Reactivity and properties of dications generated by photoionization of 2,5-norbornadiene

The production of C7Hn2+ (with n = 6, 8) dications from 2,5-norbornadiene (NBD) and their reactivity toward Xe and N2 are studied using multipole-based tandem mass spectrometry techniques coupled with ionization either by collisions with energetic electrons or with tunable VUV photons coming from the SOLEIL synchrotron facility. From the photoionization studies, an apparent double photoionization energy of 2AE(NBD) = (22.80 ± 0.08) eV is derived, while the appearance energy of the C7H62+ fragment is only slightly higher at (23.08 ± 0.10) eV and the hydrogen-depleted dication dominates the mass spectrum at photon energies above about 24 eV. Theoretical calculations predict the adiabatic 2IE(NBD) as about 1.26 eV lower than the experimental value, which is attributed to the occurrence of significant Franck–Condon effects. Concerning the formation of C7H62+, theory and experiment indicate that the dehydrogenation product corresponds to the cycloheptatrienylidene dication, as is formed upon dissociative double ionization of previously studied C7H8 isomers (namely toluene and cycloheptatriene, CHT). The C7H62+ formation results from the overall exothermic rearrangement of NBD2+ into CHT2+ and thus only little extra energy is required to afford dehydrogenation. Further, the NBD2+/CHT2+ isomerization affects the reactivity of C7H82+ and C7H62+ dications toward Xe and N2, leading to the formation of the dications C7H6Xe2+ and C7H6N22+, respectively. The high efficiency of formation of the diazo dication C7H6N22+ suggests that reactions of organic dications with N2 may play an important role in the chemistry of nitrogen and hydrocarbon-rich atmospheres of planets and moons.

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► Double photoionization of 2,5-norbornadiene is found at 22.80 eV via synchrotron radiation.
► Theoretical calculations show the adiabatic ionization energy to be much lower than this experimental value, suggesting significant Franck–Condon effects.
► The resulting C7H82+ and C7H62+ dications show ‘superelectrophilic’ reactivity with N2 and Xe gases.
► Synchrotron ionization energetics are measured against reactivity to demonstrate reaction energies.