The transition state of electrophilic aromatic substitution in the gas phase

Abstract

Electrophilic aromatic substitution without solvent molecules is a one-step reaction. This is the result of an MP2/6-31+G** analysis of the benzene–HF–BF₃ supermolecule. There is a qualitative agreement of this with an estimation given by Tomasi etal. in 1997 (Theoret. Chim. Acta, 1977, 45, 127). We report the structures of the BF₃–HF–benzene complex and the degenerate C_s transition structures of the H exchange. However, considering the zero-point vibrational energies in order to get Δ^‡H°(0), a broad transition state region including the C_2v structure becomes evident. It demonstrates the large out-of-plane mobility of the reaction cycle. The cationic subunits of the zwitterionic transition structures resembles the well-known σ-complex structure. It is shown that the prolongation of the C–H bonds at the protonated C atom of benzene may lead to a small HCH angle and a transcyclic H···H bonding in the transition structure.