Dissociation of the Anthracene Radical Cation: A Comparative Look at iPEPICO and Collision-Induced Dissociation Mass Spectrometry Results

The dissociation of the anthracene radical cation has been studied using two different methods: imaging photoelectron photoion coincidence spectrometry (iPEPCO) and atmospheric pressure chemical ionization–collision induced dissociation mass spectrometry (APCI-CID). Four reactions were investigated: (R1) C₁₄H₁₀^+• → C₁₄H₉⁺ + H, (R2) C₁₄H₉⁺ → C₁₄H₈^+• + H, (R3) C₁₄H₁₀^+• → C₁₂H₈^+• + C₂H₂ and (R4) C₁₄H₁₀^+• → C₁₀H₈^+• + C₄H₂. An attempt was made to assign structures to each fragment ion, and although there is still room for debate whether for the C₁₂H₈^+• fragment ion is a cyclobuta­[b]­naphthalene or a biphenylene cation, our modeling results and calculations appear to suggest the more likely structure is cyclobuta­[b]­naphthalene. The results from the iPEPICO fitting of the dissociation of ionized anthracene are E₀ = 4.28 ± 0.30 eV (R1), 2.71 ± 0.20 eV (R2), and 4.20 ± 0.30 eV (average of reaction R3) whereas the Δ^‡S values (in J K^–1 mol^–1) are 12 ± 15 (R1), 0 ± 15 (R2), and either 7 ± 10 (using cyclobuta­[b]­naphthalene ion fragment in reaction R3) or 22 ± 10 (using the biphenylene ion fragment in reaction R3). Modeling of the APCI-CID breakdown diagrams required an estimate of the postcollision internal energy distribution, which was arbitrarily assumed to correspond to a Boltzmann distribution in this study. One goal of this work was to determine if this assumption yields satisfactory energetics in agreement with the more constrained and theoretically vetted iPEPICO results. In the end, it did, with the APCI-CID results being similar.