The effects of molecular and lattice structures on thermotropic phase behaviour of zinc(II) undecanoate and isomeric zinc(II) undecynoates

Abstract

Molecular structures, hydrocarbon chain packing, in the crystal lattice and their effects on the thermal behaviour of saturated and isomeric zinc(II) undecynoates have been investigated by a variety of physical methods. All the compounds crystallize in a monoclinic crystal system with a being the long axis. The hydrocarbon chains adopt the fully extended all-trans conformation and are arranged as methyl–methyl overlapping bilayers within a lamellar. Furthermore, in order to enhance lattice stability, hydrocarbon chains, from different layers in the lamellar are not in the same plane but are packed in an alternating spatial arrangement and are tilted at ca. 60° to the metal basal plane. In a molecule, four carbonyl groups bind to a zinc atom, in a bridging bidentate mode, to form a three dimensional polymeric network. At elevated temperatures a highly viscous phase, a polymeric ionic mesophase, possibly a smectic C phase, is observed for 10-undecynoate only, whereas two crystal–crystal and crystal-isotropic liquid transitions are observed for the undecanoate and 9-undecynoate, respectively. Though head group coordination is nearly iso-structural, differences in molecular symmetry and lattice packing are evident. These arise from differences in the spatial orientation of the high electron density hydrocarbon chains in the crystal lattice. Indeed, it is the relative balance between head group electrostatic and van der Waals interactions at elevated temperatures that accounts for formation of the mesophase. Surprisingly, all the freshly synthesized compounds are meta-stable, only achieving complete stability over several days. Furthermore, on heating and cooling the compounds, thermotropic behaviour is altered so that on re-heating, subtle changes in phase textures are observed.