Energy release and product ion fragmentation in proton transfer reactions of N2H+ and ArH+ with acetone*.

Proton transfer reaction mass spectrometry has a so far little exploited potential for the analysis of high-purity process and inert gases. In order to protonate impurities like saturated hydrocarbons, precursor ions with very low proton affinity must be used, which in turn leads to a large energy release upon protonation of molecules with high proton affinity, potentially causing fragmentation of the product ions. To explore the potential of low-proton affinity precursor ions like ArH⁺ and N₂H⁺ for analytical purposes, we studied their gas-phase reactions and kinetics with acetone by Fourier transform ion cyclotron resonance (FT–ICR) mass spectrometry. The dominant product ion in both cases is protonated acetone, but fragment ions formed include C₂H₃O⁺, C₃H₅⁺ and CH₃O⁺, which are formed in higher abundance with ArH⁺ compared to N₂H⁺. The reaction efficiencies are determined to be close to 100%. Quantum chemical calculations reveal energetically favorable reaction pathways and explain the loss of water leading to the formation of C₃H₅⁺, as well as loss of methane to yield C₂H₃O⁺via dissociative proton attachment, which corresponds to the main product in the EI mass spectrum of acetone. Formation of protonated formaldehyde CH₃O⁺ involves rearrangement of the C–C bonds to eliminate ethylene. [ABSTRACT FROM AUTHOR]