Calorimetric study of plastically crystalline o- and m-carboranes

Abstract

The heat capacities of o- and m-carboranes were measured with an adiabatic calorimeter in the temperature range 5–310 K. m-carborane underwent two phase transitions at 170 K and 284 K with transition entropies of 12.70 and 15.52 J K−1 mol−1. The total transition entropy agreed well with the theoretical value, indicating that m-carborane is ordered at 0 K. o-carborane also exhibited two phase transitions at 160 K and 275 K with transition entropies of 3.72 and 13.72 J K−1 mol−1, and additionally a glass transition at 120 K. This glass transition does occur not in the supercooled intermediate-temperature phase but in the low-temperature phase of o-carborane. The high- and intermediate-temperature phases of o-carborane could not be quenched at the maximum cooling rate (10 K min−1) of this experiment. The total transition entropy of o-carborane was smaller than that of m-carborane by 10.8 J K−1 mol−1, suggesting that o-carborane has residual orientational disorder at 0 K. The enthalpy relaxation was examined by the temperature jump method around the glass transition of o-carborane. The results were reproduced well by the KWW function. The calorimetric relaxation time and nonexponential parameter agreed with dielectric ones at the glass transition temperature. This indicates that the glass transition of o-carborane is due to the freezing of molecular reorientation.