A selected ion flow tube, SIFT, study of the ion chemistry of H 3O +, NO + and O 2+[rad] ions with several nitroalkanes in the presence of water vapour

Abstract

We have carried out a selected ion flow tube (SIFT) study of the reactions of H 3O +, NO + and O 2 + with the following six nitroalkanes: nitromethane, nitroethane, 1-nitropropane, 2-nitropropane, 1-nitrobutane and 2-methyl-2-nitropropane. The primary purpose of this work was to extend the kinetics database to allow these compounds, M, to be analysed in air by selected ion flow tube mass spectrometry, SIFT–MS. Some nitroalkanes are used as industrial solvents and some are component of agricultural agents that are known health hazards. The initial step in all the H 3O + reactions is exothermic proton transfer to produce MH + ions, which are seen to be the only products for the two smallest nitroalkanes, but for the isomers of nitropropane and nitrobutane, fragmentation of the nascent MH + ions occurs. NO + reacts with the four smallest compounds via association resulting in NO +M product ions, whilst for the isomers of nitrobutane the C 4H 9 + hydrocarbon ion is produced. The reaction of O 2 + with nitromethane proceeds via charge transfer giving M + as the major product, whilst the O 2 + reactions with all the remaining nitroalkanes in this study lead to a single hydrocarbon ion product C n H 2 n+1 +. The secondary chemistry of the ion products with H 2O and with M, which is relevant to SIFT–MS applications, is fully described, with the interesting finding that water cluster ions of the kind MH +(H 2O) 3 containing three water molecules are formed at 300 K. The mechanisms of the reactions are described with the aid of ab initio calculations of the ion energetics that were not previously available for some of the ions involved in the chemistry.