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PIXEL Method (pixel + method)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

The lines-of-force landscape of interactions between molecules in crystals; cohesive versus tolerant and `collateral damage' contact

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010
Angelo Gavezzotti
A quantitative analysis of relative stabilities in organic crystal structures is possible by means of reliable calculations of interaction energies between pairs of molecules. Such calculations have been performed by the PIXEL method for 1108 non-ionic and 98 ionic organic crystals, yielding total energies and separate Coulombic polarization and dispersive contributions. A classification of molecule,molecule interactions emerges based on pair energy and its first derivative, the interaction force, which is estimated here explicitly along an approximate stretching path. When molecular separation is not at the minimum-energy value, as frequently happens, forces may be attractive or repulsive. This information provides a fine structural fingerprint and may be relevant to the mechanical properties of materials. The calculations show that the first coordination shell includes destabilizing contacts in ,,9% of crystal structures for compounds with highly polar chemical groups (e.g. CN, NO2, SO2). Calculations also show many pair contacts with weakly stabilizing (neutral) energies; such fine modulation is presumably what makes crystal structure prediction so difficult. Ionic organic salts or zwitterions, including small peptides, show a Madelung-mode pairing of opposite ions where the total lattice energy is stabilized from sums of strongly repulsive and strongly attractive interactions. No obvious relationships between atom,atom distances and interaction energies emerge, so analyses of crystal packing in terms of geometrical parameters alone should be conducted with due care. [source]

Conformational disorder in 4-(5,5,-dibromo-2,-chloro-4,4,-bipyridyl-2-yl)benzaldehyde: role of ,,, and halogen interactions

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2010
Mohamed Abboud
The crystal packing of the title compound, C17H9Br2ClN2O, is governed by strong ,,, stacking, where molecules are tightly bound within infinite (100) planes; these planes interact mainly through non-optimal ,,, stacking where arene rings are noticeably displaced from perfect overlap, and also through halogen,halogen interactions. The aldehyde group shows conformational disorder, with a significant population difference between the two conformers; this difference is rationalized by the energetic analysis of the crystal packing using the PIXEL method, which also allows a decomposition of intermolecular interaction energy into Coulombic, polarization, dispersion and repulsion contributions. Using such an analysis, it is found that the main reason for this unequal population of the two conformers in the crystal is two hydrogen bonds that are present only for the major conformer. [source]

A simulation study of 1D variational cloud retrieval with infrared satellite data from multiple fields of view

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 599 2004
M. D. E. Szyndel
Abstract A method for retrieving cloud-top height using one-dimensional variational analysis with multiple pixels is examined. The method is tested for a range of cloud scenarios using simulated multi-spectral infrared imagery. The skill of the multiple pixel method is compared to the skill of the single-field-of-view method. It is found that, when applied to two pixels, the multiple-field-of-view method considerably increases the skill of retrieval for the less cloudy pixel, and marginally improves the skill of retrieval for the cloudier pixel in the majority of cases. It is also found that, when applied to six pixels, the multiple-field-of-view method has a positive impact in many cases. © Crown copyright, 2004. Royal Meteorological Society [source]

Influence of Microsilica Content on the Slag Resistance of Castables Containing Porous Corundum,Spinel Aggregates

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 6 2008
Wen Yan
Corrosion of five corundum,spinel castables containing the same porous aggregates and different matrices by converter slag (C/S=3) was conducted using the static crucible test through the counting pixels method, XRD, SEM, EDAX, and so on. It was found that the corrosion and penetration resistance depend on the microsilica content. Microsilica content strongly affects liquid phase formation in the matrix, changes the porosity, pore size distribution of matrix, and viscosity of slag penetrated, and thus affects its corrosion and penetration resistance. Increasing microsilica, decreasing the porosity, or increasing the viscosity of the penetrating slag improves, whereas increasing the pore size or liquid content reduces the penetration and corrosion resistance. The penetration resistance increased abruptly with an increase of the microsilica content from 0.65 to 1.95 wt%, but changed slightly with increase of the microsilica content from 1.95 to 3.90 wt%. The corrosion resistance increased with an increase of the microsilica content from 0 to 2.60 wt%, but slightly decreased with a further increase of the microsilica content to 3.90 wt%. 1.95,2.60 wt% microsilica leads to a compromise between low corrosion and low penetration. [source]