Polymorphic crystals of oxacalix[4]arene with 1,3-alternate conformations of S ₄ and C ₂ symmetry

Calix[4]arene and oxacalix[4]arene derivatives have eight possible conformations in the up and down directions of their four aromatic rings from the mean plane of a bridged central ring, the conformations of which determine the functionality of the host frameworks. Despite being a known compound for five decades, oxacalix[4]arene, C ₂₄ H ₁₆ O ₄ , has not been characterized previously by crystallographic methods. It crystallizes from hexane/CH ₂ Cl ₂ solution to give two polymorphs, i.e. prismatic and block-shaped crystals as twisted 1,3-alternate structures with S ₄ and C ₂ symmetry, respectively. These were previously suggested as the prefered stable conformations by density functional theory (DFT) calculations.