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

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, C 5 H 8 O 2 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 C 5 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 C 1 -C 2 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. [ABSTRACT FROM AUTHOR]