Solid-state Behaviour of the Dichlorobenzenes; Actual, Semi-virtual and Virtual Crystallography.

The crystal structures of the low-melting 1,2- and 1,3-dichlorobenzene isomers have been determined by X-ray analysis and in situ crystallisation techniques. Attempts to predict these structures in advance by force-field calculations were not successful, although the known crystal structures of two of the three polymorphs of the 1,4-isomer were successfully 'a posteriori' predicted. Calculated lattice energies were supplemented with estimated lattice-vibrational entropies obtained in the rigid-body approximation. The large differences in melting point among the isomers can be largely attributed to less efficient packing of the 1,2- and 1,3-isomers.