Crystalline phase transition with a large conformational change in a thiodicarboxylic acid

A large conformational transition in a crystal of thiodipropionic acid is investigated by differential scanning calorimetry (DSC), X-ray diffraction, optical microscopy, and computer modeling. The room temperature phase (A-phase) was already known to be orthorhombic where the chains have largely zigzagged conformation with both S–C bonds in gauche conformation. A high temperature phase (C-phase) above 105°C is found by DSC. By X-ray analysis, the crystal structure of the C-phase is found to be monoclinic with molecules having nearly extended conformation. Nucleation and growth of the C-phase are clearly observed by polarizing microscope. It is found that each C-phase domain grows very fast in a preferred direction which is nearly parallel to the {110} planes of the C-phase. Molecular dynamics simulation of the transition reveals that the largely zigzagged molecules in the A-phase readily transforms to the extended all trans form at higher temperatures.