DFT Study on Nitration Mechanism of Benzene with Nitronium Ion

A reaction profile for the nitration of benzene with nitronium ion has been successfully calculated for the first time. Stationary points were located and characterized by the DFT procedures at the B3LYP/ 6-311G** level. When NO 2 + approaches the benzene molecule, a weakly bound π-complex was first produced, in which the distances between the nitrogen atom of nitronium ion to both carbon atoms C1 and C6 of benzene ring were equal. This bifurcation point was characterized as a transition state leading to a σ-complex reactant by shifting the positive ion to either single carbon C1 or C6. The σ-complex reactant (σ-R) finally reaches to the Wheland intermediate (σ-INT) via a σ-complex transition state (σ-TS). The formation of the σ-TS is the rate-determining step, and the barrier height is 8.37 kJ/mol with the correction for ZPVE. A comparison of the intensities and the shifts given by the computational infrared C−N and C−H stretching frequencies on going from σ-TS to σ-INT shows that the formation of bond C1−N13 and the cleavage of bond C1−H12 are not concerted but stepwise in the process of the nitration. This agrees well with the experimental fact of the lack of kinetic isotopic effect in most aromatic nitration reactions.