Ion chemistry at elevated ion–molecule interaction energies in a selected ion flow-drift tube: reactions of H3O+, NO+ and O2+ with saturated aliphatic ketones

Abstract

The reactions of H₃O⁺, NO⁺ and O₂⁺ ions with a homologous series of six aliphatic ketones, viz. acetone through 2-octanone, have been investigated in a helium-buffered selected ion flow-drift tube, SIFDT, in order to reveal their dependencies on ion–molecule interaction energies, E_r, and to gain insight into their mechanisms. The ultimate motivation is to allow analysis and absolute quantification of trace amounts of ketones and other volatile organic compounds in air using selected ion flow-drift tube mass spectrometry, SIFDT-MS. The reactions of H₃O⁺ with the ketone molecules, M, proceed via exothermic proton transfer producing MH⁺ ions, the collisional rate coefficients, k_c, for which can be calculated as a function of E_r and are seen to reduce by about one third over the E_r range from 0.05 eV up to 0.5 eV. The rate coefficients, k, and product ion distributions for the NO⁺ and O₂⁺ reactions with M had to be obtained experimentally relative to the calculated k_c for the H₃O⁺ reactions. The product ions of the NO⁺/ketones reactions initially proceed via the formation of excited (NO⁺M)* adduct ions that partially fragment, and the k reduces with E_r as much as four times for the acetone reactions but remains close to their respective k_c for the higher-order ketones indicating long lifetimes of the (NO⁺M)* ions with respect to the stabilising collision times with He atoms. The k for the O₂⁺/ketones dissociative charge transfer reactions are observed to be greater than their calculated k_c implying that long distance electron transfer occurs.