Intricate Phase Behavior and Crystal Structure Features of Chiral para-Methoxyphenyl Glycerol Ether Forming Continuous and Partial Solid Solutions

Abstract

Heterogeneous equilibria, crystallization, and polymorphism of chiral para-methoxyphenyl glycerol ether 1 have been inspected, and, as a result, the binary phase diagram and the Gibbs free energy vs temperature plot were constructed and analyzed. This enantiomeric system forms a stable racemic compound, which turns into an almost ideal continuous solution of the enantiomers in the crystalline phase at elevated temperatures. At room temperature the system represents a stoichiometric racemic compound and two symmetrical eutectoid invariants with partial solid solutions based on the enantiomers. Crystal structures of the true racemate, the pseudoracemate, and the pure enantiomer were investigated by single crystal X-ray diffraction. The true racemate crystallizes in the Pc space group with Z′ = 2. The pseudoracemate was solved in the Pbcn group with the only independent molecule equally disordered into two mirror-related positions. The enantiomeric crystals belong to the P21212 group and are characterized by six symmetry independent molecules (Z′ = 6), two of which undergo disordering. We also discussed possible connection between the phase behavior features and the details of the crystal structure, in particular, bilayer supramolecular organization, pseudosymmetry, high Z′, and disordered packing. General considerations about the crystalline nature of solid solutions of enantiomers were also made.