Ionic Probes of Aromaticity in Annelated Rings

Article abstract of DOI:10.1021/ja00226a001

Ionization by the deprotonation of benzene and pyridine, and by the protonation of pyridine, involves lone pairs in the ? plane without significant ? effects.In these cases annelation by a benzene ring increases the acidity or proton affinity by a constant 6+/-1 kcal/mol, ascribed to increased polarizability.In comparison, protonation of benzene and deprotonation of cyclopentadiene disrupts or creates a 6-electron aromatic system, respectively, and in their annelated derivatives, naphthalene and indene, a secondary 4-electron conjugated ? system.These cases involving ? electrons show annelation effects that are substantially larger (13.4 kcal/mol) or smaller (1.0 kcal/mol), respectively, than just the electrostatic effect.Analysis of these data suggests that the stability of secondary 4-electron system in the annelated rings is smaller by 6+/-1 kcal/mol than the aromatic 6-electron systems, in fair agreement with Herndon's structure-resonance values for these species.Annelation effects are reproduced well by Dewar's AM1 semiempirical method.