Diffraction methods study examples

Beside bimanes, some other partially saturated systems, mainly 5-6 and 6-6 bicyclic compounds, have been studied by the x-ray diffraction method. For example, (71) <87JA7104>, (72) <8lMi 83i-oi >, (73) <77AX(B)3957>, and (63) <77AX(B)2464>. 

The formation of acyl halide-Lewis acid complexes have been observed by several methods. For example, both 1 1 and 1 2 complexes of acetyl chloride, with AICI3 can be observed by NMR spectroscopy. The existence of acylium ions has been demonstrated by X-ray diffraction studies on crystalline salts. For example, crystal structure determinations have been reported for /i-methylphenylacylium and acetylium ions as SbFg salts. There is also a good deal of evidence from NMR measurements which demonstrates that acylium ions can exist in nonnucleophilic solvents. " The positive charge on acylium ions is delocalized onto the oxygen atom. This delocalization is demonstrated in particular by the short O—C bond lengths in acylium ions, which imply a major contribution from the structure having a triple bond ... 

Experimental equipment for X-ray diffraction methods has improved enormously in recent years. CCD detectors and focusing devices (Goepel mirror) have drastically reduced the data acquisition time. Cryogenic systems have been developed which allow structural studies to be extended down to the liquid helium temperature range. These developments have had important implications for SCO research. For example, fibre optics have been mounted in the cryostats for exploring structural changes effected by light-induced spin state conversion (LIESST effect). Chaps. 15 and 16 treat such studies. 

As an example for the specific case of vanadium alloys with palladium, the trend of the average atomic volume of the alloys is shown in Fig. 4.20 and compared with the phase diagram. These data were obtained by Ellner (2004) who studied the solid solutions of several metals (Ti, V, Cr, Mn, Fe, Co and Ni) in palladium. The alloys were heat treated at 800°C and water-quenched. From the unit cell parameters measured by X-ray diffraction methods, the average atomic volume was obtained Vat = c 14 (see Table 4.3). These data together with those of the literature were reported in a graph, and the partial molar (atomic) value of the vanadium volume in Pd solid solution (Fv)... 

X-ray and neutron diffraction methods and EXAFS spectroscopy are very useful in getting structural information of solvated ions. These methods, combined with molecular dynamics and Monte Carlo simulations, have been used extensively to study the structures of hydrated ions in water. Detailed results can be found in the review by Ohtaki and Radnai [17]. The structural study of solvated ions in lion-aqueous solvents has not been as extensive, partly because the low solubility of electrolytes in 11011-aqueous solvents limits the use of X-ray and neutron diffraction methods that need electrolyte of -1 M. However, this situation has been improved by EXAFS (applicable at -0.1 M), at least for ions of the elements with large atomic numbers, and the amount of data on ion-coordinating atom distances and solvation numbers for ions in non-aqueous solvents are growing [15 a, 18]. For example, according to the X-ray diffraction method, the lithium ion in for-mamide (FA) has, on average, 5.4 FA molecules as nearest neighbors with an... 

Open chain molecules have been widely studied using the electron-diffraction method and with considerable success. But quantitive barrier calculations meet with substantial difficulties. In cases with torsional barriers higher than, say 4 kJ/mol, the electron-diffraction method provides information mainly on the regions of the potential function near the minima. For lower barriers the method is usually not sufficiently sensitive to changes in the assumptions on V(0). If the barrier is, say 2 kJ/mol or less, the electron-diffraction results may in many cases be indistinguish-ably like free rotation. Accordingly in order to use the electron-diffraction method successfully for the study of torsional motion, support as to the choice of potential functions may favorably be obtained from other methods, as for example from microwave spectroscopy. 

Polymeric fullerene materials can be obtained by many methods, for example by irradiation with electrons or ions, treatment in a plasma generator, doping with alkali metals [9,28,46], direct chemical synthesis [47], or mechanical milling [29]. Because of the small penetration depth of light fullerenes can only be photopolymerized as thin films, but bulk photopolymer can be obtained by polymerization in solution [48,49]. Diffraction studies show that photopolymers usually have a disordered fee structure. Although some ordered films have been... 

Although monodentate triazenide ligands were first proposed (erroneously) by Meunier in 1900 150,152) for the copper(I) complex Cu(PhNNNPh), this mode of coordination was not finally confirmed until 1976 when Brown and Ibers 21, 22) reported the X-ray crystal structure of c/s-Pt(PhNNNPh)2(PPh3)2 C6H6 (Fig. 1). Monodentate triazenide ligands remain relatively rare and are mainly confined at present to the four-coordinate d metal ions, Rh(I), Ir(I), Pd(II), and Pt(II). To date only six examples have been characterized by diffraction methods (see Table I). In some of these, notably trans-PtH(p-tol-NNN-tol-pXPPh3)2, there is a short nonbonded contact (2.91 A) between the metal and the terminal (N-3) nitrogen atom 109). Variable-temperature proton NMR studies (see Section... 

Persistent Re(CO)3(PR3)2 radicals have been spectroscopically and even crystallographically characterized, for example, Re(CO)3(PCy3)2 (5), suggesting the square-pyramidal structure to be the most favored one see Diffraction Methods in Inorganic Chemistry). Subcarbonyls hke Re(CO)2 have been studied in rare gas matrices. 

Concentrated HCl(aq) solutions have been studied by X-ray and neutron diffraction methods (Triolo and Narten, 1975 Both etal., 2004). Comparison of our results with those works is problematic because the interpretations are not uniform, and the assumptions seem to preclude some of the questions we address. For example, the interpretations assume rigid H3O+ molecular units. 

Chemical and biochemical reactions can be viewed by diffraction methods provided the reactions are slow and the techniques for measuring them are rapid. For example, Lonsdale and coworkers studied the conversion of a photo-oxide of anthracene on exposure to Cu Ka or Mo Ka X rays at room temperature. A single mixed crystal of anthraquinone and anthrone is formed which still shows crystallinity. The crystal does not change in appearance, and the space group remains P2i/a, but the unit-cell dimensions change ... 

Early data analysis attempted to extract values of the individual structure factors from peak envelopes and then apply standard single crystal methods to obtain structural information. This approach was severely limited because the relatively broad peaks in a powder pattern resulted in substantial reflection overlap and the number of usable structure factors that could be obtained in this way was very small. Consequently, only very simple crystal structures could be examined by this method. For example, the neutron diffraction study of defects in CaF2-YF3 fluorite solid solutions used 20 reflection intensities to determine values for eight structural parameters. To overcome this limitation, H. M. Rietveld realized that a neutron powder diffraction pattern is a smooth curve that consists of Gaussian peaks on top of a smooth background... 

Moreover, the intercalation method was applied to the enantioresolution of racemic alcohols. The amide derivative of cholic add, called cholamide, serves as an excellent host for enantioresolution of 2,2-dimethyl-3-hexanol among secondary aliphatic alcohols. The intercalation method gave highly effective enantioresolution of the alcohol, as in the case of the recrystallization method. For example, dioxane molecules were exchanged with the alcohol molecules in channels of the bilayers with a high enantioselectivity of 96% ee, which was close to the value (98% ee) obtained by the recrystallization method. X-ray diffraction studies proved that the host bilayer framework has an anti-parallel arrangement on the lipophilic side in the case of dioxane, while the framework has a parallel one after intercalation of the alcohol. This indicates that the intercalation accompanies a layer-reversion. Such dynamical enantioresolution confirms the effidency of the intercalation method. 

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