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No / The predicted stability differences of the conformational polymorphs of oxalyl dihydrazide and ortho-acetamidobenzamide are unrealistically large when the modeling of intermolecular energies is solely based on the isolated-molecule charge density, neglecting charge density polarization. Ab initio calculated crystal electron densities showed qualitative differences depending on the spatial arrangement of molecules in the lattice with the greatest variations observed for polymorphs that differ in the extent of inter- and intramolecular hydrogen bonding. We show that accounting for induction dramatically alters the calculated stability order of the polymorphs and reduces their predicted stability differences to be in better agreement with experiment. Given the challenges in modeling conformational polymorphs with marked differences in hydrogen bonding geometries, we performed an extensive periodic density functional study with a range of exchange-correlation functionals using both atomic and plane wave basis sets. Although such electronic structure methods model the electrostatic and polarization contributions well, the underestimation of dispersion interactions by current exchange-correlation functionals limits their applicability. The use of an empirical dispersion-corrected density functional method consistently reduces the structural deviations between the experimental and energy minimized crystal structures and achieves plausible stability differences. Thus, we have established which types of models may give worthwhile relative energies for crystal structures and other condensed phases of flexible molecules with intra- and intermolecular hydrogen bonding capabilities, advancing the possibility of simulation studies on polymorphic pharmaceuticals.

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On account of ¡§the X-ray Phase Problem¡¨, it¡¦s impossible to get the electronic density by inverse Fourier transformation(IFT). And the crystal structure of molecule can¡¦t be solved by IFT. There are many methods to overcome the problem, such as Direct Method, Heavy Atom, and Simulated Annealing. But for macro-molecules, we still face a great challenge due to tremendous computing load. We propose a method by using wavelets to resolve the electronic density. We hope the multiresolution nature of wavelets can reduce the computing load for simulated annealing method.

Hydrated sodium-magnesium sulfate minerals are common in many continental evaporite settings around the world. The crystallization sequence of these minerals depends on such parameters as the composition of the parent brine, the temperature, the evaporation rate of the brine, and the differences in the atomic structure and water content of the minerals. The atomic structures of konyaite [Na2Mg(SO4)2·5H2O] and sodium-magnesium decahydrate [Na2Mg(SO4)2·10H2O], a newly described sulfate salt, have been determined from single-crystal X-ray diffraction experiments. The refined structures are discussed and compared to that of blödite [Na2Mg(SO4)2·4H2O]. The arrangement and importance of hydrogen bonds within all three structures are also discussed, and have been further investigated by infrared spectroscopy. Löweite [Na12Mg7(SO4)13·15H2O] was included in this experiment to provide a low-hydration end-member. Differences in water content and the importance of hydrogen bonds in the respective structures were clearly reflected in the generated infrared spectra. The growth conditions of the decahydrate, konyaite, blödite, löweite, and other phases of the Na2O-MgO-H2O system, as well as their stability relationships, were studied in a temperature-controlled crystal-growth experiment. Konyaite and the decahydrate phase were found as first precipitates over a range of temperatures and brine compositions where they are not considered to be the thermodynamically stable phase. The importance of evaporation rate in the formation of these, and other metastable phases, is discussed in relation to inland saline systems. Possible localities where the decahydrate could exist in nature are discussed, and challenges for future research are presented. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2010-09-16 09:02:33.843