Polymorphism of cis-1,4-cyclohexanediol, a new plastic crystal former. Considerations on isomeric cyclohexanediols plastic crystal forming abilities

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In this work an investigation on the polymorphism of cis-1,4-cyclohexanediol has been undertaken with emphasis on solid forms obtained in heating/cooling cycles, performed between 0°C and 115°C. A multidisciplinary investigation using differential scanning calorimetry, polarized light thermal microscopy and X-ray diffraction analysis was performed. A crystalline structure of the title compound was solved for the first time: an anisotropic solid phase, polymorph II, which gives rise on heating to a plastic crystalline phase, polymorph I, at T=(101.1±0.2)°C, which melts at T=(108.5±0.2)°C. A complex thermal behavior is observed in cooling/heating cycles performed on the molten compound, with evidence of the existence of other polymorphic forms (another ordered one, form III and form IV, tentatively assigned as plastic crystal).A comparative analysis of the crystalline structures of cis-1,4-cyclohexanediol, cis-1,2-cyclohexanediol, both plastic crystal formers, and of trans-1,4-cyclohexanediol, trans-(1S,2S)-cyclohexanediol, which do not present this mesophase, is performed by Hirshfeld surface analysis. Some similarities could be found between the plastic crystal formers which contrast with the results for the trans-isomers, namely in what concerns fingerprint plots (with much higher anisotropy for the latter), asphericity values (quite similar for the plastic crystalline formers with higher values found for the trans-isomers). It is also worth noting that in cis-1,2- and cis-1,4-cyclohexanediol, the H-bond contacts are confined in one side of the Hirshfeld surface, with the apolar segments of the molecule in the opposite side of the surface. This is also observed for trans-(1S,2S)-cyclohexanediol, but for this non-plastic crystal former, close C–H⋯H–C contacts are identified.For this class of compounds Hirshfeld surface analysis gives significant clues for their different plastic crystal forming abilities.