THE ROLE OF INTERMOLECULAR INTERACTIONS IN THE CRYSTALLISATION OF RACEMATES. By Sine Larsen and Katalin Marthi, Centre for Crystallographic Studies, University of Copenhagen, Denmark.

Crystallisation of a 1:1 mixture of enantiomers will most frequently lead to the formation of a racemic compound, crystals containing molecules of opposite chirality. In other cases a spontaneous resolution can occur, leading to a conglomerate of enantiomerically pure crystals. This difference in crystallisation behaviour of racemates reflects the difference in the free energy between the crystals of the racemic compound and the conglomerate of enantiomers.

We have found that the type of racemate obtained by crystallization can be related to differences in the intermolecular interactions. The results are based on an analysis of the crystal packing and physico-chemical properties of some closely related compounds.

The systems investigated are the ortho-, meta- and para-substituted fluoromandelic acids and some of the halogen-substituted 3-hydroxy-3-phenylpropi-onic acids. Racemates of the first class of acids crystallize as racemic compounds whereas racemates of the last class crystallize as conglomerates and racemic compounds depending on the type of halogen-substitution.

A variety of (O-H...O) hydrogen bond motifs are observed in the crystal structures of the racemic and enantiomerically pure fluoromandelic acids. Identical hydrogen bond motifs are found in the investigated 3-hydroxy-3-phenylpropionic acids. In these systems where conglomerate crystallization occurs frequently, the differences can be related to differences in weaker interactions e.g. C-H...O hydrogen bonds and electrostatic interactions. We have noticed that the racemates which form a cyclic carboxylic acid dimers tend to be higher melting than the pure enantiomer. The thermodynamic differences reflected in their binary phase diagrams are related to the observed differences in the crystal packings.