Melting-points of the meta- and para-isomers of anisylpinacolone

The generally higher melting-point of a para-disubstituted benzene relative to the corresponding meta-isomer has been ascribed to the fact that, being more symmetrical, it can pack more tightly. Exceptionally, it was observed that whereas m-anisylpinacolone melts at 58.0 degreesC, the para-isomer me lts lower at 39.5 degreesC. In this work we have attempted to understand th is apparent anomaly. The crystal structures of both isomers were determined and the packing analysed. Energy calculations of the static structures and molecular dynamics (MD) simulations at temperatures just below the respect ive melting-points were performed. The structure analyses indicate that the intermolecular contacts are comparably weak in the two cases, and do not a ppear to be the direct cause of the melting-point difference. Thermal motio n analysis, packing energies and MD simulations on minicrystals indicate th e importance of both enthalpic and entropic factors in the melting behaviou r of the two isomers. The higher melting point of the meta-isomer could ori ginate from both a smaller DeltaS(f) and higher DeltaH(f) relative to the p ara-isomer. Copyright (C) 2000 John Wiley & Sons, Ltd.