Space groups visualization

Witnauer (27,28) determined, from visually estimated intensities, that filaments of hydrated KBr-amylose have the space group 432 2 (D ) with cell constants as a = b = 10.7A, c = 16.1A. 

Nirmala and Gowda (2) have reported that tolbutamide crystallizes in the orthorhombic space group Pn2 a, with a=20.16 (2), b=9.05 (1), C=7.85 (1) A0, Z=4, Dc=1.25, Dm=1.24 Mgm 3 and U (CuKa)=1.968 mm l. The structure was solved by the Patterson search method and refined to an R factor of 0.083 for 615 visually measured reflections. The structure is stabilized by hydrogen bonding between the polar group and Van der waals interactions between the non polar groups. The... 

In a third paper by the Bernard and Holm group, visual studies (in a sand-packed capillary tube, 0.25 mm in diameter) and gas tracer measurements were also used to elucidate flow mechanisms ( ). Bubbles were observed to break into smaller bubbles at the exits of constrictions between sand grains (see Capillary Snap-Off, below), and bubbles tended to coalesce in pore spaces as they entered constrictions (see Coalescence, below). It was concluded that liquid moved through the film network between bubbles, that gas moved by a dynamic process of the breakage and formation of films (lamellae) between bubbles, that there were no continuous gas path, and that flow rates were a function of the number and strength of the aqueous films between the bubbles. As in the previous studies (it is important to note), flow measurements were made at low pressures with a steady-state method. Thus, the dispersions studied were true foams (dispersions of a gaseous phase in a liquid phase), and the experimental technique avoided long-lived transient effects, which are produced by nonsteady-state flow and are extremely difficult to interpret. 

Praseodymium dioxide crystallizes in the fluorite-type structure (space group Fm3m) with four praseodymium atoms and eight oxygen atoms per unit cell. This structure may be visualized easily as an infinite array of coordination cubes (each consisting of a Pr atom at the center with eight O atoms at the corners) stacked so that all cube edges are shared. 

The crystal structure contains some specific molecules, blocks, layers, or chains of atoms or molecules, which may be easily visualized or represented using space group symmetry in a non-standard setting. 

Visualization of space group symmetry in three dimensions... 

Let us take one example to try to visualize more precisely the above discussion. The details of a part of the XRD pattern are shown in fig. 3 for the as synthesized (SYN with tetrapropylammonium template), hydrated (WAT) and four xylene molecules adsorbed per unit-cell (XYL-1) of B-ZSM-5 sample from ref. 35 and 36. The calcined form exhibits a monoclinic structure (P2l/n space group) whereas SYN and WAT forms correspond to orthorhombic phases (Pnma space group). The striking differences in XRD patterns are seen in fig. 3. The p-xylene adsorption is known (27) to take place in two steps. The first one, up to four molecules per u.c. and designated XLY-I, corresponds to the formation of a low coverage complex and the second one (XYL-II) to a maximum of xylene adsorption with 8 molecules per u.c. The location of p-xylene could even be determined in the ZSM-5 channel by careful analysis of powder X-ray diffraction patterns using the trail error method (35) as shown in fig. 4. 

Space Groups as a Tool to Visualize Conformational Variation... 

A very sensitive piezoelectric test (2) was negative and the space group PI assumed. Data were collected at room temperature with Zr filtered MoKa radiation on a Nonius integrating Weissenberg camera. The intensities of approximately 1100 reflections were estimated visually and reduced in the normal manner to structure amplitudes. The crystal was carefully gonio-metered and its dimensions measured and an accurate correction for absorption applied. 

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