Ab initio crystal structure determination

Abstract This chapter demonstrates that it is possible to perform ab initio crystal structure determination by HREM. The various steps in a crystal structure determination recording and quantifying HREM images, analysis and processing of these data to retrieve the projected potential of the crystal and finally determine the atomic coordinates are described. 

We have seen this powder diffraction pattern several times throughout this text. The histogram collected from the nearly spherical LaNi4.85Sno.15 powder, produced by high pressure gas atomization from a melt, was used to illustrate both the quality of x-ray diffraction data and as one of the examples in the ab initio crystal structure solution. To demonstrate the Rietveld refinement of this crystal structure we will begin with the profile and unit cell parameters determined from Le Bail s algorithm Table 6.3) and the model of the crystal structure determined from sequential Fourier maps as described in section 6.9 and listed in Table 6.8. 

In analogy to the D3d and Z)4d structures of S6 and Si0 a D5d structure was initially expected for Si0. However, the crystal structure determination showed that the compound has only D2 symmetry (Figure 25).78 Ab initio calculations79 suggest that the D5A isomer is 29 kJ mol-1 higher in energy than the D2... 

Ab initio methods for polymorph, hydrate and solvate prediction are highly prized by the industry and good progress has been made in this field in recent years. This work is still a number of years from routine commercial application however, and polymorph screening experiments together with crystal structure determination, remain critical tasks for today s Pharmaceutical companies. 

Hydroxamic acids have been the subject of six papers 43 90 94 Earlier the operation of the a-effect in the reaction of p-nitrophcnyl acetate with benzohydroxamates in aqueous MeCN was discussed.43 The conformational behaviour of series of mono- (105) and di-hydroxamic acids (106) in MeOH, DMSO, and chloroform and in the solid state has been examined witii IR and NMR spectroscopy.90 X-ray crystal structure determinations of (105 X = Me, R = Me) and die monohydrate of glutarodihydroxamic acid (106 n = 3, R = H) together widi ab initio MO calculations for several hydrated and non-hydrated acids have been performed. The cis-Z conformation of the hydroxamate groups is preferentially stabilized by H-bonding witii water. 

Summary We present ab initio calculations and crystal structures of halo- and hydrido-substituted cyclodisilazanes as well as the synthesis and crystal structure of the first amino-chloro functional cyclodisilazane in the cw-conformation. Crystal structure determinations of cyclotrisilazanes are shown. 

Fagan, P. G., Roberts, K. J., Docherty, R., Chorlton, A. R, Jones, W., and Potts, G. R, An ab-initio approach to crystal structure determination using high-resolution powder diffraction and computational chemistry techniques application to 6,13-dichlorotriphendioxazine, Chem. Mater., 7, 2322, 1995. 

Corroborating evidence that heptacoordinated molecules, with either fluorine, oxygen or free valence electron pairs as ligands, possess in their ground states pentagonal bipyramidal structures with an unpuckered equatorial plane, comes from ongoing ab initio calculations and two X-ray crystal structure determinations. It has... 

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