Coordination number determination

In order to obtain data with reduced temperature smearing, experiments were also carried out at 77 K. However, such experiments could not be carried out in. situ and the catalysts were thus exposed to air before the measurements. EXAFS data of three catalysts with Co/Mo atomic ratios of 0.0., 0.25, and 0.50 were obtained. The results show many similarities with the data recorded in situ and were fitted in a similar fashion using phase and amplitude functions of the well-crystallized model compound M0S2 recorded at 77 K. The results, which are given in Table III, show that the bond lengths for the first and second coordination shell are the same for all the catalysts and identical to the values obtained for the catalyst recorded in situ (Table II). The coordination numbers for both shells appear, however, to be somewhat smaller. Although coordination numbers determined by EXAFS cannot be expected to be determined with an accuracy better than + 20, the observed reduction... 

Fig. 11. Relationship between the apparent first-shell coordination number determined from standard EXAFS analysis and the true coordination number derived from the structures obtained from molecular dynamics simulations for copper particles [adapted from Clausen et al. (34)].

Comparison of coordination numbers determined in the solid state and in solution [133]. 

Coordination and solvation numbers refiect the simple idea that the solvation of ions or molecules consists of a coordination of solute and solvent molecules. The coordination number is defined as the number of solvent molecules in the first coordination sphere of an ion in solution [103], This first coordination sphere is composed only of solvent molecules in contact with or in bonding distance of the ion such that no other solvent molecules are interposed between them and the ion. This kind of solvation is sometimes termed primary or chemical solvation. Coordination numbers, determined by different experimental techniques [103], range in water from approx. 4 for Be to approx. 9 for Th" , although the majority of the values are close to 6 e.g. for AP ). 

The need to take AFAC and inelastic scattering in to account results in a generally reduced accuracy of the coordination number determination in EXAFS. This can in part be overcome by using appropriate standards and a procedure outhned in ref. [3]. The process is described in detail in Section 7.1.4. 

The statistical error valnes obtained from a fit do not represent the true accuracies of the parameters and the error a coordination number determination can be on the order of 10 and 30 %. 

Figure 22.6 Coordination numbers determined by curve fitting analysis plotted as a function of temperature measured in TPRs. ( ) Pd-Pd (metal), (A) Pd-O, ( ) Pd Osurfacei-...

Features due to one Fe-P component per iron at 3.1-3.2 A can be observed. Since only one phosphate is bound to each enzyme molecule, the EXAFS fits require that it be bridging the diiron unit. Que and Scarrow favored an 0,0 -bridging mode for the phosphate in the porcine enzyme (la, 46), by analogy to the several examples of 0,0 -bridging phosphates in synthetic complexes. In contrast, Kauzlarich et at. (48) proposed a terminally bound phosphate for the bovine enzyme, but did not address the discrepancy between the model and the coordination number determined for P in the fit. 

Gandour. R.D. Fronezek. F.R. Gatto, V.J. Minganti. C. Schultz. R.A. White. B.D. Arnold. K.A. Mazocchi. D. Miller. S.R. Gokel, G.W. Solid-state structural chemistry of lariat ether and bible cation complexes Metal ion identity and coordination number determine cavity size. J. Am. Chem. Soc. 1986. 108. 4078-4088. 

As a general rule of thumb, the coordination number determines how many neighbors the ions can bond with, but in practice this is not always the case. For example, when the size difference is very large between the anion and cation. Let s say that the anion is so big that only two anions can physically fit around the cation. Even if it has a coordination number of 4, it can only bond with two ions. 

Nonmetallic inorganic materials are widely used for optical purposes lenses, pigments, interference filters, laser hosts, luminescent coatings, displays, solar cells, fiber optics, lamp bulbs, and tubes. For optical applications use is made of the refractory index, light absorption, luminescence, and nonlinear optical behavior of materials. These are intrinsic but may depend on the concentration of impurities. Refraction index and optical absorptivity in insulators are atomic properties and are only indirectly related to the structure, but the structure affects the selection rules and the term splitting in the atomic chromophores. The coordination number determines the intensity and wavelength of absorption and... 

Table 6.7 Coordination numbers determined from the FZ partial pair correlation functions shown in Fig. 6.8...

Table 7.4 The In-Se coordination numbers determined from both wavelengths. The running coordination numbers were calculated based up to an r value where G(r) was at a minimum (3.15 A)...

The lanthanide-nitrate interaction in anhydrous acetonitrile was investigated by means of conductimetry, vibrational spectroscopy, and luminescence measurements which are very sensitive to changes in the first solvation sphere of the metal ion (Bunzli and Choppin 1989). Conductimetiic data on 0.0001-0.01 M solutions of europium or terbium trinitrate in anhydrous acetonitrile indicate that no dissociation occurs. The vibrational spectra show the presence of coordinated acetonitrile and the nitrato group frequencies are consistent with bidentate anions of approximate C2v local symmetry (Bunzli et al. 1978). When the concentration of nitrate is increased, the formation of [R(N03) ] "b species (n>3) is clearly evidenced in the FT-IR spectra (Mabillard 1983). The coordination numbers determined for Nd, Eu, Tb and Er in solutions containing an excess of nitrate are constant (9.9 0.3) and correspond to the formation of pentanitrato [R(N03)5] species, in which the nitrate ions are bonded in a bidentate fashion and which do not contain any coordinated acetonitrile molecule. The strong nitrate/europium interaction is evidenced when water is added to acetonitrile solutions of europium trinitrate. The first two acetonitrile molecules are quantitatively replaced by water molecules, but the replacement of the remaining solvent molecules is difficult to achieve and requires... 

A comparison of our results to the preferred coordination numbers determined by Brown [35] in 1985 cf. Fig. 13) based on the smaller set of reference structures available at that time shows that for high Ac, the values determined by the approach presented here tend to be higher than those reported by Brown. This is mainly an expected result of the different approach in defining the boundary of the first coordination shell. For lower Ac the results of both studies are not significantly different, as the clear gap between the bond lengths of nearest and second-nearest neighboring shells of anions around a cation make the detailed choice of the threshold value less relevant. 

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