Sublimation Enthalpies of Substituted Pyridine N-Oxides

Sublimation Study of Six 5-Substituted-1,10-phenanthrolines by Knudsen Effusion Mass Loss and Solution Calorimetry

The vapor pressures of six solid 5-X-1,10-phenanthrolines (where X = Cl, CH₃, CN, OCH₃, NH₂, NO₂) were determined in suitable temperature ranges by Knudsen Effusion Mass Loss (KEML). From the temperature dependencies of vapor pressure, the molar sublimation enthalpies, Δ_cr^gH_m⁰(⟨T⟩), were calculated at the corresponding average ⟨T⟩ of the explored temperature ranges. Since to the best of our knowledge no thermochemical data seem to be available in the literature regarding these compounds, the Δ_cr^gH_m⁰(⟨T⟩) values obtained by KEML experiments were adjusted to 298.15 K using a well known empirical procedure reported in the literature. The standard (p⁰ = 0.1 MPa) molar sublimation enthalpies, Δ_cr^gH_m⁰(298.15 K), were compared with those determined using a recently proposed solution calorimetry approach, which was validated using a remarkable amount of thermochemical data of molecular compounds. For this purpose, solution enthalpies at infinite dilution of the studied 5-chloro and 5-methylphenantrolines in benzene were measured at 298.15 K. Good agreement was found between the values derived by the two different approaches, and final mean values of Δ_cr^gH_m⁰(298.15 K) were recommended. Finally, the standard molar entropies and Gibbs energies of sublimation were also derived at T = 298.15 K. The volatilities of the six compounds were found to vary over a range of three orders of magnitude in the explored temperature range. The large difference in volatility was analyzed in the light of enthalpies and entropies of sublimation. The latter was tentatively put in relation to the rotational contribution of the substituent group on the phenanthroline unit.