Association CH···π and No van der Waals Contacts at the Lowest Limits of Crystalline Benzene I and II Stability Regions

Molecular arrangements have been determined at the lowest limits of pressure ranges of benzene phase I, at 0.15 GPa, and phase II at 0.91 and 0.97 GPa, all at 295 K. All intermolecular contacts both in phase I and phase II to about 1.0 GPa exceed the sums of van der Waals radii; however, the transition between phases I and II does not affect the pattern of CH···π(arene) hydrogen bonds. In phase I the molecules are CH···π bonded approximately perpendicular into sheets, and there are substantial voids between the molecules within the sheets. The mechanism of transition to phase II involves a collapse of the voids, simultaneous with a shift of the CH···π bonded sheets. The thickness of sheets increases, which partly compensates the volume reduction due to the voids collapse; hence, the transition exhibits a large hysteresis of two GPa and a sluggish character at 295 K. No other phases of benzene have been observed between 0.15 and 5.0 GPa.