Polymorphism, the ability of the same molecule to crystallize in different structures, is important to the makers of pharmaceuticals and specialty chemicals. Our work aims to discover unusual polymorphs, understand how polymorphic systems crystallize, and use polymorphs as a tool to understand the process of crystallization. A polymorphic system discovered in this laboratory (ROY) has the largest number of coexisting polymorphs of solved structures. Such a system helps elucidate the origin of polymorphism and study structure-property relations.

Some questions being investigated include:

1. Why do some molecules have many polymorphs and others seemingly none?
2. Why do polymorphs grow from the same liquid at rates orders of magnitude different?
3. What determines the probability of one polymorph nucleating on another during crystallization?

Figure 1. The simple molecule ROY forms at least ten polymorphs with different colors and molecular conformations; the structures of seven polymorphs (shown) have been solved.

Figure 2. Crystal seeds in a supersaturated medium are expected to grow in the same lattice. We found, however, that seeds of one polymorph can nucleate another polymorph. D-mannitol crystallized first as the delta polymorph and then as the alpha polymorph, with alpha nucleating on delta. This phenomenon is relevant to understanding and controlling crystallization in polymorphic systems.

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