Magnetic Resonance Studies and Ab-Initio Calculations as Structure Probes for Radical Cations – Hexa-1,5-Diene Systems

Magnetic resonance (CIDNP and/or ESR) data as well as ab-initio calculations have established three distinct structure types for radical cations derived from 1,5-hexadiene sytems: a) “dissociative” radical cations (e.g. 1) containing two separate allylic moieties;b) cyclohexane-1,4-diyl radical cations (e.g. 6) in a chair conformation, the apparent products of “arrested” 1,6-cycloadditions; and c) several bridged cyclooctadiene-diyl structures (boat conformers, e.g., 2 or 3). The third radical cation type can be viewed, formally, as a cyclic conjugated “bis-homo-cyclohexatriene” structure with 5 electrons;however, the degree of interaction between the two allylic moieties appears to be quite small.The three radical cation types are striking examples of fundamental structure changesupon one-electron oxidation in systems combining and donor functions. In general, significant differences can be expected for the potential surfaces of radical cations and their neutral diamagnetic precursors.