Density Functional Theory Study on the Nitration of Halobenzene by Nitronium Ion

The rate determining steps of isomeric ortho, meta and para nitrations of fluorobenzene and chlorobenzene have been theoretically investigated at the B3LYP/6‐311G* level. Stationary points of the step involving reactant, transition state and intermediate complexes were successfully located and characterized without any restriction on the internal coordinates. Their molecular geometries, electronic structures, IR spectra, and the FMO symmetries of two initial aromatic compounds and the NO2+ suggest that the halogens are ortho‐para directive, and also activating substituents in the gas phase. The blue shift of the C‐N stretching vibration and the red shift of the tetrahedral C‐H stretching vibration from transition state to Wheland intermediate demonstrate that the formation of the C‐N and the cleavage of the C‐H are not concerted but stepwise at the rate determining step. This provides theoretical evidence for the experimental fact of the absence of primary kinetic isotopic effect in most aromatic nitration reactions. The fully optimized geometries of nitro‐chlorobenzene complexes show that the C‐Cl bond in nitro‐chlorobenzene complexes become much shorter than in chlorobenzene molecules and appears as a double bond. On the contrary, the C‐F bond in nitro‐fluorobenzene complexes is still a single bond, and even lengthens slightly as the NO2+ moves toward the fluorobenzene. This reveals that there is a special interaction, namely inductomeric effect, between the chlorine and nitro group in nitro‐chlorobenzene complexes. The inductomeric effect may cause a marked increase in the percentage of ortho product for the nitration of chlorobenzene.