Reaction of phenylperoxy radicals with NO2 at 298 K

Capillary atmospheric pressure electron capture ionization (cAPECI): a highly efficient ionization method for nitroaromatic compounds

We report on a novel method for atmospheric pressure ionization of compounds with elevated electron affinity (e.g., nitroaromatic compounds) or gas phase acidity (e.g., phenols), respectively. The method is based on the generation of thermal electrons by the photo-electric effect, followed by electron capture of oxygen when air is the gas matrix yielding O2(-) or of the analyte directly with nitrogen as matrix. Charge transfer or proton abstraction by O2(-) leads to the ionization of the analytes. The interaction of UV-light with metals is a clean method for the generation of thermal electrons at atmospheric pressure. Furthermore, only negative ions are generated and neutral radical formation is minimized, in contrast to discharge- or dopant assisted methods. Ionization takes place inside the transfer capillary of the mass spectrometer leading to comparably short transfer times of ions to the high vacuum region of the mass spectrometer. This strongly reduces ion transformation processes, resulting in mass spectra that more closely relate to the neutral analyte distribution. cAPECI is thus a soft and selective ionization method with detection limits in the pptV range. In comparison to standard ionization methods (e.g., PTR), cAPECI is superior with respect to both selectivity and achievable detection limits. cAPECI demonstrates to be a promising ionization method for applications in relevant fields as, for example, explosives detection and atmospheric chemistry.

Hydroxyl radical regeneration in the photochemical oxidation of glyoxal: kinetics and mechanism of the HC(O)CO + O(2) reaction

Glyoxal, HC(O)CHO, is an important trace component of the Earth's atmosphere, formed in biomass burning and in the photooxidation of volatile organic compounds (VOCs) like isoprene and aromatic hydrocarbons. The HC(O)CO free radical is the primary product of the glyoxal + OH reaction, and this study uses computational chemistry to show that the HC(O)CO radical can react with O(2) to regenerate the hydroxyl radical (OH) in the atmosphere. Master equation simulations indicate that the HC(O)C(O)O(2) peroxy radical adduct proceeds directly to CO(2) + CO + OH in a chemically activated mechanism, with minor collisional deactivation of the relatively unstable HC(O)C(O)O(2) peroxy radical. The reaction of HC(O)CO with O(2) is found to be competitive with thermal decomposition to HCO + CO at tropospheric temperatures and pressures, accounting for ca. 40% or more of the total yield. The present process provides a new mechanism for OH regeneration in the troposphere, which involves the decomposition of unstable alpha-formylperoxy radicals.

CO Formation in the 254 nm Gas Phase Photolysis of Nitrosobenzene-Oxygen Mixtures at Room Temperature

Large CO yields were detected in the gas phase photolysis of nitrosobenzene at 254 nm in the presence of O2 and N2 by long-path IR absorption in a 200 L photoreactor made of quartz. CO yields were measured at 298 K for partial pressures of O2 between 1 μbar and 1 bar and total pressures from 10 μbar to 1 bar. Depending on reaction conditions, the CO yield varied between ≤ 1 % (at p N2 = 1 bar, p O2 ≤ 120 μbar) and ≈ 100 % (for pO2 = 20 μbar, p N2 = 0). The dependencies of the CO yield (i) from the O2 partial pressure in O2-N2-mixtures at a total pressure of 1 bar, (ii) from the O2 pressure in the absence of N2, and (iii) from the total pressure at constant O2 pressures show very different behaviour. They can be reproduced well by a mechanism involving vibrationally excited phenyl (C6H5*), phenylperoxy (C6H5O2*) and phenoxyl (C6H5O*) radicals where collisional relaxation of the excited radicals competes with O2 addition for C6H5* and with both unimolecular decomposition and reaction with O2 for C6H5O2* and/or C6H5O*. The measured CO yields allow to select reaction conditions where nearly exclusively either highly vibrationally excited C6H5, C6H5O2 and/or C6H5O radicals (for CO yields ≈ 1) or thermally distributed C6H5, C6H5O2 and/or C6H5O radicals (for CO yields ≤ 1 %) are formed.