Coordination shells

As well as charge-ordering in the system, out to two chloride shells, the specibc local structure shows strong interactions between the chloride and the ring hydrogens, as well as some interaction between the methyl groups of adjacent imidazoli-um cations. This is consistent with the crystal structure and implies that the molecular paclbng and interactions in the brst two or three coordination shells are similar in both the crystal and the liquid. 

C. Hydrated monovalent cation approaching an area of the membrane where an amphiphilic carrier is located with its lipid side in contact with the lipid layer and with polar oxygens directed outward into solution. On close approach to the carrier, water molecules in the first coordination shell become replaced by carrier oxygens. As the ion becomes enclosed, the carrier moves into the lipid layer. 

D. Hydrated monovalent cation approaching the carbonyl oxygens of a transmembrane channel. The carbonyl oxygens at the mouth replace water in the first coordination shell. As the ion moves through the channel, it retains one bound water molecule preceding and following it and the walls of the channel provide for lateral coordination. (Parts A through D reproduced with permission from Ref. 6>. 

There had been doubts about the utility of palladacycles in asymmetric catalysis, raised by the failure to achieve enantioselectivity as a result of a slow release of low ligated Pd(0) (naked Pd) [54]. However, recent success of several planar chiral palladacycles in highly enantioselective aza-Claisen reactions and in a number of other applications proves that the coordination shell of the Pd(II) species is not necessarily destroyed during the catalytic action. 

In a mono-atomic solid, the EXAFS function x k) is the sum of the scattering contributions of all atoms in neighboring coordination shells ... 

Each coordination shell contributes a sine function multiplied by an amplitude, which contains the number of neighbors in a coordination shell, Nj, as the most desirable information ... 

As regards intimate mechanism, electron transfer reactions of metal complexes are of two basic types. These have become known as outer-sphere and inner-sphere (see Chapter 4, Volume 2). In principle, an outer-sphere process occurs with substitution-inert reactants whose coordination shells remain intact in... 

The Cr(II) reduction of H2O2 involves transfer of one oxygen atom from the peroxide into the coordination shell of the resulting hexaaquochromium(III)" It would seem that 0-0 fission is an inner-sphere process, viz. 

Theories of EXAFS (26.30) based on the scattering of an ejected photoelectron by atoms in the coordination shells surrounding the central absorbing atom give an expression for xW of the following form ... 

The copper EXAFS of the ruthenium-copper clusters might be expected to differ substantially from the copper EXAFS of a copper on silica catalyst, since the copper atoms have very different environments. This expectation is indeed borne out by experiment, as shown in Figure 2 by the plots of the function K x(K) vs. K at 100 K for the extended fine structure beyond the copper K edge for the ruthenium-copper catalyst and a copper on silica reference catalyst ( ). The difference is also evident from the Fourier transforms and first coordination shell inverse transforms in the middle and right-hand sections of Figure 2. The inverse transforms were taken over the range of distances 1.7 to 3.1A to isolate the contribution to EXAFS arising from the first coordination shell of metal atoms about a copper absorber atom. This shell consists of copper atoms alone in the copper catalyst and of both copper and ruthenium atoms in the ruthenium-copper catalyst. 

This discussion of EXAFS on ruthenium-copper clusters has emphasized qualitative aspects of the data analysis. A quantitative data analysis, yielding information on the various structural parameters of interest, has also been made and published (8). Of particular Interest was the finding that the average compo tion of the first coordination shell of ruthenium and copper atoms about a ruthenium atom was about 90% ruthenium, while that about a copper atom was about 50% ruthenium. Details of the methods Involved in the quantitative analysis of EXAFS data on bimetallic clusters can be obtained from our original papers (8.12-15). 

There appears to be concentration of rhodium in the surface of the iridium-rhodium clusters, on the basis that the total number of nearest neighbor atoms about rhodium atoms was found to be smaller than the nunber about iridium atoms in both catalysts investigated. This conclusion agrees with that of other workers (35) based on different types of measurements. The results on the average compositions of the first coordination shells of atoms about iridium and rhodium atoms in either catalyst Indicate that rhodium atoms are also incorporated extensively in the interiors of the clusters. In this respect the iridium-rhodium system differs markedly from a system such as ruthenium-copper (8), in which the copper appears to be present exclusively at the surface. 

In a recent paper we used the temperature sequence of EXAFS measurements of the reduced catalyst In H2 to determine the temperature dependence of the disorder. (7 ) Comparable data was obtained for Ft metal over the same temperature range. The analysis proceeded by fitting the 1st coordination shell catalyst data to a 2-shell model In which the 1st shell was assumed to be that part of the Ft cluster... 

A. The structure of the first coordination shell in liquid water. Science 2004, 304, 995-999. 

Such a function exhibits peaks (Fig. 9C) that correspond to interatomic distances but are shifted to smaller values (recall the distance correction mentioned above). This finding was a major breakthrough in the analysis of EXAFS data since it allowed ready visualization. However, because of the shift to shorter distances and the effects of truncation, such an approach is generally not employed for accurate distance determination. This approach, however, allows for the use of Fourier filtering techniques which make possible the isolation of individual coordination shells (the dashed line in Fig. 9C represents a Fourier filtering window that isolates the first coordination shell). After Fourier filtering, the data is back-transformed to k space (Fig. 9D), where it is fitted for amplitude and phase. The basic principle behind the curve-fitting analysis is to employ a parameterized function that will model the... 

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