Sublimation thermodynamics of pyrazinoic, dipicolinic and quinolinic acids: Experiment and theoretical prediction

The thermophysical characteristics and decomposition of pyrazinoic (POA), dipicolinic (DPA), and quinolinic (QNA) acids have been studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and mass-spectrometry. The transpiration method has been used to measure the vapour pressures as a function of the POA, DPA or QNA temperature. It has been found that QNA remains thermally stable in the solid form up to the temperature of 367.15 K, and further heating causes its decarboxylation and irreversible conversion to nicotinic acid. The standard molar enthalpies, entropies, and Gibbs energies of sublimation for POA, DPA and QNA have been calculated. A comparative study of five calculation schemes for sublimation enthalpy estimation (periodic Density Functional Theory computations (DFT-D3), Quantum Theory of Atoms in Molecules (QTAIM), Gavezzotti’s Coulomb-London-Pauli model (CLP and PIXEL) and CrystalExplorer CE-B3LYP) has been conducted. The sublimation thermodynamic functions of the compounds studied by the correlation method have been estimated. When applied to the objects of this study, the method proved to be a quick, convenient and inexpensive way to evaluate the sublimation thermodynamic functions of molecular crystals using only the melting temperature of the compound.