Polymorphism of 1,3-cyclohexanediols: molecular structure and plastic crystal formation of cyclohexanediol isomers

In this work, solid state characterization and the investigation of the thermal behavior of cis -1,3 and trans -1,3 cyclohexanediol isomers were carried out. A plastic crystal phase could be identified only for the trans isomer, which vitrifies on cooling, while for the cis isomer two monotropically related ordered crystal phases have been found. These results allow gaining of comprehensive knowledge of the six cyclohexanediol isomers' plastic crystal forming ability; plastic crystal phases were observed for three of the six cyclohexanediol derivatives, coinciding with the axial-equatorial isomers cis -1,2, trans -1,3 and cis -1,4-cyclohexanediols. The molecular structure of all the possible conformations of the isolated molecules of the six cyclohexanediol isomers was optimized at the MP2/aug-cc-pVDZ level of theory. The axial-equatorial isomers have several conformers with relevant abundance, which can have greater entropy of mixing, which may have a role in stabilizing the disordered plastic phases. For trans -1,4-cyclohexanediol, the conformer population breakdown is 66.4% bi-equatorial and 33.6% bi-axial. The bi-axial conformations of trans -1,4-cyclohexanediol are preserved in the solid forms. The Hirshfeld surface analysis of the parent anisotropic phase was confirmed as a valuable tool to assess the plastic crystal forming ability in this family of compounds, and the asphericity parameter to be a good indicator. The fingerprint plots highlight frequent intermolecular H-H close contacts for cis -1,3-cyclohexanediol, also found in other non-plastic crystal former cyclohexanediols. Different plastic crystal formation abilities of cyclohexanediol isomers interpreted with Hirshfeld surfaces.