Thermodynamic properties of alkyl 1H-indole carboxylate derivatives: A combined experimental and computational study

Abstract

The standard (po=0.1MPa) molar enthalpies of formation, in the crystalline phase, of methyl 1H-indole-3-carboxylate and ethyl 1H-indole-2-carboxylate, at T=298.15K, were derived from measurements of the standard massic energies of combustion using a static bomb combustion calorimeter. The Knudsen effusion technique was used to measure the vapour pressures as a function of the temperature, which allowed determining the standard molar enthalpies of sublimation of these compounds. The standard (po=0.1MPa) molar enthalpies of formation, in the gaseous phase, at T=298.15K, were calculated by combining, for each compound, the standard molar enthalpy of formation, in the crystalline phase, and the standard molar enthalpy of sublimation, yielding −(207.6±3.6)kJ·mol−1 and −(234.4±2.4)kJ·mol−1, for methyl 1H-indole-3-carboxylate and ethyl 1H-indole-2-carboxylate, respectively.Quantum chemical studies were also conducted, in order to complement the experimental study. The gas-phase enthalpies of formation were estimated from high level ab initio molecular orbital calculations, at the G3(MP2) level, for the compounds studied experimentally, extending the study to the methyl 1H-indole-2-carboxylate and ethyl 1H-indole-3-carboxylate. The results obtained were compared with the experimental data and were also analysed in terms of structural enthalpic group contributions.