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

• Published in: The Journal of chemical thermodynamics 1998-12, Vol.30 (12), p.1441-1453
• Main Authors: Sorai, Michio, Kimura, Keisuke, Weiss, Alarich, Strauss, Roman
• Format: Article
• Language: English
• Subjects: Chemical thermodynamics; Chemistry; Elements, mineral and organic compounds; Exact sciences and technology; General and physical chemistry; heat capacity; orientationally disordered crystal; phase transition; plastic crystal; tetrakis(methylthio)methane; Thermodynamic properties
• ISSN: 0021-9614; 1096-3626
• DOI: 10.1006/jcht.1998.0442

The heat capacities of tetrakis(methylthio)methane C(SCH3)4were determined at the temperaturesT=(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,Ttrs=296.23 K, ΔtrsHm=7.09 kJ·mol−1, ΔtrsSm=23.93 J · K−1·mol−1; transition from phase II to I,Ttrs=318.76 K, ΔtrsHm=7.29 kJ · mol−1, ΔtrsSm=22.87 J · K−1·mol−1; transition from phase I to liquid,Tfus=338.89 K, ΔfusHm=3.31 kJ · mol−1, ΔfusSm=9.77 J · K−1·mol−1. The highest 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 notI4/mmm(D174h) reported so far by X-ray diffraction, butI4 (S24). This conclusion supports the assignment previously derived from Raman Spectroscopy.