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Multipole Parameters (multipole + parameter)

Distribution by Scientific Domains
Chemistry100%

Selected Abstracts

WinXPRO: a program for calculating crystal and molecular properties using multipole parameters of the electron density

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2002
Adam Stash
The computer program WinXPRO enables the calculation of crystal and molecular properties using the multipole parameters of the electron density. The list of properties includes the electron density and its topological and electric field characteristics, the local kinetic and potential energies, the electron localization function, and the effective crystal potential. WinXPRO works under the Windows operating system and can utilize any existing graphics program to display output. [source]

Experimental charge-density study of paracetamol , multipole refinement in the presence of a disordered methyl group

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2009
Joanna M. B
A high-resolution single-crystal X-ray study of paracetamol has been performed at 85,K. Different approaches to modeling the experimental electron density (ED) were tested for the dynamically disordered portions of the molecule in order to check to what extent it is possible to obtain a proper ED distribution in the ordered part. Models were examined in which the methyl-group ED was built from pseudoatoms taken from the University at Buffalo Pseudoatom Databank or the Invariom database, with multipole parameters for the remaining atoms being obtained from free refinement. The ,, restricted multipolar model (KRMM) and free ,, refinements were compared; restriction of the ,, parameters was essential in order to obtain values of the electrostatic interaction energy consistent with the results of theoretical single-point periodic calculations. After simultaneous use of KRMM refinement and the databases to model the methyl group, the bond critical point properties and interaction electrostatic energy values were found to be closer to those obtained from theory. Additionally, some discrepancies in the ED distribution and dipole moment among transferred aspherical atom model refinements utilizing both theoretical databases and parameters from theoretical periodic calculations are shown. Including the influence of the crystal field in the periodic calculations increases the ED in the hydroxyl and amide groups, thus leading to higher values of the electrostatic interaction energy, changes in the electrostatic potential values mapped on the isodensity surface and changes in the shape of the anisotropic displacement parameters with respect to results found for both database models. [source]

Accurate charge density of the tripeptide Ala-Pro-Ala with the maximum entropy method (MEM): influence of data resolution

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2007
Andreas Hofmann
The accurate electron density of Ala-Pro-Ala is determined by the maximum entropy method (MEM), employing the same reflection data measured at 100,K which was used for a multipole refinement by Kalinowski et al. [(2007), Acta Cryst. Accepted for publication]. Properties of the electron density are compared with the corresponding properties of the static electron density from the multipole model and to the dynamic MEM electron density of trialanine at 20,K. It is thus shown that the increased thermal smearing at 100,K leads to lower electron densities in the bond critical points and atomic charges closer to zero for Ala-Pro-Ala than has been obtained for trialanine at 20,K. The influence of the resolution of the data is investigated by a series of MEM calculations. Atomic charges and atomic volumes are found not to depend on the resolution, but the charge density in the BCPs decreases with decreasing resolution of the dataset. The origin of this dependence is found to lie mostly in the more accurate estimate of the atomic displacement parameters (ADPs) for the higher-resolution datasets. If these effects are taken into account, meaningful information on chemical bonding can be obtained with data at a resolution better than dmin = 0.63,Å. Alternatively, low-resolution X-ray diffraction data can be used in accurate electron-density studies by the MEM, if another source of accurate values of the ADPs is available, e.g. from refinements with multipole parameters from a database of transferable multipole parameters. [source]

Charge-density study on cyclosporine A

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 3 2009
S. K. J. Johnas
Two single-crystal X-ray diffraction data sets of cyclosporine A were measured to high resolution using synchrotron radiation at temperatures of 5 and 90,K. They allowed an accurate determination of its molecular and electronic structure. Three electron-density models based on pseudoatom scattering factors were compared in terms of derived bond topological properties and in terms of electron-density differences on a grid. In one model multipole parameters were freely refined, whereas in the other two models the density was built up from fixed database parameters from the invariom database and University at Buffalo Databank. The data quality not only allowed benchmarking of the quality of both databases with the refined density, but also judgement of the feasibility of a multipole refinement of a larger oligopeptide structure such as cyclosporine A. Both databases performed equally well and reproduced the experimentally determined charge density satisfactorily. [source]