Spectroscopy crystal diffraction

The methods available for structure determination are surveyed. Those that are applicable to the gas phase, i.e. electron diffraction and rotational spectroscopy, are suitable mainly for small molecules. Data for the crystalline phase are usually relatively straightforward to obtain, but acquiring reliable structural data for silicon compounds as liquids or in solution by diffraction methods or liquid crystal NMR spectroscopy remains a challenge. 

Adsorption of e.g. rosin (abietic acid) at the pigment surface may - depending on the concentration of the rosin - reduce or accelerate the crystal growth. The presence of an excess amount of rosin during the production of diarylide yellow pigments of the Pigment Yellow 13 type affords an additional crystal modification, which can be identified by X-ray diffraction spectroscopy [4],... 

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. 

The single-crystal to single-crystal nature and the steric course of the photodimerization of thiocoumarin (102) to (+)-a t/-head-to-head dimer (103) in the inclusion complex were investigated by x-ray crystallographic analysis and x-ray powder diffraction spectroscopy [54],... 

Protein structure The three-dimensional structure of a protein can be determined almost to the determination atomic level by the techniques of X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy. In X-ray crystallography a crystal of the protein to be visualized is exposed to a beam of X-rays and the resulting diffraction pattern caused as the X-rays encounter the protein crystal is recorded on photographic film. The intensities of the diffraction maxima (the darkness of the spots on the film) are then used to mathematically construct the three-dimensional image of the protein crystal. NMR spectroscopy can be used to determine the three-dimensional structures of small (up to approximately 30 kDa) proteins in aqueous solution. 

Nitromethane, CH -NOf. The equilibrium structure of singlet nitromethane has been studied at several levels of theory [3,60,64-71]. Two conformations are possible for nitromethane, staggered (Is) and eclipsed (le), but the eclipsed form has been characterized as a transition structure at MP2/6-31G with an imaginary frequency of 30 cm 1 [3]. Rotation around the H3C-NO2 bond occurs essentially without barrier the estimated value is only 0.01 kcal/mol. This is in accordance with a microwave study, which reports a C-N rotation barrier of only 6 cal/mol [72,73]. The C-N bond length of nitromethane has been estimated with X-ray single crystal diffraction [74], neutron diffraction [46,75], microwave spectroscopy [72,73], MP2/6-31G [3], and B3LYP/6-31+G [71] at respectively 1.449, 1.486, 1.489, 1.485, and 1.491 A, showing that the theoretical estimates compare very well with those determined by experimental methods. The experimentally reported vibrational frequencies of nitromethane... 

Unfortunately, specialists in the field of single crystal NMR spectroscopy are not specialists in single crystal neutron diffraction, and, as with solid-state infrared spectroscopy, good experiments using both methods on the same crystalline compound are relatively rare, cf. [246, 247]. 

Reactions that involve cis-protected Mo2(DAniF)2 units have led to a variety of square structures with dicarboxylate anions. The linkers that have been used are oxalate (121), fumarate (122), ferrocenedicarboxylate (123) and 4,4 -biphenyldicarboxylate (124). The midpoints of the M02 units constitute a square, and their structures have been confirmed by H NMR spectroscopy and X-ray studies. Similar molecular squares containing Rh2 + units have been prepared with linkers such as oxalates (120), bicyclo[l.l.l]pentane-1,3-dicarboxylate (125), tetrafluoroterephthalate (126), 1,4-cubanedicarbboxylate (127), terephthalate (128), fumarate (129) and trans 1,4-cyclohexanedicarboxylate (130). The structures of these molecules have been established by spectroscopic and single-crystal diffraction studies. Detailed electrochemical investigations of these complexes have also been undertaken. ... 

Both EXAFS spectroscopy and X-ray single crystal diffraction have been used to study uranyl complexes with 18-crown-6 and dicylohexyl-18-crown-6 (dch-18-crown-6) in the presence of the... 

Despite the wealth of information provided by NMR spectroscopy, the importance of X-ray crystallography in placing these results on firm foundations cannot be overemphasized. Single-crystal diffraction analysis has thus been utilized extensively for characterizations of the complexes described herein. Examples of selected structures have been included throughout this chapter, and a comprehensive list of structurally characterized metallacarbaborane species among the compounds 1 to 63 is given in Table I. 

TABLE 7. Molecules studied by joint analysis of electron diffraction and liquid crystal NMR spectroscopy. 

XRD X-ray diffraction spectroscopy (used in the determination of asbestos, silica, and other particulates, and to analyze metals and study structures of crystals)... 

Extended X-ray absorption fine structure (EXAFS) spectroscopy is a powerful complementary technique since single crystals are not required. EXAFS gives element-specific information and has a higher resolution than single crystal diffraction albeit restricted to a range of a few Angstroms from the... 

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