EPR and flash photolysis investigations of some p-quinone radical cations

Abstract

EPR and flash photolysis investigations of some p-benzoquinone radical cations in a trifluoroacetic acid–trifluoroacetic anhydride solvent mixture (5:1 vol.) are reported. Coupling constants and g-values are estimated for the radical cations of p-benzoquinone, duroquinone, tetrachloro- and tetrafluoro-p-benzoquinone and anthraquinone. The electron transfer rate between p-benzoquinone radical cation and N,N,N′,N′-tetramethyl-p-phenylenediamine was measured by flash photolysis and found to be 4 × 10⁹ dm³ mor^–1 s^–1. A protonation constant of 2.3 × 10^–5 dm³ mol^–1 for p-benzoquinone in trifluoroacetic acid-trifluoroacetic anhydride was found from conductivity measurements. Quantum chemical MNDO, AM-1 and ab initio calculations lead to the conclusion that the SOMO of the p-benzoquinone radical cation is σ-type orbital.