Mechanism of Isostructural Phase Transition in Isobutane

Gaseous isobutane, when isothermally compressed at 295 K, condenses into a liquid at 0.32 MPa and freezes either at 1.41 GPa into the crystalline phase α or at 1.55 GPa into phase β. Their structures are similar, and an isostructural transition has been observed, preserving the space-group symmetry Pnma, while displaying considerable hysteresis and reversed relation of the crystal volumes. The compression of the two new phases has been analyzed through in situ isothermal and isochoric crystallizations, X-ray diffraction structural determinations, and a description of subtle differences in the molecular arrangement. The implications of this phase transition have been discussed in the context of molecular dynamics, emphasizing the interplay between pressure, temperature, and structural changes. The volume relations have rationalized the exemption of isobutane and n-butane isomers from Carnelley’s rule, reflecting molecular close packing and interactions. However, the isothermal crystallization of isobutane and n-butane proceeds according to Carnelley’s rule; i.e., the higher-symmetric isomer, isobutane, requires lower pressure than the lower-symmetric n-butane, despite its markedly higher density.