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

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• 作者：Miroslav Polášek ; Emilie-Laure Zins ; Christian Alcaraz ; Ján Žabka ; Věra Křížová ; Linda Giacomozzi ; Paolo Tosi ; Daniela Ascenzi
• 刊名：Journal of Physical Chemistry A
• 出版年：2016
• 出版时间：July 14, 2016
• 年：2016
• 卷：120
• 期：27
• 页码：5041-5052
• 全文大小：644K
• 年卷期：0
• ISSN：1520-5215

文摘

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.