Double ionization of cycloheptatriene and the reactions of the resulting C7Hn2+ dications (n = 6, 8) with xenon

Abstract

The formation and fragmentation of the molecular dication C₇H₈²⁺ from cycloheptatriene (CHT) and the bimolecular reactivities of C₇H₈²⁺ and C₇H₆²⁺ are studied using multipole-based tandem mass spectrometers with either electron ionization or photoionization using synchrotron radiation. From the photoionization studies, an apparent double-ionization energy of CHT of (22.67 ± 0.05) eV is derived, and the appearance energy of the most abundant fragment ion C₇H₆²⁺, formed via H₂ elimination, is determined as (23.62 ± 0.07) eV. Analysis of both the experimental data as well as results of theoretical calculations strongly indicate, however, that an adiabatic transition to the dication state is not possible upon photoionization of neutral CHT and the experimental value is just considered as an upper bound. Instead, an analysis via two different Born–Haber cycles suggests ²IE(CHT) = (21.6 ± 0.2) eV. Further, the bimolecular reactivities of the C₇H_n²⁺ dications (n = 6, 8), generated via double ionization of CHT as a precursor, with xenon as well as nitrogen lead, inter alia, to the formation of the organo-xenon dication C₇H₆Xe²⁺ and the corresponding nitrogen adduct C₇H₆N₂²⁺.