On the Structure of Phenonium Ions:  The Important Role of Back-Bonding Interaction in Carbocation Chemistry

Binding in phenonium ions [C 6 H 5 −C 2 (OH) 4 ] +, [C 6 H 5 −C 2 (CH 3 ) 4 ] +, [C 6 H 5 −C 2 F 4 ] +, [ p -NC−C 6 H 4 −C 2 H 4 ] +, [ p -OHC−C 6 H 4 −C 2 H 4 ] +, [C 6 H 5 −C 2 H 4 ] +, [ p -F−C 6 H 4 −C 2 H 4 ] +, [ p -H 3 C−C 6 H 4 −C 2 H 4 ] +, [ p -HO−C 6 H 4 −C 2 H 4 ] +, [C 6 H 5 −C 2 (CHO) 4 ] +, and [C 6 H 5 −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, [C 6 H 5 −C 2 H 4 ] +, 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 π systems merge with each other. The important stabilization gained from this process determines the orthogonal conformation of the phenonium ion.