A theoretical study on nitration mechanism of benzene and solvent effects

Both π and σ transition states on the potential energy surface of the nitration of benzene with nitronium ion have been successfully optimized using unrestricted hybrid DFT procedure B3LYP with the triple-zeta 6-311G** basis set without any assumption. Subsequently, a σ reactant-complex and a σ intermediate (Wheland intermediate) were located by the intrinsic reaction coordinate (IRC) calculation. The reaction pathway and the experimental observation lacking the kinetic isotopic effect in the title nitration were confirmed on geometries, atomic charges, energies, IR spectra and thermodynamic properties of all stationary points. The activation energy of 8.370 kJ/mol in the gas phase and the order of 1010 mol@L-1@s-1 of rate constant were obtained for the reaction. The results both in thermodynamics and kinetics show that the electrophilic substitute mechanism is more preferable than the electron transfer mechanism of radical pairs. The solvent effect on the geometries of stationary points and the reaction mechanism were systematically studied for the nitration of benzene with nitronium by self-consistent reaction field (SCRF) technique with different dielectric constants of 5.0, 25.0, 50.0 and 78.5. It was then found that the solvent effect would depress the activation energy and finally make the formation of σ-TS without energy barrier in aqueous solution. Furthermore, the linear correlations given by charge migrations of NO2 group, dipole moments of solute, gaps of HOMO and LUMO and solvent stabilization energies in different solvents were demonstrated for both theoretically and experimentally concerned Wheland intermediate.