On the structure of phenonium ions:: The important role of back-bonding interaction in carbocation chemistry

Binding in phenonium ions [C6H5-C-2(OH)(4)](+), [C6H5-C-2(CH3)(4)](+), [C6H5-C2F4](+), [p-NC-C6H4-C2H4](+), [p-OHC-C6H4-C2H4](+), [C6H5-C2H4](+), [p-F-C6H4-C2H4](+), [p-H3C-C6H4-C2H4](+) [p-HO-C6H4-C2H4](+), [C6H5-C-2(CHO)(4)](+), and [C6H5-C-2(CN)(4)](+) was investigated with the B3LYP/6-31G* method. The analysis of the Kohn-Sham orbitals and a Bader analysis of the computed electron density of the phenonium ion, [C6H5-C2H4](+), clearly show that back-bonding from the phenyl cation moiety to the ethylene fragment determines the formation of the three-membered cycle, rendering the shielding of the ipso carbon atom similar to that for an sp(3) C, while an extension of the conjugation occurs as both pi systems merge with each other. The important stabilization gained from this process determines the orthogonal conformation of the phenonium ion.