Heat capacity and phase transitions of the orientationally disordered crystal C(SCH3)4

✍ Scribed by Michio Sorai; Keisuke Kimura; Alarich Weiss; Roman Strauss

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 208 KB
• Volume: 30
• Category: Article
• ISSN: 0021-9614
• DOI: 10.1006/jcht.1998.0442

✦ Synopsis

The heat capacities of tetrakis methylthio methane C SCH were determined at the 3 4 Ž . temperatures T s 13 to 360 K by using an adiabatic calorimeter. Three first-order phase transitions were observed, whose temperatures and enthalpy and entropy gains were determined as follows: transition from phase III to II, T s 296.23 K, ⌬ H s trs trs m . y1 . y1 . y1 7.09 kJ mol , ⌬ S s 23.93 J K mol ; transition from phase II to I, T s 318.76 K, trs m trs . y1 . y1 . y1 ⌬ H s 7.29 kJ mol , ⌬ S s 22.87 J K mol ; transition from phase I to liquid, trs m trs m . y1 . y1 . y1 T s 338.89 K, ⌬ H s 3.31 kJ mol , ⌬ S s 9.77 J K mol . The highest fus fus m fus m temperature crystalline phase I can be assigned as an orientationally disordered state, which is established by two-step phase transitions. The entropy gains at the phase transitions occurring in the solid state are well accounted for in terms of thermal excitation of the reorientational molecular motions in the crystal lattice. Based on the transition entropies, the present study clearly predicts that the most reasonable space group of phase II is not 17 2 Ž . Ž . I4rmmm D reported so far by X-ray diffraction, but I4 S . This conclusion supports the 4h 4 assignment previously derived from Raman Spectroscopy.