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Are the crystal structures of enantiopure and racemic mandelic acids determined by kinetics or thermodynamics?

MPS-Authors
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Schulze,  Eric
Molecular Simulations and Design, Max Planck Institute for Dynamics of Complex Technical Systems, Max Planck Society;
International Max Planck Research School (IMPRS), Max Planck Institute for Dynamics of Complex Technical Systems, Max Planck Society;

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Lorenz,  Heike
Physical and Chemical Foundations of Process Engineering, Max Planck Institute for Dynamics of Complex Technical Systems, Max Planck Society;

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Seidel-Morgenstern,  Andreas
Physical and Chemical Foundations of Process Engineering, Max Planck Institute for Dynamics of Complex Technical Systems, Max Planck Society;
Otto-von-Guericke-Universität Magdeburg, External Organizations;

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Stein,  Matthias
Molecular Simulations and Design, Max Planck Institute for Dynamics of Complex Technical Systems, Max Planck Society;

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Citation

Hylton, R., Tizzard, G. J., Threlfall, T. L., Ellis, A. L., Coles, S. J., Seaton, C. C., et al. (2015). Are the crystal structures of enantiopure and racemic mandelic acids determined by kinetics or thermodynamics? Journal of the American Chemical Society, 137(34), 11095-11104. doi:10.1021/jacs.5b05938.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0028-4DFF-8
Abstract
Mandelic acids are prototypic chiral molecules where the sensitivity of crystallized forms (enantiopure/racemic com-pound/polymorphs) to both conditions and substituents provides a new insight to the factors that may allow chiral separation by crystallization. The determination of a significant number of single crystal structures allows the analysis of 13 enantiopure and 30 racemic crystal structures of 21 (F/Cl/Br/CH3/CH3O) substituted mandelic acid derivatives. There are some common phenyl packing motifs between some groups of racemic and enantiopure structures, although they show very different hydrogen bonding motifs. The computed crystal energy landscape of 3-chloromandelic acid, which has at least 2 enantiopure and 3 racemic crystal polymorphs, reveals that there are many more possible structures, some of which are predicted to be thermodynamically more favorable as well as slightly denser than the known forms. Simulations of mandelic acid dimers in isolation, water and toluene do not differentiate between racemic and enantiopure dimers, and also suggest that the phenyl ring interactions play a major role in crystallization mechanism. The observed crystallization behavior of mandelic acids does not correspond to any simple “crystal engineering rules” as there is a range of thermodynamically feasible structures with no distinction between the enantiopure and racemic forms. Nucleation and crystallization appear to be determined by the kinetics of crystal growth with a statistical bias, but the diversity of the mandelic acid crystallization behavior demonstrates that the factors that influence the kinetics of crystal nucleation and growth are not yet adequately understood.