Protonation–oxidation manifold in large PAHs. Benzo[a]coronene and benzo[ghi]perylene; stable ion studies in superacid media and AM1 calculations

Abstract

Low temperature protonations of benzo[a]coronene 5 and benzo[ghi]perylene 3 have been studied in FSO₃H–SO₂ClF and CF₃SO₃H–SO₂ClF superacids. For 5, rapid competing oxidation to the radical cation (RC) prevents the observation of 5H⁺ by NMR spectroscopy; the RC was probed by EPR spectroscopy. For 3, competing oxidation is less problematic and a persistent arenium ion 3H⁺ could be seen by NMR spectroscopy which was line-broadened due to presence of the RC.

Protonation of a mixture of 5 and 4,5-dihydropyrene 8 produced the C-3 protonated 8H⁺ and 5H˙⁺. Addition of 8 to the superacid solution containing 3H⁺ and 3˙⁺ led to detection by NMR spectroscopy of C-3 protonated 8H⁺ and the disappearance of 3H⁺.

Arenium ion energies (ΔΔHion – neutral) and changes in carbon charges [ΔQ=q_c(ion)–q_c(neutral)] for protonation of 5 and 3 were probed by the AM1 method. The singlet oxidation dication of 5 was also calculated. The charge delocalization mode in the PAH arenium ions are discussed and compared.