Chemistry deriving from OOQOOH radicals in alkane low-temperature oxidation: A first combined theoretical and electron-ion coincidence mass spectrometry study

Abstract

While there is consensus on the fact that OOQOOH radicals, produced by two oxygen additions from alkyl radicals, are the heart of the low-temperature oxidation of alkanes, the determination of the isomeric distribution and the quantification of their derived products (ketohydroperoxides and diones) are still a challenge. For the first time, heavy oxygenated products produced during alkane oxidation have been investigated using electron/ion coincidence mass spectrometry. The investigated prototype reaction is n-pentane oxidation carried out in a jet-stirred reactor (temperatures from 585 to 665 K, pressure of 1.1 bar, lean mixture). Identification attempts were made for m/z 100 and 118 species using coincident mass-tagged Slow PhotoElectron Spectra obtained by electron-ion coincidence mass spectrometry combined with first principle computations, consisting in the determination of their adiabatic ionization energies and the Franck-Condon envelope of the photoionization spectra. 4-hydroperoxypentan-2-one has been confirmed as the dominant obtained ketohydroperoxide, as predicted by up-to-date kinetic models. However, difficulties due to fragmentation has made impossible the identification of the ketohydroperoxides present in lower amounts. In parallel, C5H8O2 isomers were identified, showing the possible formation, in addition to diones, of species with a ketone and an enol function. In addition, we provide new information on the first steps of the fragmentation pathways of C5 ketohydroperoxides. From the shape of their corresponding peaks on mass spectra and the energy and temperature dependence of their signal, ions at m/z 43, 57 and 85 have been identified as fragments from ketohydroperoxides. Taking into account these fragmentations lowers, by more than a factor of 10, the previously observed deviation between experiments and modeling for ketohydroperoxide mole fractions. The formation of the C1-C2 carboxylic acids, predicted from Korcek decomposition, was also observed, but with a favored formation of acetic acid versus formic acid that what was predicted for propane.