Structure determination, experimental single crystal diffraction

Organic NLO materials have been intensively studied recently due to their potential application in various fields, such as telecommunications and optical information processes. A series of dicyanoisophorones have been synthesized and structurally determined by single-crystal X-ray diffraction by Kolev and coauthors [421-428] on the basis of Lemke s [444] pioneering studies. Experimental and computational studies on the vibrational spectra of 2-[5,5-dimethyl-3-(2-phenyl-vinil)-cyclohex-2-enylidene]-malononitrile have been reported [445]. However, the presence of aromatic and conjugated systems in the structures of these compounds makes the assignment of individual bands difficult using IR spectra. Therefore, this chapter deals with the solid-state IR-LD analysis of 2-[5,5-dimethyl-3-(2-phenyl-vinyl)-cyclohex-2-enylidene]-malononitrile (/), 2- 5,5-dimethyl-3-[2-(2-methoxyphenyl) vinyl]cyclo-hex-2-enylidene malononitrile (2), and 2-[3-[2-(2,4-dimethoxyphenyl)... 

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... 

It is worth noting that practically all non-traditional methods for solving crystal structures have been initially developed for both powder and single crystal diffraction data to manage intrinsic incompleteness or poor quality that cannot be improved experimentally. Despite a variety of structure solution approaches, traditional direct phase determination methods appear to be the most common and successful when powder diffraction data are adequate. Patterson methods also work quite well but they require the presence of a heavy atom and, perhaps, more extensive crystallographic expertise. The non-traditional methods are generally employed when other techniques fail and their use is somewhat restricted by both the complexity and limited availability of computer codes. 

The molecular structure of uranocene has been determined by single-crystal x-ray diffraction and is shown in Fig. 22.15 [130,131]. The molecule possesses rigorous Dg), symmetry with the eight-membered rings arranged in an eclipsed conformation. The mean U-C bond distance is 2.647(4) A, and the mean C-C bond distance is 1.392(13) A. TheCgHi rings are planar to within experimental error. 

Del] Delattre, J.L., Stacy, A.M., Siegrist, T., Structure of Ten-Layer Orthorhombic Ba5Fc50i4 (BaFe02.g) Determined from Single Crystal X-Ray Diffraction , J. Solid State Chem., 177(3), 928-935 (2004) (Crys. Structure, Experimental, 31)... 

X-Ray diffraction from single crystals is the most direct and powerful experimental tool available to determine molecular structures and intermolecular interactions at atomic resolution. Monochromatic CuKa radiation of wavelength (X) 1.5418 A is commonly used to collect the X-ray intensities diffracted by the electrons in the crystal. The structure amplitudes, whose squares are the intensities of the reflections, coupled with their appropriate phases, are the basic ingredients to locate atomic positions. Because phases cannot be experimentally recorded, the phase problem has to be resolved by one of the well-known techniques the heavy-atom method, the direct method, anomalous dispersion, and isomorphous replacement.1 Once approximate phases of some strong reflections are obtained, the electron-density maps computed by Fourier summation, which requires both amplitudes and phases, lead to a partial solution of the crystal structure. Phases based on this initial structure can be used to include previously omitted reflections so that in a couple of trials, the entire structure is traced at a high resolution. Difference Fourier maps at this stage are helpful to locate ions and solvent molecules. Subsequent refinement of the crystal structure by well-known least-squares methods ensures reliable atomic coordinates and thermal parameters. 

The most important experimental task in structural chemistry is the structure determination. It is mainly performed by X-ray diffraction from single crystals further methods include X-ray diffraction from crystalline powders and neutron diffraction from single crystals and powders. Structure determination is the analytical aspect of structural chemistry the usual result is a static model. The elucidation of the spatial rearrangements of atoms during a chemical reaction is much less accessible experimentally. Reaction mechanisms deal with this aspect of structural chemistry in the chemistry of molecules. Topotaxy is concerned with chemical processes in solids, in which structural relations exist between the orientation of educts and products. Neither dynamic aspects of this kind are subjects of this book, nor the experimental methods for the preparation of solids, to grow crystals or to determine structures. 

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