Electrostatic matching in phase IV of chloroiodomethane: a new aggregation pattern in the isostructural classes of dihalomethanes

Abstract

The new structure of chloroiodomethane, CH₂ClI, has been determined at ambient pressure and 180 K. This polymorph IV is orthorhombic, space group Pna2₁ with Z = 8. It is a unique symmetry among all classes of isostructural dihalomethane crystals determined so far in low-temperature or at high pressure, despite the fact that the electrostatic matching between the net charges of the molecules is a common feature to the CH₂ClI phase IV and all the other dihalomethane crystals. Likewise, the exceptional arrangement of CH₂ClI molecules in phase IV is governed by electrostatically attractive halogen⋯halogen and C–H⋯halogen directional interactions. Their directional character has been shown to result from the specific distribution of the electrostatic potential on the molecular surface. The formation of small voids in CH₂ClI phase IV and its density (2.83 g cm⁻³), much lower than that of phase III (3.09 g cm⁻³), can be explained by the packing efficiency of the two competing patterns of the electrostatically aggregated molecules.