Selected ion flow tube studies of the reactions of H3O+, NO+, O2+˙ and O−˙ ions with alkanes in He and N2 carrier gases at different temperatures

Abstract

The kinetics of the reactions of H₃O⁺, NO⁺, O₂⁺˙ and O⁻˙ with n-hexane, 3-methylpentane, 2,5-dimethylhexane and 2,3-dimethylheptane were studied experimentally under several selected ion flow tube (SIFT) conditions: in a Profile 3 instrument in He and N₂ carrier gases at 300 K and in the Voice200 instrument in N₂ carrier gas at 300 and 393 K – where the effect of the extraction lens voltage was also assessed. It was found that H₃O⁺ ions react differently than expected, with reaction rates slower than collisional. Instead of transferring a proton, they associate and form fragment product ions [M–H]⁺. NO⁺ ions react via hydride ion transfer. O₂⁺˙ ions react via charge transfer followed by fragmentation that is highly sensitive to the temperature and the ion extraction lens voltage. Negative ions did not react, except for the O⁻˙ ion, which reacted via an associative detachment process. Computational analysis using Density Functional Theory (DFT) provided insights into the exothermicities and exergodicities of these reactions. A notable result is that proton transfer from H₃O⁺ does not take place despite its potential exothermicity; this is important for the interpretation of proton transfer reaction (PTR) and SIFT mass spectrometry data.