Pressure-induced structural change of the tetrafluoro-p-benzoquinone (p-fluoranil) crystal from ab initio total energy calculations

The pressure-induced structural change of the tetrafluoro-p-benzoquinone (p-fluoranil, C₆F₄O₂) crystal has been investigated by means of periodic density functional theory calculations with plane waves up to a pressure of 4 GPa. The calculations indicate that an anisotropic deformation of the unit cell of the crystal occurs under the effect of hydrostatic compression at pressure regimes above 1 GPa. The predicted change of the unit cell axial lengths follows the order b > c > a. A rationale for this trend was obtained by comparing the strengths and orientations of the principal interaction motifs that characterize this molecular crystal. Two types of dipolar interactions involving the carbonyl groups of p-fluoranil are operative on the bc face of the crystal unit cell. The stronger intermolecular interaction (-4.2 kcal/mol) arises from an antiparallel motif characterized by a pair of equivalent C=O ··· C=O contacts, whereas a crossed interaction motif with a single short C=O ··· C=O contact possess about half the strength (-2.1 kcal/mol) of the former. A weaker interaction energy (-1.2 kcal/mol) characterizes two F ··· F interactions between a pair of neighboring p-fluoranil molecules aligned along the a axis. Some similarities between the structural changes induced by variations of pressure and temperature are also discussed.