Why this chapter is important

Polymorphism is a widely spread phenomenon in solid substances (Bernstein, 2002, Hilfiker, 2006, Brittain, 2009). A substance exhibits polymorphism when it can exist in more than one crystalline state. These various crystalline states consequently have a different thermodynamic potential, and therefore a different solubility in a given solvent. These various states also possess different physical and chemical properties. Polymorphs have, for example, different crystal shapes and can have different colors and tastes.

Occurrence and consequences

For simple substances, as in many mineral compounds, the basic entity has a fixed atomic, molecular or ionic structure and the different lattice structures result from different packing arrangements in the crystal lattice.

An example of an inorganic mineral substance is calcium carbonate, which can crystallize in three polymorphic crystalline forms, calcite (trigonal), aragonite (orthorombic) and vaterite (hexagonal), depending on the crystallization conditions (see Figure 14.1). An amorphous phase can also be directly precipitated from highly supersaturated solutions. The most stable form at ambient conditions is calcite.

Molecular structures of organic compounds are often flexible and expose rotational degrees of freedom around a single bond. They are therefore prone to polymorphism. An isolated molecule in the gas phase has one or more equilibrium structures. These equilibrium structures are at a different local or total minimum in potential energy and are called conformers. To go from one conformer to another, an energy barrier has to be crossed. A particular arrangement of atoms in a molecular crystal lattice cannot be far from an equilibrium structure in the gas phase. Its conformation in a crystal lattice is only adjusted to minimize the sum of the intra- and inter-molecular energy. A variation of any torsion angle of a molecule in a crystal lattice is a new conformation. In this way different polymorphs can be formed by only a conformational adjustment with respect to the gas-phase conformation. If, in addition to a change in torsion angle, there is a change in potential energy well in the new conformation, the new conformation is also a conformer.