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 Cs 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 C2v 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.