Quantitative crystallographic analysis

For quantitative crystallographic analysis the intensity of the diffraction spots has to be measured as a function of the primary beam energy respective of the voltage, that is, the so-called I-V curves. The theory for dynamic LEED analysis has been developed over many years [73, 76-79], Kinematic theory fails in the... 

Thermolysis of the pentacoordinate 1,2-thiazetidine 1-oxide 56, which was synthesized for the first time and characterized by X-ray crystallographic analysis, gave the corresponding aziridine 57 and a cyclic sulfinate almost quantitatively <06OL4625>. 

In the solid-state photoreaction of 24c, a more chemoselective reaction occurred and only p-thiolactam 25c was obtained almost quantitatively. Of particular importance is the finding that the solid-state photoreaction of 24c involves a crystal-to-crystal nature where the optically active p-thiolactam 25c is formed in specific yield. Furthermore, the X-ray crystallographic analysis revealed that the crystals of 24c are chiral, and the space group is P2j. Irradiation of crystals at 0 °C exclusively gave optically active P-thiolactam 25c, in 81% yield at 100% conversion (entry 5). As expected, the thiolactam 25c showed optical activity (81% ee). This reaction exhibited good enantioselectivity throughout the whole reaction, where a small difference was observed in the ee value from 97 to 81% ee with increasing conversion from 20 to 100% (entries 5 and 6). The solid-state photoreaction also proceeded without phase separation even after 100% reaction conversion. The crystal-to-crystal nature of the transformation was confirmed by X-ray diffraction spectroscopy. 

Moreover, the cyclization of the azetidinonc 11a, performed with AT-bromosuccinimide in acetonitrile, afforded the bicyclic product 13a only in quantitative yield, which is an intermediate in the synthesis of carbapenems. Under the same reaction conditions, lib afforded 13b in quantitative yield. The stereochemistry of both 13a and 13b was assigned either by NOE experiments or X-ray crystallographic analysis. The reaction proceeds via the intermediate /V-brominated compound 12. Bromine then migrates to the double bond and the amidic nitrogen attacks the formed halonium ion to give 13237. 

Quantitative chemical analysis using the characteristic x-rays emitted by the specimen in the electron microscope is considered in Chapter 7. Since the basic principles are the same as for the electron microprobe (which will be familiar to most geologists and mineralogists), emphasis is placed on those aspects of the technique that are related to the use of thin foil specimens. The intensities of the characteristic x-ray peaks depend on the crystallographic orientation of the specimen with respect to the electron beam this effect is the basis of the relatively new technique ALCHEMl (atom location by channeling enhanced microanalysis), which under certain conditions can be used to determine the location of minor element atoms in a crystal structure. This technique is potentially very useful for mineralogical studies and is therefore discussed in some detail. 

Reconstructions similar to the ones observed on the Pt-Sn system have been observed in some other cases of binary alloys. For instance for Cu-Al(lll) [74] the quantitative LEED analysis [75, 76] showed that the topmost layer is a mixed plane of the same structure of the reconstructed PtsSnClll) surface. Also a (v X /3) R30°reconstruction has been observed for the (111) surface of the random substitutional Al-6.5at% Li alloy, [77] (Quantitative crystallographic data not available). Nothing comparable to the pyramidal structures observed by STM on the PtsSnClOO) system has been reported so far for other alloy systems. 

The origins of the chemical shifts are probably not sufficiently well understood (as yet), to allow a quantitative discussion of aromatic character in the annulenes. If such a concept is considered meaningful it would probably best be defined in terms of the degree of bond alternation therein, which is of pivotal importance to the jr-electron properties (see Sections B and C). Apart from theoretical calculations, a number of physical methods have demonstrated their ability to estimate the extent of bond alternation in annulenes (crystallographic analysis, electronic/vibronic spectral analysis, diamagnetic anisotropy/susceptibility exaltation measurements and of course n.m.r.), see ref. > for a full discussion. (Furthermore the known correlation between n.m.r. vicinal coupling constants and carbon-carbon bond orders is of potential utility in any determination of bond alternation 65>). 

The determination of the depth of information in zeolite materials is difficult as the termination of the oxidic surface is not equal to that of a bulk crystallographic unit cell. A film of strongly bovmd adsorbates (mostly water) and possible contamination from synthesis, catalytic appUcation or ion exchange procedures (salts) will cover the zeolite surface and compensate for the significant polarity of the surface which is an unavoidable consequence of the termination of the crystal at its surface. In quantitative elemental analysis of zeolite materials, there will always be a significant excess of oxygen and carbon and often of alkali ions. This does not mean that these excess atoms are uniformly distributed... 

From on-line coupled TG-MS and TG-FTIR measurements, in combination with quantitative chemical analysis, the chemical formula for an unknown bismuA oxalate compound was deduced to be Bi(NH4)(C204)2 3.71(6)H20 by Vanhoyland et al. [78], Solution of the crystallographic structure on the basis of X-ray powder data proved this formula to be correct. Bi is 8-fold coordinated by oxygen from the oxalate anions. Because these BiOg polyhedra do not share any edges or vertexes, an open framework is formed with water and ammoniiun molecules between. As a result, water can easily be removed, which is clearly indicated by the rapid initial mass loss in the TG cxu e. 

---