Transition metal systems, chemical

Our studies on graphite - transition metal systems [11] have shown the methods of chemical deposition of nano-scaled metal particles on the surface of graphite supporter to be powerful technique for production of CM with well dispersed, nanoscaled homogeneously distributed modifier component. 

A range of chemical analogs of the catalytic centers of Mo and W dithiolene-containing enzymes (pterins) have been prepared. In particular, the rich chemistry of multisulfur transition metal systems allows ligand redox, internal electron transfer, and intermediate redox states. Such redox flexibility may facihtate coupled proton/electron transfer and/or 0x0-transfer mechanisms, which are employed by Mo and W enzymes. 

The wealth of quantitative vibrational information available from NRVS also facilitates unusually detailed and quantitative comparison with theoretical predictions. NRVS data on model compounds " " and on proteins have been successfully reproduced by adjusting force constants in empirical potentials. Moreover, NRVS provides a particularly rigorous test of vibrational predictions from quantum chemical models, which may otherwise be difficult to test for transition metal systems. DFT methods have now reproduced vibrational frequencies, amplitudes, and... 

In contrast with empirical models, quantum chemical methods do not provide adjustable force constants. It is therefore not unexpected that quantitative discrepancies appear when quantum chemical predictions are compared in detail with the results of NRVS measurements. NRVS results thus provide a benchmark for development of quantum chemical methods for transition metal systems. Using quantum chemical results as starting input in empirical calculations may be a valuable approach for future work. Meanwhile, however, reproduction is sufficiently accurate to guide the understanding of observed vibrational features. Mode descriptions given in the previous section largely rely on comparison with quantum chemical predictions. 

Quantum Chemical Studies on the Structure and Spectroscopy of Large Transition Metal Systems. ... 

The carbides of the early transition metals exhibit chemical and catalytic properties that in many aspects are very similar to those of expensive noble metals [1], Typically, early transition metals are very reactive elements that bond adsorbates too strongly to be useful as catalysts. These systems are not stable under a reactive chemical environment and exhibit a tendency to form compounds (oxides, nitrides, sulfides, carbides, phosphides). The inclusion of C into the lattice of an early transition metal produces a substantial gain in stability [2]. Furthermore, in a metal carbide, the carbon atoms moderate the chemical reactivity through ensemble and ligand effects [1-3]. On one hand, the presence of the carbon atoms usually limits the number of metal atoms that can be exposed in a surface of a metal carbide (ensemble effect). On the other hand, the formation of metal-carbon bonds modifies the electronic properties of the metal (decrease in its density of states near the Fermi level metal—>carbon charge transfer) [1-3], making it less chemically active... 

The solid-state polymerization of the following classes of monomers is discussed diacetylenes, monoacetylenes, mono- and dienes, ring-opening cyclic monomers, and transition metal systems. With particular emphasis on open questions and current problems, the chemiced, structural, and physical properties of the resultant polymers are sununarized. A brief prognosis is offered. 

Another area where significant work has been done is in the prediction of activation barriers of chemical reactions. Truhlar and co-workers have assembled a set of 44 barrier heights [65] for assessment of the performance of theoretical methods for thermochemical kinetics . Finally, a hmited set of thermochemical information has been collected for molecules containing third-row main group elements and for transition metal systems [27-29,73,74]. 

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