Protonation of fused aromatic systems: Ab initio study of some model Wheland intermediates

Structure and properties of Wheland's intermediates are studied in some model systems. The effect of the annelated small ring is simulated by bending two vicinal CH bonds in benzene toward each other. The extent of bending describes the ring‐size effect of a small fused fragment molecule. The electrophilic substituent is represented by a proton. Calculations at the HF/6‐31G* and MP2/HF/6–31G* levels of sophistication confirm the Mills–Nixon (MN) hypothesis. It is conclusively shown that ß‐complexes are more stable than are α‐protonated species. The results are interpreted in terms of the ground‐state density distribution dictated by rehybridization and of the transition structure π‐electron redistribution triggered by protonation. Their effects are additive to a large extent as far as CC bond distances are concerned. Energetic properties are determined by the interplay of two characteristic π‐electron localization patterns, one caused by the initial angular strain and the accompanying rehybridization related to the ground state and the other one occurring during formation of the proton σ‐complex. This interplay destabilizes α‐intermediates relative to ß‐intermediates. © 1994 John Wiley & Sons, Inc.