Crystal packing features of potentially mesomorphic organic compounds; phase transitions in 4-(tetradecyloxy)phenacyl thiocyanate NCS-CH₂-C(O)-C₆H₄-OC₁₄H₂₉

Abstract

4-(Tetradecyloxy)phenacyl thiocyanate NCS—CH₂—C(O)—C₆H₄—OC₁₄H₂₉ was studied by differential scanning calorimetry (DSC) and X-ray diffraction. According to the DSC analysis, this compound does not form a mesophase upon melting or cooling of the isotropic melt. The crystal-I—crystal-II phase transition with a small enthalpy, which is not accompanied by mechanical degradation of the crystal, occurs at 68.8 °C prior to the melting of the crystals (82.5 °C). The crystal and molecular structure of the compound was studied by X-ray diffraction at temperatures below (22 °C) and above (73 °C) the temperature of the phase transition in the crystal. Both crystalline phases have nearly identical structures. The crystal packing consists of alternating loosely packed aliphatic and closely packed aromatic regions, which is typical of crystals of mesomorphic compounds. In the aromatic regions of the crystal packing, there are intermolecular S…S contacts. In the low-temperature phase crystal-I, the S…S distances equal to 3.458 Å correspond to a non-covalent interaction, through which the molecules are linked into centrosymmetric dimers. In the high-temperature phase, the S…S distances are increased to 3.623 Å, which corresponds to twice the van der Waals radius of sulfur. The absence of mesomorphism in the compound is attributed to the fact that, upon heating, the crystal undergoes a transition to another phase, which contains no structural elements responsible for the structuring of the melt.