Calculation of ionization potentials and C-H bond dissociation energies of toluene derivatives

We present ab-initio and density functional theory calculations (with both local and nonlocal exchange-correlation functionals) on the ionization potentials and C-H bond dissociation energies in para-substituted toluene derivatives and compare the results with available experimental data. We find that with the Hartree-Fock formalism the inclusion of correlation and spin projection is necessary to achieve satisfactory agreement with experiment. The DFT calculations lead to a good reproduction of the ionization potentials with all exchange-correlation functionals investigated, while for the C-H bond energies nonlocal exchange-correlation functionals are necessary. In all cases the B3LYP functional leads to results that are closest to experiment. In agreement with experimental data, we find that the ionization potentials are sensitive to the ring substituents, whereas the bond dissociation energies are hardly affected. The significance of these findings for the reaction mechanism of the chloroperoxidase-dependent oxidation of toluene derivatives is discussed.