Thermodynamic consideration on a series of compounds (CH3),CCl4-n: Correlation between molecular sphericity, configurational order/disorder, and molecular packing in plastically crystalline and liquid phases

Heat capacities of tert-butyl chloride and 2,2-dichloropropane were measure d with adiabatic calorimeters in the temperature ranges between 13 and 300 K and between 6 and 300 K, respectively, and the standard thermodynamic fun ctions were evaluated. In a series of compounds, (CH3)(n)CCl4-n (n = 0-4), the absolute entropy, after subtraction of the contribution due to the exch ange between positions of the methyl group and chlorine atom within each mo lecule, showed its n dependence of a curve with a downward convex in both t he plastically crystalline and liquid phases. The volume per molecule, as t aken from literature data, showed its n dependence of a curve with a simila r downward convex in both phases. These were interpreted to indicate that t he positional and/or orientational short-range order of molecules develops with the increasing deviation of the molecular shape from spherical in both phases. Taking also into consideration the fact that the entropy of fusion decreased gradually as the shape of molecule deviated from a spherical one , it is indicated further that the development is more remarkable in the li quid phase without any long-range order than in the plastically crystalline phase with long-range positional order of molecules. It follows, from thes e considerations for nonspherical molecules, that provided a particular mol ecule is fixed in its orientation, the time-averaged orientational configur ation of the surrounding molecules would not fulfill, at the lattice site o f the particular molecule, the site symmetry expected from the space group of the plastically crystalline phase.