MOLECULAR REARRANGEMENT DUE TO CRYSTAL PACKING. Jan C. A. Boeyens, Centre for Molecular Design, Chemistry Department, University of the Witwatersrand, Private Bag 3,Wits, 2050, South Africa
Conclusive evidence for the decisive effect of intermolecular interactions on molecular conformation in the solid state is provided by an analysis of the phase relationships between isomeric crystals of ([[eta]]5 - C5H4Me)Re(CO)2Br2(I). This is interpreted as further evidence in support of the conjecture that molecular shape is not an inherent property of small molecules, but more likely the result of environmental pressure. The stable form of I in solution and in molecular mechanics is the lateral isomer which on refluxing in toluene converts to the diagonal isomer [1]. Both isomers have been isolated as single crystals and studied crystallographically [2]. The diagonal isomer now has the lower triclinic symmetry and undergoes a phase transition above 100deg.C into the monoclinic form of the lateral isomer. The thermally induced isomerisation in solution is therefore seen to be linked to an intramolecular interaction, whereas in the crystal it is driven by an intermolecular interaction. The intermolecular interaction is readily identified by noting that the sequence of events described here is restricted to compounds with small substituents, compatible with a characteristic packing in sheets in the triclinic crystal, stabilised by polar interactions that operate only if the molecule has the diagonal conformation. This novel result has an important bearing on the old question of conformational changes between solution and crystal.
1. L. Cheng and N. J. Coville, Organometallics, 1996, in press.
2. J. C. A. Boeyens, L. Cheng, N. J. Coville, D. C. Levendis and K. McIntosh, unpublished results.