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Charge-density Study (charge-density + study)
Selected AbstractsCharge-density study on cyclosporine AACTA CRYSTALLOGRAPHICA SECTION D, Issue 3 2009S. 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] Experimental charge-density study of paracetamol , multipole refinement in the presence of a disordered methyl groupACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2009Joanna 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] Experimental and theoretical charge-density study of a tetranuclear cobalt carbonyl complexACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2009Jacob Overgaard Details of the complex bonding environment present in the molecular centre of an alkyne-bridged dicobalt complex have been examined using a combination of experimental and theoretical charge-density modelling for two compounds which share a central Co2C2 tetrahedral moiety as their common motif. Topological analysis of the experimental electron density illustrates the problem of separating the Co,C bond-critical points (b.c.p.s) from the intervening ring-critical point (r.c.p.), due largely to the flat nature of the electron density in the CoC2 triangles. Such a separation of critical points is immediately obtained from a topological analysis of the theoretical electron density as well as from the multipole-projected theoretical density; however, the addition of random noise to the theoretical structure factors prior to multipole modelling leads to a failure in consistently distinguishing two b.c.p.s and one r.c.p. in such close proximity within the particular environment of this Co2C2 centre. [source] Weak intra- and intermolecular interactions in a binaphthol imine: an experimental charge-density study on (±)-8,-benzhydrylideneamino-1,1,-binaphthyl-2-olACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2009Louis J. Farrugia The charge density in (±)-8,-benzhydrylideneamino-1,1,-binaphthyl-2-ol (1) has been studied experimentally using Mo,K, X-ray diffraction at 100,K, and by theory using density-functional thoery (DFT) calculations at the B3LYP/6-311++G** level. The nature of the weak intramolecular peri -C...N, CH...,, H...H and C(,)...C(,) interactions has been examined by topological analysis using the Quantum Theory of Atoms in Molecules (QTAIM) approach. An analysis of the density ,(r), the Laplacian of the density ,2,(rb) and other topological properties at the bond-critical points were used to classify these interactions. The study confirms the presence of the intramolecular CH..., interaction in (1), which was previously suspected on geometrical grounds. An analysis of the ellipticity profiles along the bond paths unambiguously shows the ,-delocalization between the imine unit and one N -phenyl group. The weak intermolecular interactions in the crystal of (1) were examined experimentally and theoretically through the pairwise interactions of the seven independent dimeric pairs of (1) responsible for the set of unique intermolecular interactions, and also through examination of the Hirshfeld surface dnorm property. The theoretical dimeric-pair calculations used the BLYP-D functional which supplements the exchange-correlational functional with an empirical dispersion term to provide a more accurate determination of the energies for the weak intermolecular interactions. [source] | ||||||||||