Crystal structures of mucochloric and mucobromic acids: pseudosymmetry, polymorphism, and intermolecular interactions

Abstract

The crystal structure of the known triclinic modification of mucochloric acid (MCA) was refined significantly, as well as its earlier unknown orthorhombic modification was discovered. In both forms, the compound is represented by two independent molecules with a pseudosymmetric relationship between them. Despite different symmetries of two crystal modifications of MCA, the triclinic centrosymmetric and rhombic chiral crystals display analogous layers formed by classical hydrogen bonds and stabilized additionally through the C–H...O and C=O...C=O (n(O)→π(C=O)*) interactions. The layers formally differ in the crystallographic status of symmetry elements: the triclinic crystal has crystallographic symmetry center, pseudosymmetric screw axes 21, and pseudoplanes of sliding reflection; while, in the rhombic crystal, the axes 21 are crystallographic and the center of symmetry and glide planes are pseudosymmetric. The fundamental difference between the structures of polymorphs consists in the manner of layer joint: parallel in the triclinic crystal and antiparallel in the orthorhombic crystal. The triclinic crystal structure of mucobromic acid (MBA) isostructural to the well-known polymorph of MCA was established for the first time. The possible reason for the pseudosymmetry in the crystals of MCA and MBA is a specificity of the optimum geometry necessary for the Hal...Hal contacts to be realized, which results in the displacement of analogous layers relative to each other. The specific nature of Cl...Cl contacts in the MCA crystals was studied additionally by DFT/PBE0/6-31G(d,p).