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†
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, Er, 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, kc, for which can be calculated as a function of Er and are seen to reduce by about one third over the Er 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 kc 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 Er as much as four times for the acetone reactions but remains close to their respective kc 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 kc implying that long distance electron transfer occurs.