X-Ray crystallographic and computational study on uranyl-salophen complexes bearing nitro groups.

In the solid state, salophen-UO2 complexes bearing one, two, or three NO2 groups lack the pronounced ligand curvature that represents a structural hallmark for this class of compounds. A detailed structural study based on single-crystal X-ray crystallography and computational methods, comprising molecular dynamics, gas-phase Hartree Fock, and DFT calculations, was carried out to investigate the coordination properties of the uranyl cation.

A top-down approach to crystal engineering of a racemic Δ2-isoxazoline.

The crystal structure of racemic dimethyl (4RS,5RS)-3-(4-nitrophenyl)-4,5-dihydroisoxazole-4,5-dicarboxylate, C13H12N2O7, has been determined by single-crystal X-ray diffraction. By analysing the degree of growth of the morphologically important crystal faces, a ranking of the most relevant non-covalent interactions determining the crystal structure can be inferred. The morphological information is considered with an approach opposite to the conventional one: instead of searching inside the structure for the potential key interactions and using them to calculate the crystal habit, the observed crystal morphology is used to define the preferential lines of growth of the crystal, and then this information is interpreted by means of density functional theory (DFT) calculations. Comparison with the X-ray structure confirms the validity of the strategy, thus suggesting this top-down approach to be a useful tool for crystal engineering.

Potassium caffeate/caffeic acid co-crystal: the rat race between the catecholic and carboxylic moieties in an atypical co-crystal.

The vast literature concerning caffeic acid and its derivatives lacks any reference to the solid state structures of its inorganic salts as these crystals are quite difficult grow. Most of the already published works deal with computational studies of these compounds as well as investigations of their behaviour in solution. Having obtained good quality potassium caffeate/caffeic acid co-crystals, we solved their structure and used a robust approach, already applied to caffeic acid alone, to compare the X-ray structure with the one inferred by Molecular Dynamics (MD), focusing our attention on the structure-property relationships. The reliability of this method is confirmed by the overall agreement extended up to the anisotropic displacement parameters calculated, on one hand, by means of MD and the ones gathered, on the other hand, by X-ray measurements. Moreover, the lack of experimental evidence of an enthalpically favored polymorph, arising from the MD calculations, were explained on the basis of the Shannon's entropy.

An integrated X-ray and molecular dynamics study of uranyl-salen structures and properties.

Uranium complexes of bis(p-tert-butyl-salicylidene)-1,2-diphenylethylenediamine (1) and bis(salicylidene)-1,2-diphenylethylenediamine (2) have been synthesized and investigated by X-ray single crystal diffraction and MD calculations in Periodic Boundary Conditions. Both compounds form crystals which are densely packed and do not provide voids accessible to solvent molecules. The configurations adopted by 1 and 2 are determined by well defined T-shaped and π-stacking non covalent interactions between phenyl groups of adjacent molecules as well as by a network of hydrogen bonds. These interactions and the relative arrangements of the molecules, explain the packing in the crystal structures. Each uranyl moiety shows a penta-coordination in the equatorial plane perpendicular to the trans oxygens giving rise, in both compounds, to a bypiramidal geometry. As usual for this class of compounds, the 5th position is characterized by the presence of the coordinated solvent. The in silico simulations confirm this hypothesis in very fine details. Moreover, in 1, even the partial occupancy of the solvent molecule determined from the crystal structure refinement, was shown to be due to a constrained freedom of motion of the solvent molecule that can be reproduced by molecular dynamics. This suggests that the reported disorder is not due to a poor quality of the harvested crystals but to a structural feature. In further agreement with the above mentioned results, DFT calculations demonstrated that the molecular orbital configuration and energies suit the described properties of complexes 1 and 2 suggesting a potential enantioselective activity as already shown by molecules belonging to this class of compounds.

Molecular dynamics (MD) simulations and large-angle X-ray scattering (LAXS) studies of the solid-state structure and assembly of isotactic (R)-poly(2,2'-dioxy-1,1'-binaphthyl-)phosphazene in the bulk state and in the cast film.

The intrachain conformation, molecular structure and interchain assembly of isotactic (R)-poly(2,2'-dioxy-1,1'-binaphthyl)phosphazene (P-DBNP) both in the bulk state (I) and in the cast film (II) were studied by molecular dynamics (MD) simulations of models, as implemented by a bias potential for the analysis of the radial distribution function (RDF) obtained from large-angle X-ray scattering (LAXS) data. The microscopic structure and order extension of the polymer changed from I to II, as qualitatively shown in the shapes of their experimentally measured RDF curves. With the use of a bias potential, the MD simulations provided a much more accurate analysis of the models, as seen in the reproduction of the RDFs. The chiral P-DBNP chain was found to be consistent with helix conformations in both the I and the II samples. The predominant interchain clustering motif was best reproduced with a seven-chain model. In the case of I, the maximum chain length was 18 monomeric -R(2)NP- units, while in the case of the cast film II the chain was more elongated, up to distances of approximately 100 A, equivalent to over 48 monomeric -R(2)NP- units. The seven-chain assembly was accounted for in terms of nonbonded interactions favouring the minimum voids area between the seven tubular structures of the material. The results validate our earlier finding that MD analysis with implementation of a biasing potential for the RDFs can provide quantitative information on the structural and conformational features of amorphous solids. The combined theoretical and experimental approach was found to be a useful tool to detect, locate and evaluate the intra- and intermolecular modifications of materials subsequent to their phase transformation and, as in the present case, changes in their microscopic structures or preparation methods.