High-pressure crystal structure of methyl iodide: molecular aggregation in the crystals of halomethanes and their isostructural relations,

Abstract

Molecular association of methyl iodide, CH₃I, is dominated by electrostatic interactions in the crystalline state. Similar interactions govern the molecular association in dihalogen crystals, Cl₂, Br₂ and I₂, and they are the reason of their isostructural relations with the crystals of halomethanes. The prototypic Cmca space group symmetry of the dihalogens reduces to Cmc2₁ for CH₃Cl and to Pnma for CH₃Br and CH₃I, due to the reduced molecular symmetry and domination of the C–H⋯halogen interactions in CH₃Cl and halogen⋯halogen interactions in CH₃Br and CH₃I. High pressure X-ray diffraction data recorded for CH₃I in situ crystallized at 0.53, 1.16 and 2.53 GPa (all at 295 K) confirm its monomorphic structure, orthorhombic space groupPnma, in this pressure range, with the crystals of CH₃I at 193 K and CD₃I at 4.5 K. The CH₃I crystal compression is consistent with the directions of halogen⋯halogen and C–H⋯halogen intermolecular electrostatic interactions in the structure.