Crystallographic techniques neutrons, technique

Although we felt the distances in the Pt-oxo unit were unequivocal based on the disorder-free X-ray structures, we endeavored to use an additional structural method to assess this unprecedented structure, neutron diffraction. As an independent structural technique, neutron diffraction can determine not only the absolute structure of molecules but also the location of hydrogens. The latter are almost never located in even the best X-ray crystallographic structure determinations of polytungstates. 

The vast majority of crystal structures published in the literature have been solved using X-ray diffraction. However, it is also possible to use neutron diffraction for crystallographic studies. It is a much less commonly used technique because very few sources of neutrons are available, whereas X-ray diffractometers can be housed in any laboratory. It does have advantages for certain structures, however. 

The crystallographic study of organic conductors has involved a number of techniques. Three main types of radiation—x-rays, neutrons, elec-... 

The techniques of single-crystal diffractometry have been discussed by Arndt and WUlis (51). We should note that extinction is a very serious problem in the determination of accurate magnetic intensity data from single crystals. Although extinction must always be accounted for in conventional crystallographic studies, it is particularly important to make proper correction in polarized neutron experiments where the ratio of magnetic to nuclear structure factors is determined. 

Some authors have postulated the existence, beside ordinary H-bonds, of bifurcated H-bonds or three-centre H-bonds that can be occasionally encountered and consist of a single X-H donor group that simultaneously interacts with two acceptor groups Y and Y. These multicentred H-bonds lose the properties of directionality of ordinary H-bonds. These authors are mainly crystallographers (2 pp. 136-146) who measured, by X-ray or neutron diffraction, distances X- Y and X- -Y that are nearly the same in an appreciable number of systems. The existence of such H-bonds has never been experimentally put into evidence by other techniques, particularly IR spectroscopy, the most sensitive method to observe H-bonds (Chs. 4 and 5). It has been occasionally used as an ad hoc explanation of some... 

The architecture of the ribosome is being studied by a variety of physical techniques such as electron microscopy (figure 3.20(b)) and neutron, as well as X-ray, diffraction and NMR. Some progress has been made in the X-ray crystallographic analysis of some of the individual proteins in the ribosomal subunits (for a review see Liljas (1982)). These proteins are, however, very difficult to crystallise. The structures of some of the smaller ones are now also being determined by two-dimensional NMR methods. (For a review of two-dimensional NMR see Wuthrich (1986).)... 

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