Thermodynamic Insights on the Structure-Property Relationships in Substituted Benzenes: Are the Pairwise Interactions in Tri-Substituted Methyl-Nitro-Benzoic Acids Still Valid?

Abstract

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.