Unimolecular reaction chemistry of a charge-tagged beta-hydroxyperoxyl radical

Abstract

β-Hydroxyperoxyl radicals are formed during atmospheric oxidation of unsaturated volatile organic compounds such as isoprene. They are intermediates in the combustion of alcohols. In these environments the unimolecular isomerization and decomposition of β-hydroxyperoxyl radicals may be of importance, either through chemical or thermal activation. We have used ion-trap mass spectrometry to generate the distonic charge-tagged β-hydroxyalkyl radical anion, ˙CH2C(OH)(CH3)CH2C(O)O(-), and investigated its subsequent reaction with O2 in the gas phase under conditions that are devoid of complicating radical-radical reactions. Quantum chemical calculations and master equation/RRKM theory modeling are used to rationalize the results and discern a reaction mechanism. Reaction is found to proceed via initial hydrogen abstraction from the γ-methylene group and from the β-hydroxyl group, with both reaction channels eventually forming isobaric product ions due to loss of either ˙OH + HCHO or ˙OH + CO2. Isotope labeling studies confirm that a 1,5-hydrogen shift from the β-hydroxyl functionality results in a hydroperoxyalkoxyl radical intermediate that can undergo further unimolecular dissociations. Furthermore, this study confirms that the facile decomposition of β-hydroxyperoxyl radicals can yield ˙OH in the gas phase.

Observation of isoprene hydroxynitrates in the southeastern United States and implications for the fate of NO<sub><i>x</i></sub>

“…Therefore, the overall relative uncertainty in our IN yield is +36 %/−33 % and we report our total IN yield to be 9(+4/−3) % to encompass all the measurement uncertainties. This result lies in the middle of the 4-14 % range of IN yields determined from previous experiments (Chen et al, 1998;Patchen et al, 2007;Lockwood et al, 2010;Paulot et al, 2009;Sprengnether et al, 2002;Tuazon and Atkinson, 1990). Previous IN studies conducted in our group using GC methods consistently resulted in lower IN yields (Chen et al, 1998;Lockwood et al, 2010).…”

“…Reaction of these RO 2 radicals with NO proceeds primarily via two reaction pathways (Reaction R2a and b). Laboratory studies suggest that the nitrate formation channel (Reaction R2a) is minor compared to the alkoxy radical (RO) formation channel (Reaction R2b), with reported total IN yields ranging from 4 to 14 % (Chen et al, 1998;Patchen et al, 2007;Lockwood et al, 2010;Paulot et al, 2009;Sprengnether et al, 2002;Tuazon and Atkinson, 1990). Reaction (R2a) leads to the formation of eight IN isomers, including four β-IN isomers and four δ-IN isomers (Table 1).…”

“…During the BEARPEX 2009 study conducted in the Sierra Nevada of California, IN constituted 38 % of the total organic nitrates (Beaver et al, 2012). Methods to quantify organic nitrates include infrared spectroscopy (IR), thermal-dissociation laser-induced fluorescence (TD-LIF) spectroscopy, chemiluminescence, gas chromatography (GC)-based separation and detection techniques, and mass spectrometry (MS) (Rollins et al, 2010;Tuazon and Atkinson, 1990;Sprengnether et al, 2002;Day et al, 2002;O'Brien et al, 1995;Beaver et al, 2012;Lee et al, 2014a;Lockwood et al, 2010;Paulot et al, 2009;Giacopelli et al, 2005;Grossenbacher et al, 2004;Patchen et al, 2007;Hartsell et al, 1994;Kwan et al, 2012;Teng et al, 2015). IR, TD-LIF, and chemiluminescence can only measure total organic nitrates because they respond solely to the nitroxy functional group (Day et al, 2002;Rollins et al, 2010;Tuazon and Atkinson, 1990;Sprengnether et al, 2002;O'Brien et al, 1995;Hartsell et al, 1994).…”

“…Methods to quantify organic nitrates include infrared spectroscopy (IR), thermal-dissociation laser-induced fluorescence (TD-LIF) spectroscopy, chemiluminescence, gas chromatography (GC)-based separation and detection techniques, and mass spectrometry (MS) (Rollins et al, 2010;Tuazon and Atkinson, 1990;Sprengnether et al, 2002;Day et al, 2002;O'Brien et al, 1995;Beaver et al, 2012;Lee et al, 2014a;Lockwood et al, 2010;Paulot et al, 2009;Giacopelli et al, 2005;Grossenbacher et al, 2004;Patchen et al, 2007;Hartsell et al, 1994;Kwan et al, 2012;Teng et al, 2015). IR, TD-LIF, and chemiluminescence can only measure total organic nitrates because they respond solely to the nitroxy functional group (Day et al, 2002;Rollins et al, 2010;Tuazon and Atkinson, 1990;Sprengnether et al, 2002;O'Brien et al, 1995;Hartsell et al, 1994). GC-and MSbased methods can speciate organic nitrates and have been employed previously to quantify IN in both laboratory and field studies (Lockwood et al, 2010;Patchen et al, 2007;Giacopelli et al, 2005;Paulot et al, 2009;Lee et al, 2014a;Grossenbacher et al, 2004;Beaver et al, 2012;Kwan et al, 2012).…”

Highly efficient gas-phase reactivity of protonated pyridine radicals with propene

“…Although each reaction is energetically barrierless overall, reflection of reaction flux from [dehydro-N-pyridinium + propene] •+ pre-reactive complexesback to reactants may account for the differences in measured rates between radical isomers, and the observation of reaction efficiencies lower than 100%. Involvement of pre-reactive complexes have been shown to significantly influence radical reaction kinetics in other systems,[55][56][57] however further investigation is needed to quantify their effect on the dehydro-N-pyridinium + propene system. Computational kinetic modelling (RRKM-ME) studies of the phenyl radical + propene system 42 indicates back-reaction of intermediates to reactants accounts for up to 11% of reaction flux at 7.6 Torr and 1500 K. The study reports increasing re-dissociation of intermediates to reactants with decreasing pressure, calculated from 7600 Torr to the lower limit of 7.6 Torr.…”