Disorder, occupational, determination

However, given the uncertainty in the experimental determinations and the necessary assumption that the best LEED pattern corresponds to complete occupation of all available sites, the unambiguous assignment of a coverage to the 2 x 2 structure can only be done with a LEED intensity calculation. Order-disorder transitions at elevated temperatures have been reported for the 2 x 2 structures formed on Ru(001) (148) as well as on Rh(l 11) (146). In the latter case this process is irreversible. 

Toraya s WPPD approach is quite similar to the Rietveld method it requires knowledge of the chemical composition of the individual phases (mass absorption coefficients of phases of the sample), and their unit cell parameters from indexing. The benefit of this method is that it does not require the structural model required by the Rietveld method. Furthermore, if the quality of the crystallographic structure is poor and contains disordered pharmaceutical or poorly refined solvent molecules, quantification by the WPPD approach will be unbiased by an inadequate structural model, in contrast to the Rietveld method. If an appropriate internal standard of known quantity is introduced to the sample, the method can be applied to determine the amorphous phase composition as well as the crystalline components.9 The Rietveld method uses structural-based parameters such as atomic coordinates and atomic site occupancies are required for the calculation of the structure factor, in addition to the parameters refined by the WPPD method of Toraya. The additional complexity of the Rietveld method affords a greater amount of information to be extracted from the data set, due to the increased number of refinable parameters. Furthermore, the method is commonly referred to as a standardless method, since the structural model serves the role of a standard crystalline phase. It is generally best to minimize the effect of preferred orientation through sample preparation. In certain instances models of its influence on the powder pattern can be used to improve the refinement.12... 

The technique of channeling-enhanced X-ray emission (CHEXE) has enabled cation site occupancies to be determined in various minerals, including transition metal ions in spinels and ferromagnesian silicates (Taftp, 1982 Taftp and Spence, 1982 Smyth and Taftp, 1982 McCormick etal., 1987). The method, which is based on relative intensities of X-ray peaks measured on crystals with diameters as small as 50 nm under the electron microscope, is particularly useful for determining site occupancies of minor elements with concentrations as low as 0.05 atom per cent in a structure. The most important criterion for the determination of element distribution in a mineral by this technique is that the cation sites should lie on alternating crystallographic planes. In order to make quantitative site population estimates, additional information is required, particularly the occupancy of at least one element in one of the sites or in another site that lines up with one of the sites of interest (McCormick et al., 1987). For example, cation site occupancies by CHEXE measurements have been determined from X-ray peak intensity ratios of Si to Ni, Mn, Cr and Fe in forsterite, as well as thermal disordering of these cations in heated olivines (Smyth and Taftp, 1982). 

The oxygen atom was found to be disordered at the crystallographic inversion center and therefore refined with half occupation factors on each side of the B-B bond (model A, Space Group C2/c, No. 15). This disorder is connected with the centrosymmetry in the crystal lattice, which is also strictly valid for the C(SiMe3)3 substituents. In spite of the disorder of the oxygen atom in the crystal, the bond length of the B-B bond could be determined with satisfactory precision [1.601 (7)... 

X-Ray structures of bis(thiophenes) and bis[benzo(7)thiophenes] have been determined. In compounds 1,4-bis-(2-thienyl)butadiyne 34a, both the thiophene rings are disordered. Thus the position of sulfur and carbon atoms in thiophene rings, adjacent to the diacetylenic backbone, are interchangeable but with unequal occupations. The thiophene rings are planar and the dihedral angle between them is 65.6°. The diacetylene chains are inclined to the shortest axis, that is, the -axis, by 40.5°, and the perpendicular distance between the adjacent chains is 3.823 A, as against the respective values of 45° and 3.4 < Aj < 4.0 A, required for solid-state polymerization. 

Here Ti(T) ordered implies calculation of the sum [Zj0H ny(T)(r/6)] in (6) using the determined values of equilibrium occupation numbers ni(T), n2(T) and n3(T), while Ti(T) disordered implies replacement of the above sum by the expression [c Yj0H (r7 6)] at all temperatures. As it can be seen from Fig. 4b the difference delta seems to be very small to be registered in the measurements. 

The first structure of yMIPS was solved with only partial occupancy of NAD+ in the active site. Residues 352-409 were not built into the model and were thought to be disordered in the structure (Stein and Geiger, 2002). This encompasses al3, al4, al5, and (315. Subsequent structures were determined both of the apo enzyme and the fully occupied NAD+-bound enzyme. yMIPS bound to NAD+ has been crystallized and its structure determined from three crystal forms, a C2 form with two independent molecules in the asymmetric unit, a P2j2j2 crystal form with four molecules in the asymmetric unit, and a P2j form, also with four monomers per asymmetric unit (Jin and Geiger, 2003, Kniewel et al., 2002). 

Many minerals contain minor elements whose distribution among possible sites can be ordered or disordered. Olivine, for example, in addition to Mg, Si, and O usually contains Fe and smaller amounts of Mn and Ni, which substitute for Mg atoms in the two nonequivalent Ml and M2 sites. The distribution of Mg, Fe, Mn, and Ni in Ml and M2 sites in a specimen of San Carlos olivine was determined by Tafto and Spence (1982) using ALCHEMI, and more recently, McCormick, Smyth, and Lofgren (1987) have used the same technique to determine the site-occupancies of these minor elements in synthetic olivines as a function of equilibration temperature. Recall from Section 7.4 that ALCHEMI is not fully quantitative for the determination of site-occupancy in some structures because of the so-called localization effect. However, because these studies of olivine indicate that planar ALCHEMI can be used with confidence in this important mineral, it is therefore appropriate to consider this particular application of the technique as an example. 

Clinical history should include both medical and occupational history to determine a relationship between exposure to a specific substance and development of respiratory hypersensitivity. Relevant information includes aggravating factors both in the home and workplace, the onset and progress of the disease, family history and medical history of the patient in question. The medical history should also include a note of other allergic or airway disorders from childhood, and smoking history. 

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