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, HOOCH 2 OCHO) 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. Based on experimentally observed ionization thresholds and fragmentation appearance energies, interpreted with the aid of ab initio calculations, we have identified HPMF and its

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

“…Therefore, we provide the most reliable mole fraction profiles possible for CH 3 OOH and HPMF using the theoretically calculated PICS, as described in the previous section. In the last part, we provide further validation targets for the HPMF chemistry in the form of the temperature profiles for the isomeric CH 2 O 3 intermediates, which were identified as the HPMF decomposition products performic and carbonic acid [HC(O)OOH and HOC(O)OH] in ref 6. This section of the paper also includes a discussion of the limitations of our approach to determine PICS with respect to the separation and quantification of isomeric species.…”

“…Note that 1,3-dioxetane was identified as an isomer on the same mass. 6 This intermediate species was not considered in the data evaluation due to its very low concentration, and the resulting error is considered to be within 1% on the methyl formate concentration based on the photoionization efficiency (PIE) curve reported in ref 6. Interestingly, the Nancy mech is capable of predicting the double-peak feature in the low-temperature regime measured in the experiment for CH 3 OCHO. In addition, a smaller second peak in the intermediate-temperature range can be seen at 820 K, which was also observed in the study of Rodriguez et al 5 This peak is currently only predicted by the Nancy mechanism.…”

“…58 The signal increase above 11.3 eV [visible in Figure 6a] falls together with the lowest-lying excited electronic state of the cation for CH 3 OOH, which was calculated to be 1.6 eV above the ground electronic state of the cation. This increase could also be caused by the isomeric methanediol (calcd IE = 11.10 eV), 6 but its presence in the low-temperature oxidation regime of DME is unknown. Both facts do not influence our determination of the necessary cross section values in the chosen energy range.…”

“…6 All of these appearance energies are within the first 1.0−1.5 eV above the ionization energy of HPMF (∼10.0 eV), and resulting fragmentation pattern therefore has to be considered in the determination of HPMF's photoionization cross section.…”

“…The applicability of the previously presented method to determine photoionization cross sections is limited if two or more isomers contribute to the PIE curve within the first 1.5 eV over the lowest threshold. Performic acid and carbonic acid, both identified as intermediates in the low-temperature oxidation regime of DME, 6 represent such a case. At photon energies 1.5 eV above the threshold of performic acid both species contribute to the measured PIE curve on m/z = 62.000 u (calcd IEs: performic acid 10.87 eV; carbonic acid 11.29 eV).…”