Competing C═O···C═O, C–H···O, Cl···O, and Cl···Cl Interactions Governing the Structural Phase Transition of 2,6-Dichloro-p-benzoquinone at Tc=122.6 K

2,6-Dichloro-p-benzoquinone (DCBQ) has been investigated by single-crystal X-ray diffraction experiments in the T range of 300−21 K and quantum-mechanical simulations. A reversible monoclinic (high-T) to triclinic (low-T) phase transition has been detected at Tc = 122.6(5) K. The various noncovalent interactions (NCIs) that determine the solid-state self recognition of DCBQ have been characterized as a function of T through the quantum theory of atoms in molecules. On lowering T, carbonyl−carbonyl interactions progressively strengthen, inducing a change in the crystal structure, while the dipolar C−Cl···O=C NCIs and the relatively strong Cl···Cl halogen bonds (XBs) and CH···O hydrogen bonds play an essential, but ancillary, role. Dispersive forces cooperate with other closed-shell dipolar NCIs, and particularly with XBs, in determining their overall attractive character, even when bulky and positively charged chlorine atoms are drawn closer and closer at low temperatures. The intermolecular interaction energies have been evaluated above and below Tc as sums of electrostatic, repulsion, and dispersion contributions.