Selective Generation of the Radical Cation Isomers [CH3CN]•+ and [CH2CNH]•+ via VUV Photoionization of Different Neutral Precursors and Their Reactivity with C2H4

Experimental and theoretical studies have been carried out to demonstrate the selective generation of two different C2H3N+ isomers, namely, the acetonitrile [CH3CN]•+ and the ketenimine [CH2CNH]•+ radical cations. Photoionization and dissociative photoionization experiments from different neutral precursors (acetonitrile and butanenitrile) have been performed using vacuum ultraviolet (VUV) synchrotron radiation in the 10-15 eV energy range, delivered by the DESIRS beamline at the SOLEIL storage ring. For butanenitrile (CH3CH2CH2CN) an experimental ionization threshold of 11.29 ± 0.05 eV is obtained, whereas the appearance energy for the formation of [CH2CNH]•+ fragments is 11.52 ± 0.05 eV. Experimental findings are fully supported by theoretical calculations at the G4 level of theory (ZPVE corrected energies at 0 K), giving a value of 11.33 eV for the adiabatic ionization energy of butanenitrile and an exothermicity of 0.49 for fragmentation into [CH2CNH]•+ plus C2H4, hampered by an energy barrier of 0.29 eV. The energy difference between [CH3CN]•+ and [CH2CNH]•+ is 2.28 eV (with the latter being the lowest energy isomer), and the isomerization barrier is 0.84 eV. Reactive monitoring experiments of the [CH3CN]•+ and [CH2CNH]•+ isomers with C2H4 have been performed using the CERISES guided ion beam tandem mass spectrometer and exploiting the selectivity of ethylene that gives exothermic charge exchange and proton transfer reactions with [CH3CN]•+ but not with [CH2CNH]•+ isomers. In addition, minor reactive channels are observed leading to the formation of new C-C bonds upon reaction of [CH3CN]•+ with C2H4, and their astrochemical implications are briefly discussed.