Enthalpies of formation and strain of chlorobenzoic acids from thermochemical measurements and from ab initio calculations

Molar enthalpies of sublimation of 2-chloro-, 3-chloro-, and 4-chlorobenzoic acids were obtained from the temperature dependence of the vapor pressure measured by the transpiration method. Thermochemical investigations of chlorobenzoic acids available in the literature were collected and combined with own experimental results to obtain their reliable standard molar enthalpies of formation at T = 298.15 K in the gaseous state. Ab initio calculations of chlorobenzoic acids have been performed using the G3(MP2) theory, and results from the homodesmic reactions are in excellent agreement with experiment. New results help us to resolve the uncertainty in the available thermochemical data on chlorobenzoic acids. The strain enthalpies of chlorobenzoic acids have been assessed using an isodesmic reaction procedure.