]. The second reduction of [Fe3(CO)x2] is variously described as (1) chemically irreversible in acetone or CH2CI2 at a platinum electrode, leading to cluster fragmentation (137) (2) not fully reversible in dry CH2CI2 even at cyclic voltammetric scan rates as high as 8 V sec (159) and (3) reversible at a scan rate of 100 mV sec" (138). 

Polarographic and cyclic voltammetric studies showed the formation of dianions, with one of the dianionic complexes exhibiting further chemical reactivity. Bulk electrolysis and infrared spectroelectrochemistry were used to confirm the decomposition of the dianionic tetranuclear clusters to stable tricobalt anions. 

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