Gas-phase acidity, bond dissociation energy and enthalpy of formation of fluorine-substituted benzenes: A theoretical study

A variety of theoretical methods have been used to study the gas-phase acidity of benzene and its eleven fluorine-substituted derivatives: fluorobenzene, three isomers of difluorobenezene, three isomers of trifluorobenzene, three isomers of tetrafluorobenzene and 1,2,3,4,5-pentafluorobenzene. The high-level ab initio methods, G3//B3-LYP and CBS-QB3, are shown to reproduce experimental data to within an average of 1.9 and 1.4 kcal mol−1, respectively. Of the lower-cost methods studied, M05-2X and MP2 showed the best overall performance with mean absolute deviations of just 1.2 and 1.1 kcal mol−1, respectively. The effect of substitution and position on the acidity of the protons in the various compounds are studied and the structure–reactivity trends in these heterolytic C–H bond dissociation energies (BDEs) are compared with the corresponding homolytic C–H BDEs for the same species.