Detection and Identification of the Keto-Hydroperoxide (HOOCH₂OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether

Abstract

In this paper we report the detection and identification of the keto-hydroperoxide (hydroperoxymethyl formate, HPMF, HOOCH₂OCHO) and other partially oxidized intermediate species arising from the low-temperature (540 K) oxidation of dimethyl ether (DME). These observations were made possible by coupling a jet-stirred reactor with molecular-beam sampling capabilities, operated near atmospheric pressure, to a reflectron time-of-flight mass spectrometer that employs single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation. On the basis of experimentally observed ionization thresholds and fragmentation appearance energies, interpreted with the aid of <i>ab initio</i> calculations, we have identified HPMF and its conceivable decomposition products HC­(O)­O­(O)­CH (formic acid anhydride), HC­(O)­OOH (performic acid), and HOC­(O)­OH (carbonic acid). Other intermediates that were detected and identified include HC­(O)­OCH₃ (methyl formate), <i>cycl</i>-CH₂–O–CH₂–O– (1,3-dioxetane), CH₃OOH (methyl hydroperoxide), HC­(O)­OH (formic acid), and H₂O₂ (hydrogen peroxide). We show that the theoretical characterization of multiple conformeric structures of some intermediates is required when interpreting the experimentally observed ionization thresholds, and a simple method is presented for estimating the importance of multiple conformers at the estimated temperature (∼100 K) of the present molecular beam. We also discuss possible formation pathways of the detected species: for example, supported by potential energy surface calculations, we show that performic acid may be a minor channel of the O₂ + ĊH₂OCH₂OOH reaction, resulting from the decomposition of the HOOCH₂OĊHOOH intermediate, which predominantly leads to the HPMF