Pressure-frozen benzene I revisited.

The crystal structure of benzene, C6H6, in situ pressure-frozen in phase I, has been determined by X-ray diffraction at 0.30, 0.70 and 1.10 GPa, and 296 K. The molecular aggregation within phase I is consistent with van der Waals contacts and electrostatic attraction of the positive net atomic charges at the H atoms with the negative net charges of the C atoms. The C-H...aromatic ring centre contacts are the most prominent feature of the two experimentally determined benzene crystal structures in phases I and III, whereas no stacking of the molecules has been observed. This specific crystal packing is a likely reason for the exceptionally high polymerization pressure of benzene. The changes of molecular arrangement within phase I on elevating the pressure and lowering the temperature are analogous.