Crystallography and kinetics: structure correlations in the light of Marcus rate-equilibrium theory

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 (115 -CsR;Me)Re(C0)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 rei1uxing 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 lOO"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 intem1olecular 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 beating on the old question of conformational changes between solution and crystaL 1. L.

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