Four nitration trajectories of benzene and toluene with nitronium ion have been calculated. Totally twelve stationary points including four transition states were successfully located for the first time by using hybrid DFT procedure B3LYP with 6-31G* * without any assumption. Mechanistic studies on structures, atomic charges, energies and IR spectra for stationary points are presented to reveal the dual roles of incoming methyl group, which impacts on positional selectivity of electrophilic aromatic nitration. The order in activation energy for concerned nitration reactions was found as p- > o- > PhH > m-. Another order in isodesmic energy showing the relative stability of EDA complexes was derived as p-ArCH3-NO2+ > o-ArCH3-NO2+ > m-ArCH3-NO2+ > PhH-NO2+. In addition, absence of kinetic isotope effect in most electrophilic aromatic substitutions could also be suggested by our results.

Key words: METHYLBENZENE, NITRIFICATION, DFT, ACTIVATION ENERGY, STABILITY