Unimolecular Dissociation of 1‑Methylpyrene Cations: Why Are 1‑Methylenepyrene Cations Formed and Not a Tropylium-Containing Ion?

1-Methylpyrene radical cations undergo the loss of a hydrogen atom at internal energies above the first dissociation threshold. Imaging photoelectron photoion coincidence spectroscopy was employed in combination with RRKM modeling to determine a 0 K activation energy of 2.78 ± 0.25 eV and an entropy of activation of 6 ± 19 J K^–1 mol^–1 for this H-loss reaction. The ionization energy of 1-methylpyrene was measured by mass-selected threshold photoelectron spectroscopy to be 7.27 ± 0.01 eV. These values were found to be consistent with calculations at the CCSD/6-31G­(d)//B3-LYP/6-31G­(d) level of theory showing that the formation of the 1-methylenepyrene cation (resulting from H loss from the methyl group) is kinetically more favorable than the formation of a tropylium-containing product ion that is structurally analogous to the formation of the tropylium cation in H loss from ionized toluene. The shift away from a tropylium-containing structure was found to be due to the increased ring strain imposed on the C7 moiety when it is bound to three fused benzene rings. The RRKM results allow for the derivation of the Δ_f<i>H</i>₀^o (1-methylenepyrene cation) of 945 ± 31 kJ mol<sup>–1