Crystals Diffracting

Many scientifically and teclmologically important substances caimot be prepared as single-crystals large enough to be studied by single crystal diffraction of x-rays and, especially, neutrons. If a sample composed of... 

Flarker D and Kasper J S 1948 Phases of Fourier coefficients directly from crystal diffraction data Aota Crystallogr. 70-5... 

The Wealth of Information from Single-Crystal Determinations. The amount of information that is determined from a crystal stmcture experiment is much greater and more precise than for any other analytical tool for stmctural chemistry or stmctural molecular biology. Indeed, almost all of the stmctural information that has been deterrnined for these two fields has been derived from x-ray single crystal diffraction experiments. 

Bragg-Brentano Powder Diffractometer. A powder diffraction experiment differs in several ways from a single-crystal diffraction experiment. The sample, instead of being a single crystal, usually consists of many small single crystals that have many different orientations. It may consist of one or more crystalline phases (components). The size of the crystaUites is usually about 1—50 p.m in diameter. The sample is usually prepared to have a fiat surface. If possible, the experimenter tries to produce a sample that has a random distribution of crystaUite orientations. 

The single-crystal diffraction technique is much more elaborate and gives much richer information. The first task is to grow a perfect single crystal of the sample. Whereas that task is usually straightforward for simple inorganic solids, it... 

X-ray was discovered in 1895 by W.K. Rontgen. After its discovery, studies of this radiation were expanded when in 1912 Laue and Friedrich found that the atoms in crystals diffracted... 

The possibility of obtaining single crystal diffraction patterns from regions of very small diameter can obviously be an important addition to the means for investigating the structures of catalytic materials. The difficulty arises that data on individual small particles is usually, at best, merely suggestive and at worst, completely meaningless. What is normally required is statistical data on the relative frequencies of occurrence of the various structural features. For adequate statistics, it would be necessary to record and analyse very large numbers of diffraction patterns. 

There are many variants of this system which can be envisaged as means by which the current possibilities for automation in data collection can be applied for specific purposes. There are considerable dangers in this approach in that it may be all too easy to build in restrictions which predetermine the results. These dangers, however, are not likely to be worse than those normally encountered in electron microscopy or single crystal diffraction where the one particularly "good-looking picture is taken as being "typical" of a sample. 

In addition to microwave plasma, direct current (dc) plasma [19], hot-filament [20], magnetron sputtering [21], and radiofrequency (rf) [22-24] plasmas were utilized for nanocrystalline diamond deposition. Amaratunga et al. [23, 24], using CH4/Ar rf plasma, reported that single-crystal diffraction patterns obtained from nanocrystalline diamond grains all show 111 twinning. 

Stewart, R.F. and Craven, B.M. (1993) Molecular electrostatic potentials from crystal diffraction The Neurotransmitter y -aminobutyric acid, Biophys. J., 65, 998-1005. 

Blessing, R.H. (1987) Data reduction and error analysis for accurate single crystal diffraction intensities, Cryst. Rev., 1, 3-58. 

The facial complexes (PMe3)3lr(CH3)(H)(SiR3), (55), (R = EtO, Ph, Et) result from the oxidative addition of the corresponding silane to MeIr(PMe3)4.69 On heating (55) in which R = OEt and Ph, reductive elimination of methane forms iridasilacycles, as shown in reaction Scheme 6. The structure of compound (55) in which R = Ph is confirmed by single-crystal diffraction studies. 

Photolysis of complex (189), R = mesityl, causes geometric isomerization to (190), with the structure (190) established by single-crystal diffraction studies.354 Reaction of Ph2P(CH2)2Si(Me)2-Si(Me)2(CH2)2PPh2 with t/Y// ,v-[Ir(PPh3)2(CO)Cl] yields complex (191) via an oxidative addition reaction to the Si-Si linkage.355... 

X-ray single-crystal diffraction yields precise three-dimensional structure, bond distances, and angles for small molecules with the same information... 

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