X-ray diffraction, for determination

Virtually all parts of the spectrum of electromagnetic radiation, from x rays to radio waves, have some practical application for the study of organic molecules. The use of x-ray diffraction for determination of the structures of... 

The molecular structures of several [TpBut]ZnX derivatives have been determined by x-ray diffraction. For example, x-ray diffraction studies confirm that the acetate ligand in [TpBut]Zn(r)1-02CMe) is bound to zinc in a unidentate mode, similar to that proposed for [TpBut]Mg(7j1-02CMe), but in contrast to the bidentate coordination proposed for the copper analogue [TpBut]Cu(T)2-02CMe) (86,87). Such a change in coordination mode for copper and zinc derivatives is to be anticipated on the basis of structural studies on the nitrate derivatives [TpBut]M(N03) (M = Co, Ni, Cu, Zn), as described in Section V,B,2,e. The thioacetate [TpPh]-Zn V-SC(0)Me (81), and cyanoacetate [Tp lZnlr -C CCH N) (88) derivatives also exhibit unidentate coordination. 

Fig. 19 The structure of 35 2Zn(II) OH (CF3SO3 )3 CH3OH as determined by X-ray diffraction. For clarity, the triflate counter ions and methanol solvate are not shown.

Two structures have been determined by X-ray diffraction for stereochemical determination. The structure of one enolate derived from Meyers oxazolidinone was reported <1996JA7429> as well as structure of (2S,5R,7aR)-5-(benzotriazol-l-yl)-3-phenyl[2,l-A]oxazolopyrrolidine which was obtained as only one diastereomer <1998TL1697, 1999JOC1979>. 

Water on Vermiculite. For low water contents (that is, one or two water layers), the evidence for highly structured water in the interlayer spaces of smectites and vermiculites is most easily seen in X-ray diffraction structure determinations of ordered hydrate structures such as the two-water layer hydrate of Ca-vermiculite (14. 15) and Na-vermiculite (15., 16). 

Another advantage of d5mamic scattering is that, for acentric zones, Friedel s law breaks. This allows for an easy way, much more reliable than for X-ray diffraction, to determine the absolute structure configuration[8j. 

While most superoxo complexes—in contrast to peroxo compounds— have been assigned a bent, end-on coordination mode [9], the superoxide ligand of Tp Cr(02)Ph was suggested to exhibit the more unusual side-on (r] ) coordination [10]. The reactivity of the complex did not allow for the determination of its molecular structure however, close analogs could be isolated, crystallized and structurally characterized by X-ray diffraction. For example, the reaction of [Tp Cr(pz H)]BARF (pz H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis(3,5-bis(trifiuoromethyl)phenyl)borate) with O2 produced the stable dioxygen complex [Tp Cr(pz H)( ] -02)]BARF (Scheme 3, bottom), which featured a side-on bound superoxide ligand (do-o = 1.327(5) A, vo-o = 1072 cm ) [11]. Other structurally characterized... 

In order to use X-ray diffraction for the determination of absolute configuration, two conditions must be met ... 

The theoretical density of a crystal can be obtained from the volume of the unit cell and the mass of the unit cell contents. The results of an X-ray diffraction structure determination gives both of these data, as the unit cell dimensions are accurately measured and the type and number of formula units in the unit cell are also determined. An example of this type of calculation for FeO follows ... 

Mineralogy. The most important mineral constituents were determined by X-ray diffraction for five samples (four of them from NR-10). Quartz and opal (mainly CT) are important components in all five samples. In a marl (NR-10, 81.5 m), calcite, kaolinite, pyrite, gypsum and illite/smectite mixed-layer minerals occur in addition to those. 

Since isomerically pure polymers were not available, three different kinds of BR, each relatively high in one of the three kinds of base units were used as standards [35]. The band near 1308 cm 1 was identified [38,39] with the cis isomer and used for analyses [43]. The 1308 cm 1 band is weak and relatively broad, with the appearance of an unresolved doublet (1306,1311 cm 1). The cis band at 730 cm 1 is more frequently used in spite of some difficulties. Relatively pure, crystalline stereoregular polymers have been prepared and structures were determined by X-ray diffraction for cis [44], trans [45] and syndiotactic vinyl [46] and isotactic vinyl [47]. Infrared spectra [48-50] have been published for the four stereoregular polybutadienes, with detailed analyses of the spectra and band assignments for cis [51], trans [51] and syndiotactic vinyl [51] polymers. For the spectrum of isotactic vinyl BR, bands at 1232, 1225, 1109, 943, 876, 807 and 695 cm"1... 

Whereas two methyl N-substituents are inefficient in transmitting the chiral information encoded at the rear side of the heterocyclic ligand (Entry 1 in Table 1), the stereoselectivity is improved by the introduction of N-benzyl substituents (Entry 2). The steric repulsion between the ferf-butyl and benzyl groups leads to a C2 symmetric arrangement of the latter with respect to the carbene donor function as is apparent in the molecular structure determined by X-ray diffraction for the silver(I) complex 20 [51]. In this way the chirality in the heterocycle is transmitted towards the reaction center. Introduction of methoxy groups in the meta-position of the phenyl rings of the benzyl substituents slightly increases the selectivity of the catalyst (Entry 3). 

Since the Braggs first determination, thousands of structures, most of them far more complicated than that of sodium chloride, have been determined by x-ray diffraction. For covalently bonded low molecular weight species (such as benzene, iodine, or stannic chloride), it is often of interest to see just how the discrete molecules are packed together in the crystalline state, but the crystal structures affect the chemistry of such substances only to a minor degree. However, for most predominantly ionic compounds, for metals, and for a large number of substances in which atoms are covalently bound into chains, sheets, or three dimensional networks, their chemistry is very largely determined by the structure of the solid. 

High-accuracy molecular dimensions for the parent monocyclic heterocycles have been determined by micro-wave spectroscopy and these can be found in Section 2.3.3.2 (Table 7). The ring dimensions obtained by X-ray diffraction for the 2-carboxylic acid derivatives of furan, thiophene, selenophene, and tellurophene (Table 3) are generally in good agreement with those obtained for the parent heterocycles using microwave spectroscopy (Table 7). 

Linus Pauling (1901-1994) was the most famous American chemists of the 20th century and the author of the classic The nature of the chemical bond. His early work pioneered the application of X-ray diffraction to determine the structure of complex molecules he then went on to apply quantum theory to explain these observations and predict the bonding patterns and energies of new molecules. Pauling, who spent most of his career at Cal Tech, won the Nobel Prize for Chemistry in 1954 and the Peace Prize in 1962. 

Gulik-Krzywicki, T., and Delacroix, H. (1994). Combined use of freeze fracture electron microscopy and x-ray diffraction for the structure determination of three-dimensionally ordered specimens. Fio/. Cell 80, 193-201. 

A recent study (I) of Egyptian glass used atomic absorption in conjunction with x-ray diffraction to determine the elements responsible for color and opacity. Atomic absorption (AA) is possibly the most accurate instrumental method for determining elemental composition, although it is insensitive to certain elements and each element must be determined separately (2). X-ray photoelectron spectroscopy (XPS) (3) is a technique developed relatively recently that can go beyond simple elemental constitution and examine the oxidation state of particular elements. We have extended the atomic absorption study of Cowell and Werner (1) to include further Egyptian glass materials, and we have... 

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