The heat capacity of crystalline pentachloronitrobenzene was measured by adiabatic calorimetry between 6 and 303 K. A large step in heat capacity was detected around 185 K; this is attributed to a glass transition corresponding to freezing of structural disorder. The observed glass transition temperature was similar to that estimated from the temperature dependence of the dielectric relaxation due to molecular reorientation around the "sixfold axis" of the benzene core.
(Bull. Chem. Soc. Jpn., 74, 1221-1224 (2001))
The heat capacities of crystalline and liquid p-bromochlorobenzene
were measured by adiabatic calorimetry between 6 K and 350 K.
The crystal melts at 337.99 K with an enthalpy and an entropy of fusion
of 18390 J mol-1 and 55.41 J K-1 mol-1,
Neither an ordering phase transition nor a glass transition was
detected in the solid state,
contrary to an expectation based on the apparent symmetry of the molecule
at room temperature due to an orientational disorder.
The calorimetric entropy of the gas was estimated
using the vapor pressure data.
A comparison with spectroscopic entropy based on
a quantum chemical calculation shows that
the PBCB crystal has a residual entropy of about Rln2
(R: gas constant),
which is compatible with the orientational disorder at room temperature.
The absence of a sizeable anomaly due to a glass transition
suggests two possibilities:
an undetectably small thermal anomaly around the glass transition,
or a glass-transition temperature higher than the temperature of fusion.
(Bull. Chem. Soc. Jpn., 73, 2279-2282 (2000))
Heat capacities of the disordered and ordered crystalline phases of p-chloronitorobenzene were measured by adiabatic calorimetry
between 6 K and 303 K.
The disordered phase stable at room temperature shows
a glass transition around 245 K,
which arises from the freezing of molecular reorientation
as evidenced from the magnitude of the residual entropy.
The ordered phase that is stable below 279 K was obtained
through annealing the disordered phase around 260 K for a long time.
The thermodynamic quantities concerning
the first-order phase transition between the ordered and disordered phases
were determined as Ttrs = 279.0 K,
DeltatrsH = (4468 +- 5) J mol-1 and
DeltatrsS = (16.01 +- 0.05) J K-1
Standard thermodynamic functions are tabulated.
(J. Chem. Phys., 112, 2355-2360 (2000))