Thermodynamic studies of phase transitions in chloropentamethylbenzene

Heat capacities of solid chloropentamethylbenzene were measured from 3 to 300 K. A first-order phase transition (phase IV to III) at 84.0±0.1 K (ΔHt=0.93±0.01 kJ mol−1, ΔSt=11.1±0.1 J K−1 mol−1), and a transition (III to II) at 153.5±0.1 K (ΔHt= 1.33±0.04 kJ mol−1, ΔSt=8.8±0.3 J K−1 mol−1) were found on the sample cooled from room temperature at a rate of about −5 K min−1. A slow exothermic phenomenon was observed below the transition at 153.5 K, which spontaneously transformed III into III′; III′ showed a transition at 187.2±0.2 K (ΔHt=1.50±0.04 kJ mol−1, ΔSt=8.5±0.3 J K−1 mol−1). The temperature dependence of the exothermic phenomenon was analyzed. Another monotropic transition occurred just above 153.5 K, to give a yet new phase II′, which showed a transition at 180.4±0.2 K (ΔHt=1.33 ±0.04 kJ mol−1, ΔSt=7.7±0.3 J K−1 mol−1). The stability relations among the phases were determined in terms of the Gibbs energies. Raman spectra of phase II showed a gradual change in lattice modes. Nuclear quadrupole resonance (35Cl), measured for phases IV and III as a function of temperature between 78 and 145 K, showed only one resonance line. No resonance was observed in phase II. Phase relations are discussed in connection with crystal structure and molecular motion.