Acentric structure

BUSTER has been run against the L-alanine noisy data the structure factor phases and amplitudes for this acentric structure were no longer fitted exactly but only within the limits imposed by the noise. As in the calculations against noise-free data, a fragment of atomic core monopoles was used the non-uniform prior prejudice was obtained from a superposition of spherical valence monopoles. For each reflexion, the likelihood function was non-zero for a set of structure factor values around this procrystal structure factor the latter acted therefore as a soft target for the MaxEnt structure factor amplitude and phase. 

At each stage of the refinement of a new set of parameters, the hat matrix diagonal elements were calculated in order to detect the influential observations following the criterium of Velleman and Welsh [8,9]. The inspection of the residues of such reflections revealed those which are aberrant but progressively, these aberrations disappeared when the pseudo-atoms model was used (introduction of multipoler coefficients). This fact confirms that the determination of the phases in acentric structures is improved by sophisticated models like the multipole density model. 

A first map of this kind is shown in Fig. 5.8. The X-N deformation density is thermally averaged, and has limited resolution as the summation in Eq. (5.17) is truncated at the limit of the experimental observations. Since both Fobs and Fcalc are complex for an acentric structure, the structure factor phases are continuously variable, and must be considered. Expression (5.17) can be rewritten as... 

The true phases of the structure factors will, in general, be different from the phases calculated with the independent-atom model. In centrosymmetric structures, with phases restricted to 0 or n, only very few weak reflections are affected. In acentric structures, only the reflections of centrosymmetric projections, such as the hkO, hOl, and Okl reflections in the space group P212,21, are invariant. 

If the best donors and acceptors couple, and the common aminopyrimidine bidentate hydrogen bonds form, then the following acentric structure is conceivable. 

DM also applies to acentric structures, where a phase angle oc(hkl) must be found for each reflection. Typically, the possible phase angles are divided into 15° increments, so that 24 possible phase angles must be considered per reflection. About 90% of all structures can be solved in a day or two by direct methods. 

Figure 8.3 Probability distribution function of normalized structure factor ampli tudes for centrosymmetric (centric) and non centro symmetric (acentric) structures.

An attractive way of self-assembling highly EO-active organic materials is by using supramolecular interactions, such as intermolecular hydrogen bonding, to achieve highly ordered, noncovalently bound, acentric structures under mild... 

The acentric structure consists of isolated (Si207) double tetrahedra and Pr ions, which are arranged in four sheets perpendicular to the c axis (Fig. 26). Within each of the four adjacent sheets, which are related through Idle 90° rotation of the 4i axis, the (Si207) units and the heavy atoms form parallel rows in the [110] direction. Within those rows there are always two of the double tetrahedra which are almost related by a centre of symmetry, with the comer oxygens forming some kind of closest packing. The heavy atoms are attached to each side of the sheets and provide a connection to the adjacent sheet. 

Lu, J. Y Runnels, K. A. Norman, C. A new metal-organic polymer with large grid acentric structure created by unbalanced inclusion species and its electrospun nanofibers. Inorgan. Chem., 2001, 40,4516-4517. 

Figure 6.9 Acentric structure of 3,5 dinitrobenzoic acid and 4-aminobenzoic acid created upon grinding and from sciutionA "...

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