Single crystals diffraction experiments

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. 

In our laboratory we have obtained a number of inclusion compounds using cadmium(II) cyanide or isopolycyanocadmate(II) as the hosts and several organic molecules as the guests. The host structures so far determined by single crystal diffraction experiments have been classified into three groups in general, 3-cristobalite-like [1], clay-like, and zeolite-like [2]. 

Hydrogen atoms can be located in a single crystal diffraction experiment. 

Metal-hydrogen and hydrogen-hydrogen distances can sometimes be measured to accuracies of 0.005 A or more in a single-crystal diffraction experiment. 

Historically, all the early neutron sources were of the conventional reactor type, and a large monochromating crystal was commonly employed (for single-crystal diffraction experiments) to select neutrons with a small wavelength spread in order to produce a monochromatic neutron beam. The rest... 

As is obvious from columns 2 and 3 in Table 5.24, different indexing algorithms result in different choices of the unit cell for the same lattice and, therefore, unit cell reduction is especially important to compare the results in triclinic symmetry. The unit cell dimensions, reduced using the WLepage program, are listed in Table 5.24 in columns 6-8. Obviously, all of them are represented by the same unit cell, except the incorrect solution shown in row 2. The triclinic unit cell was confirmed by a single crystal diffraction experiment, as shown in row 6. 

All interatomic distances are within normal limits and the Fourier map calculated after the completion of Rietveld refinement confirms that no additional atoms are present in the unit cell of this material. The refined coordinates of all atoms are nearly identical to those determined from a single crystal diffraction experiment for the Sm5Ge4 compound. Thus, we conclude that the crystal structure of GdsGe4 is solved correetly and it belongs to the SmsGe4-type of crystal structure in which Gd atoms occupy Sm-positions and Ge atoms are distributed in the corresponding Ge-sites of the prototype. 

A single-crystal diffraction experiment generates thousands of diffracted beams whose intensities may be measured in correspondence to the reciprocal lattice points ... 

Several small-molecule single-crystal diffraction experiments have thus been conducted using the ID9 beamline, the most celebrated of which is the photo-induced paraelectric (neutral) to ferroelectric (ionic) structural phase transition in... 

X-ray diffraction analysis was used for crystal structure determination. Single crystal diffraction experiments were carried out using autodiffractometer Syntex PI... 

For single-crystal diffraction experiments the most common type of detector is the CCD. The area of the detector is typically of the order of 25-100 cm and it is held in a fixed position, so the sample must be rotated in order to collect more of the diffraction pattern. For neutron diffraction experiments it is common to fix the orientation of the sample and therefore use a bank of multiple CCD detectors, the data from which are then merged into one file. CCDs can also be used in electron diffraction experiments, but some modifications are required. First, the diffraction pattern contained in the beam of electrons must be converted to an optical signal, which is typically done by positioning a phosphor screen in front of the detector. Secondly, the diffracted beam of electrons must be removed before it strikes the surface of the detector, or charge build-up on the surface will result. This can be achieved by the addition of a second, conducting layer, such as a thin film of aluminum metal. 

Osaki, K. and Schmidt, G.M.J., Single-crystal diffraction experiments in a search for direct structural evidence for soHd-state reactions, Isr. J. Chem., 10, 189, 1972. 

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