Practical computational techniques of molecular crystal structures is strongly expected for obtaining significant knowledge for molecular crystallographic designs of functional materials and pharmaceutical compounds, and also useful information on adaptive control of solid state phenomena, such as nucleation, growth, polymorphism, and phase transition. To cope with those expectations, we have been developing a computational chemistry program �CONFLEX� to crystallographic analysis and prediction of conformational and packing polymorphism based on theoretical evaluations for many energy minima of crystal structures and their dynamics behaviors. In this conference, our computational investigation on polymorphic transition between two aspirin crystal forms is proposed as an extended application of CONFLEX/MMFF94s.According to a series of reports, aspirin crystal has two polymorphic forms (two packing arrangements), called �form I� and �form II�. The second form was first predicted computationally, observed by X-ray diffraction analysis, and after that, aspirin crystal has been carefully determined as possibly many turnovers of domain within a single crystal. In order to quantitatively elucidate the aspirin crystal polymorphism without purported ambiguity, we have calculated the potential energy profiles of this transformations by using CONFLEX crystal calculations.In comparison with their free energies of two polymorphic forms at room temperature and 100 K, the slightly-predominant stability of �form I� (?G = -1.8 (rt) and -0.5 (100K) kcal/mol) has been computationally confirmed as well as the experimental. Potential energy profile of the polymorphic reversible transitions between form I and II are also evaluated to demonstrate the dynamical behaviors of aspirin crystal polymorphs. As the results, we found that this polymorphism is not understood only by a heterogeneous reaction coordinate but also including competitive homogeneous transformations. Precise analyses will be proposed in poster session.

Potential energy profile of polymorphic transformation in aspirin crystal

243rd National Meeting (2012)

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