scholarly journals Crystal Structures of Organic Compounds

10.5772/48536 ◽  
2012 ◽  
Author(s):  
Nader Noroozi
Keyword(s):  
Crystal Structures ◽  
Organic Compounds

scholarly journals FIDEL: A new method for SDPD of nanocrystalline organic compounds

2014 ◽  
Vol 70 (a1) ◽  
pp. C134-C134
Author(s):  
Martin Schmidt ◽  
Stefan Habermehl ◽  
Philipp Moerschel ◽  
Pierre Eisenbrandt
Keyword(s):  
Crystal Structures ◽  
Rietveld Refinement ◽  
Organic Compounds ◽  
Structure Data ◽  
Structural Model ◽  
Lattice Energy ◽  
Lattice Parameters ◽  
Low Energy ◽  
Powder Pattern ◽  
Field Methods

Rietveld refinements generally fail, if the lattice parameters of the structural model differ more than slightly from the correct lattice parameters and the simulated reflections do not overlap with the experimental ones. For molecular crystals, we have developed a more robust fitting algorithm, which uses the cross-correlation function of calculated and experimental powder patterns, and allows a FIt with DEviating Lattice parameters (FIDEL). The method is also successful for nanocrystalline organic compounds showing only 10-20 peaks in their powder diagrams. The FIDEL method has proven to be useful for various applications, including refinements starting from (1) structure data of an isostructural chemical derivative; (2) structure data of an isostructural hydrate or solvate; (3) structure data from measurements at another temperature (e.g. for fitting a room-temperature powder diagram starting with a structure determined from a single-crystal measurement at 100K). FIDEL is also used for determining crystal structures from non-indexed powder diagrams of nanocrystalline organic compounds. Three steps are performed: (1) Prediction of possible crystal structures in various space groups using global lattice-energy minimizations by force-field methods. (2) FIDEL fit of 100 to 600 low-energy structures to the experimental powder pattern. The structure candidate leading to the correct structure results in a significantly better fit than all other structures. (3) Rietveld refinement. The FIDEL method was used to determine the hitherto unknown crystal structure of the nanocrystalline alpha-phase of 2,9-dichloroquinacridone (C20H10Cl2N2O2). The upper part of the figure shows the experimental powder pattern and the simulated powder diagram of one of the predicted low-energy structures before any fitting. The lower part displays the result of the FIDEL fit, before the Rietveld refinement.

scholarly journals Likelihood of atom–atom contacts in crystal structures of halogenated organic compounds

IUCrJ ◽  
2015 ◽  
Vol 2 (3) ◽  
pp. 327-340 ◽  
Author(s):  
Christian Jelsch ◽  
Sarra Soudani ◽  
Cherif Ben Nasr
Keyword(s):  
Chemical Composition ◽  
Crystal Structures ◽  
Organic Compounds ◽  
Chemical Species ◽  
Halogen Bonding ◽  
Molecular Surface ◽  
Analysis Tool ◽  
Halogenated Organic Compounds ◽  
High Propensity ◽  
Crystal Contacts

The likelihood of occurrence of intermolecular contacts in crystals of halogenated organic compounds has been analysed statistically using tools based on the Hirshfeld surface. Several families of small halogenated molecules (containing organic F, Cl, Br or I atoms) were analysed, based on chemical composition and aromatic or aliphatic character. The behaviour of crystal contacts was also probed for molecules containing O or N. So-called halogen bonding (a halogen making short interactions with O or N, or a π interaction with C) is generally disfavoured, except when H is scarce on the molecular surface. Similarly, halogen...halogen contacts are more rare than expected, except for molecules that are poor in H. In general, the H atom is found to be the preferred partner of organic halogen atoms in crystal structures. On the other hand, C...C interactions in parallel π-stacking have a high propensity to occur in halogenated aromatic molecules. The behaviour of the four different halogen species (F, Cl, Br, I) is compared in several chemical composition contexts. The analysis tool can be refined by distinguishing several types for a given chemical species, such as H atoms bound to O or C. Such distinction shows, for instance, that C—H...Cl and O—H...O are the preferred interactions in compounds containing both O and Cl.

Molecular coordination numbers in crystal structures of organic compounds

2000 ◽  
Vol 56 (3) ◽  
pp. 501-511 ◽  
Author(s):  
E. V. Peresypkina ◽  
V. A. Blatov
Keyword(s):  
Crystal Structures ◽  
Organic Compounds ◽  
Arbitrary Shape ◽  
Short Range ◽  
Close Packing ◽  
Geometrical Analysis ◽  
Coordination Numbers ◽  
Topological Parameters ◽  
Packing Model

The calculation of molecular coordination numbers (MCNs), and topological and geometrical analysis of the environment of molecules in the crystal structures of 23 067 organic compounds, shows that Kitaigorodskii's close-packing model, assuming the predominance of MCN = 12, works correctly in only a few cases, whereas MCN = 14 is the most frequent. To explain this fact the close-packing model is extended with the model of the thinnest covering of space by deformable molecules. It is shown that the packing of molecules of arbitrary shape and composition can be better described with geometrical, but not topological, parameters of their short-range environment, which is conveniently characterized by molecular Voronoi–Dirichlet polyhedra.

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