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Modeling solid-fluid equilibrium of chiral molecules

Monica D Navarro, University of Massachusetts Amherst


This work provides understanding on crystallization of chiral molecules, through the study of theoretical models with hard core interactions, using Monte Carlo simulations. Several issues are addressed. The first is to understand the reasons why chiral molecules prefer to crystallize as racemic compound crystals instead of pure enantiomer solids. We investigate crystal structures at close-packing using a five-site hard-core model, with a focus on the role of shape in packing. Crystal structures at close-packing were found using an algorithm developed during this work. The aim of the algorithm is a density maximization search, using an annealing scheme in pressure and allowing the unit cell of the system to change using the Parrinello-Rahman scheme. The algorithm was tested against analytical results and a protocol to use it was established. Two versions of the algorithm were developed, one to search for close packed structures with a fixed space group and another with a variable space group. We have studied five-site hard core models of chiral molecules and in particular the packing arrangements of these molecules in the solid phase. Close-packed structures for different versions of the model allow the comparison of close-packed densities for racemic compound crystals and pure enantiomer solids for all cases, without the restrictions found in experimental settings. Our results agree with Wallach's rule, in that racemic compounds tend to be denser than their chiral counterparts. We found that the stability of the racemic compound crystals increases with the molecular volume, the same trend found in experimental results. We have also found lower nearest-neighbor distances in these crystals. We have investigated the properties of non-additive hard sphere systems as models for chiral molecule systems. We have found that such models can describe some of the features of the more complex models including the stability of racemic compounds. Using this idea we have also developed a very simple model for diastereomeric salts using hard dumbbells with non-additive interactions. Finally, we used the algorithms developed to search for crystal structures at close-packing to study mixtures of additive hard sphere with a focus on searching for substitutionally ordered packing arrangements.

Subject Area

Chemical engineering

Recommended Citation

Navarro, Monica D, "Modeling solid-fluid equilibrium of chiral molecules" (2010). Doctoral Dissertations Available from Proquest. AAI3427560.