Low temperature diffraction

Our knowledge of the channel inclusion complexes of TTP has reached a stage of tantalising incompleteness. The overall structure of the complexes has been determined and appears rather simple and straightforward. However, the major chemical characteristic of these complexes, namely the aromatic nature of the guests, is quite without an established explanation while the structural basis for many unusual details of the diffraction patterns remains outside our grasp. It seems probable that only low-temperature diffraction studies will provide enough additional information to enable us to unravel these conundrums. 

A variety of instmmental techniques may be used to determine mineral content. Typically the coal sample is prepared by low temperature ashing to remove the organic material. Then one or more of the techniques of x-ray diffraction, infrared spectroscopy, differential thermal analysis, electron microscopy, and petrographic analysis may be employed (7). 

On an industrial scale PCI3 is sprayed into steam at 190 and the product sparged of residual water and HCl using nitrogen at 165. Phosphorous acid forms colourless, deliquescent crystals, mp 70. T, in which the structural units shown form four essentially linear H bonds (O - H 155-I60pm) which. stabilize a complex 3D network. The molecular dimensions were determined by low-temperature single-crystal neutron diffraction at 15K.f - ... 

Sample cells include Lindemann/capillary tubes (normally < 1 mm in diameter) and aluminium holders. In the latter, thin aluminium windows sandwich the sample in a cylindrical aluminium sample holder. The diffraction from the aluminium is observed in this case, and may be used as a calibration standard. For low-temperature materials, the aluminium window can be replaced by the polymer Kapton. Beryllium may also be used [14]. Sample volumes of between 50 and 100 pL are typically required. 

This mechanism is also confirmed by X-ray diffraction measurements [94, 100]. It is also mentioned that the absence of reflexes belonging to Li3NbC>4, in X-ray diffraction patterns obtained for mixtures treated at relatively low temperatures, could be explained by the formation of an amorphous material at the very beginning of the process [103]. 

Most surface area measurements are based on the interpretation of the low temperature equilibrium adsorption of nitrogen or of krypton on the solid using the BET theory [33,269,276—278]. There is an extensive literature devoted to area determinations from gas adsorption data. Estimates of surfaces may also be obtained from electron micrographs, X-ray diffraction line broadening [279] and changes in the catalytic activity of the solid phase [ 280]. 

F. Mo, private communication on the low-temperature X-ray diffraction analysis of the 1,2-bis(methylsulfonyl)ethane crystal. 

From X-ray diffraction experiments28 it is known that in the crystalline phase the erythrodiisotactic poly(l,2-dimethyltetramethylene) has a (g+aaa g aaa)n structure as shown in Fig. 13. The bold printed letters in the denotation give the conformation of the CH—CH bond. In agreement with this structure and low temperature solution state spectra of 2,3-dimethylbutane, 3,4-dimethylhexane, and 4,5-dimethyloctane 29 30) in which the CHCH bond rotation is frozen the crystalline signals can be assigned conclusively. Like for the crystalline state of poly(l,2-... 

In addition, an interesting, although negative, result has come from powder diffraction studies of the hexachloro compounds. We have examined Debye—Scherrer photographs of several samples known to contain predominantly hexachlorodibenzo-p-dioxins and have identified the patterns of at least three crystalline phases therein. (There are 10 possible isomers of hexachlorodibenzo-p-dioxin.) These patterns have been checked carefully against the calculated d-spacings and intensities of the 1,2,3,7,8,9-hexa isomer described by Cantrell, Webb, and Mabis (I) and also against an observed pattern supplied by Cantrell and believed to be from the low temperature phase of the same material. Yet to date we... 

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