Thermodynamic properties of 1,2-dihydronaphthalene: Glassy crystals and missing entropy

Measurements leading to the calculation of the standard thermodynamic properties for gaseous 1,2-dihydronaphthalene (Chemical Abstracts registry number [447-53-0]) are reported. Experimental methods include oxygen combustion-bomb calorimetry, adiabatic heat-capacity calorimetry, vibrating-tube densitometry, comparative ebulliometry, and inclined-piston gauge manometry. 1,2-Dihydronaphthalene decomposed significantly when heated to temperatures above T = 480 K. Consequently, the critical temperature, critical pressure, and critical density were estimated. Standard molar entropies, standard molar enthalpies, and standard molar Gibbs free energies of formation were derived at selected temperatures between T = 250 K and 500 K. The standard state is defined as the ideal gas at the pressure p = p^° = 101.325 kPa. Standard entropies are compared with those calculated statistically on the basis of assigned vibrational spectra from the literature for the vapor phase. A large and near constant difference between the entropies calculated statistically and those determined calorimetrically was observed over the entire temperature range studied. Two glass-like features are observed in the heat capacity against temperature curve for the solid state, indicating that the crystals are disordered. A quantitative accounting for the entropy discrepancy is proposed based on possible molecular orientations of 1,2-dihydronaphthalene. Results are compared with experimental values reported in the literature.