Thermodynamic Insights on the structure‐property relationships in substituted benzenes: are the pairwise interactions in tri‐substituted methyl‐nitro‐benzoic acids still valid?

A comprehensive experimental thermochemical study of nine methyl‐substituted nitrobenzoic acids was carried out, leading to the final standard molar enthalpies of formation in the gas phase. The combustion energies were measured using high‐precision combustion calorimetry, and the enthalpies of formation of the crystal phase were derived. The sublimation enthalpies were obtained from the vapor pressure‐temperature dependencies measured using the classic Knudsen effusion mass loss and the transpiration methods. The standard molar enthalpies of vaporisation were derived from the temperature dependence of the mass‐loss rates measured using the non‐isothermal thermogravimetry. The thermal behaviour, including melting temperatures and standard molar enthalpies of fusion, was investigated by DSC. The high‐level quantum chemical G* methods were used for the mutual validation of the experimental and theoretical gas phase enthalpies of formation of methyl‐substituted nitrobenzoic acids. The consistent set of experimental properties at the reference temperature T = 298 K was evaluated and recommended for thermochemical calculations. The pairwise interactions of the substituents on the benzene ring were derived from nitro‐toluenes, methyl‐benzoic acids and nitro‐benzoic acids available in the literature, and the additivity of the contributions when three substituents are placed simultaneously in the benzene ring was discussed.