Standard molar enthalpies of formation and of sublimation of the terphenyl isomers

Abstract

The standard ( p ∘ = 0.1 MPa) molar enthalpies of formation in the crystalline phases of ortho, meta and para-terphenyl isomers, at T = 298.15 K, were derived from the standard molar energies of combustion, measured by mini-bomb combustion calorimetry. The Knudsen mass-loss effusion technique was used to measure the dependence of the vapour pressure of the crystals with the temperature, thus deriving their standard molar enthalpies of sublimation by means of the Clausius–Clapeyron equation. Combining the standard molar enthalpies of formation and sublimation of the crystalline terphenyls, the standard molar enthalpies of formation in the gaseous state, at T = 298.15 K, were derived for the three isomers. Compound - Δ c U m ∘ ( cr ) / ( kJ · mol - 1 ) Δ cr g H m ∘ / ( kJ · mol - 1 ) Δ f H m ∘ ( g ) / ( kJ · mol - 1 ) o-Terphenyl 9257.8 ± 2.8 103.0 ± 0.4 285.5 ± 3.6 m-Terphenyl 9236.6 ± 3.0 118.6 ± 0.7 279.9 ± 3.9 p-Terphenyl 9234.1 ± 2.4 125.6 ± 0.8 284.4 ± 3.5 The results show small but detectable isomerization enthalpies between the terphenyls, indicating the following relative enthalpic stabilities: m- > p- ≈ o-terphenyl.