Self-reaction Of Peroxy Radicals Research Articles

Dimeric Product of Peroxy Radical Self-Reaction Probed with VUV Photoionization Mass Spectrometry and Theoretical Calculations: The Case of C2H5OOC2H5

Organic peroxy radicals (RO2) as key intermediates in tropospheric chemistry exert a controlling influence on the cycling of atmospheric reactive radicals and the production of secondary pollutants, such as ozone and secondary organic aerosols (SOA). Herein, we present a comprehensive study of the self-reaction of ethyl peroxy radicals (C2H5O2) by using advanced vacuum ultraviolet (VUV) photoionization mass spectrometry in combination with theoretical calculations. A VUV discharge lamp in Hefei and synchrotron radiation at the Swiss Light Source (SLS) are employed as the photoionization light sources, combined with a microwave discharge fast flow reactor in Hefei and a laser photolysis reactor at the SLS. The dimeric product, C2H5OOC2H5, as well as other products, CH3CHO, C2H5OH and C2H5O, formed from the self-reaction of C2H5O2 are clearly observed in the photoionization mass spectra. Two kinds of kinetic experiments have been performed in Hefei by either changing the reaction time or the initial concentration of C2H5O2 radicals to confirm the origins of the products and to validate the reaction mechanisms. Based on the fitting of the kinetic data with the theoretically calculated results and the peak area ratios in the photoionization mass spectra, a branching ratio of 10 ± 5% for the pathway leading to the dimeric product C2H5OOC2H5 is measured. In addition, the adiabatic ionization energy (AIE) of C2H5OOC2H5 is determined at 8.75 ± 0.05 eV in the photoionization spectrum with the aid of Franck-Condon calculations and its structure is revealed here for the first time. The potential energy surface of the C2H5O2 self-reaction has also been theoretically calculated with a high-level of theory to understand the reaction processes in detail. This study provides a new insight into the direct measurement of the elusive dimeric product ROOR and demonstrates its non-negligible branching ratio in the self-reaction of small RO2 radicals.

Peroxy self-reaction leading to the formation of furfural.

Interest in alternative fuels to petroleum and classical fuels has been growing very rapidly in recent years. Furan and its alkyl derivatives, such as methylfuran (2MF), have been identified as valid alternative biofuels. This study focuses on the self-reaction of the peroxy radical generated in the first oxidation step of 2MF, initiated by Cl atoms at 323 K and 4 Torr. The experiments have been carried out by a multiplexed synchrotron photoionization mass spectrometer (mSPIMS) at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory (USA). The presence of a peak at m/z = 96 reveals that furfural is the dominant product of 2MF oxidation. Various reaction mechanisms for furfural formation are proposed here. The potential energy surfaces for singlet and triplet spin states have been mapped using quantum mechanical methods, such as CCSD(T), DFT-B3LYP, and composites models (CBS-QB3), to optimize the products, transition states, and intermediates. Experimental and theoretical results provide evidence that furfural does not form by primary reaction chemistry. Self-reaction of the peroxy radical generated in the first oxidation step of 2MF has been proposed as the pathway leading to the formation of furfural. Among various reaction channels, we indentified some entirely exothermic pathways involving oxygen-oxygen coupling and the formation of ROOOOR Russell intermediates.

Diurnal variation of non-methane hydrocarbons in the subantarctic atmosphere

Environmental context. The ocean surface is known to be supersaturated in some non-methane hydrocarbons and particularly alkenes. This oceanic source, though small on a global scale, can be a dominant component of the background atmosphere in remote areas. Attempts have been made to quantify this source, in order to estimate its magnitude in the budgets of these gases in the water column and the atmosphere. A main difficulty is to determine the production processes involved under the effects of plankton activity and solar and UV radiation penetration in the water column. Abstract. Non-methane hydrocarbons (NMHCs) play a key role in the photochemistry of the remote atmosphere. They are oxidised by OH radicals and subsequently lead to a net formation of peroxy radicals, which have a crucial role in the budget of tropospheric ozone. Whereas in polluted areas, the effect of light hydrocarbons results in a net formation of ozone, in non- or low-polluted areas, the self-reaction of peroxy radicals dominates and leads eventually to ozone destruction, which in turn acts significantly on the OH budget. In remote environments, the origin of the NMHC background level is clearly attributed to a marine production, but a great uncertainty still exists about its geographical, seasonal and diurnal variability. Here, we present in situ measurements of NMHCs and particularly of alkenes in subantarctic areas, which show very systematic diurnal trends in agreement with an origin clearly dependent on photochemical processes on the surface seawater. The diurnal variability of alkene atmospheric mixing ratios appears strongly related to solar irradiance at the ocean surface. The magnitude of this marine source is deduced from a simple 1-D model of the alkene budget in the marine boundary layer. It appears that the required source must be approximately one order of magnitude greater than the source deduced from concentration measured at 1-m depth, and consistent with probable high concentration gradients close to the ocean surface.

Atmospheric Chemistry of a Model Biodiesel Fuel, CH3C(O)O(CH2)2OC(O)CH3: Kinetics, Mechanisms, and Products of Cl Atom and OH Radical Initiated Oxidation in the Presence and Absence of NOx

Relative rate techniques were used to study the kinetics of the reactions of Cl atoms and OH radicals with ethylene glycol diacetate, CH3C(O)O(CH2)2OC(O)CH3, in 700 Torr of N2/O2 diluent at 296 K. The rate constants measured were k(Cl + CH3C(O)O(CH2)2OC(O)CH3) = (5.7 +/- 1.1) x 10(-12) and k(OH + CH3C(O)O(CH2)2OC(O)CH3) = (2.36 +/- 0.34) x 10(-12) cm3 molecule-1 s-1. Product studies of the Cl atom initiated oxidation of ethylene glycol diacetate in the absence of NO in 700 Torr of O2/N2 diluent at 296 K show the primary products to be CH3C(O)OC(O)CH2OC(O)CH3, CH3C(O)OC(O)H, and CH3C(O)OH. Product studies of the Cl atom initiated oxidation of ethylene glycol diacetate in the presence of NO in 700 Torr of O2/N2 diluent at 296 K show the primary products to be CH3C(O)OC(O)H and CH3C(O)OH. The CH3C(O)OCH2O* radical is formed during the Cl atom initiated oxidation of ethylene glycol diacetate, and two loss mechanisms were identified: reaction with O2 to give CH3C(O)OC(O)H and alpha-ester rearrangement to give CH3C(O)OH and HC(O) radicals. The reaction of CH3C(O)OCH2O2* with NO gives chemically activated CH3C(O)OCH2O* radicals which are more likely to undergo decomposition via the alpha-ester rearrangement than CH3C(O)OCH2O* radicals produced in the peroxy radical self-reaction.

Dimethyl Ether Oxidation at Elevated Temperatures (295−600 K)

Dimethyl ether (DME) has been proposed for use as an alternative fuel or additive in diesel engines and as a potential fuel in solid oxide fuel cells. The oxidation chemistry of DME is a key element in understanding its role in these applications. The reaction between methoxymethyl radicals and O(2) has been examined over the temperature range 295-600 K and at pressures of 20-200 Torr. This reaction has two product pathways. The first produces methoxymethyl peroxy radicals, while the second produces OH radicals and formaldehyde molecules. Real-time kinetic measurements are made by transient infrared spectroscopy to monitor the yield of three main products-formaldehyde, methyl formate, and formic acid-to determine the branching ratio for the CH(3)OCH(2) + O(2) reaction pathways. The temperature and pressure dependence of this reaction is described by a Lindemann and Arrhenius mechanism. The branching ratio is described by f = 1/(1 + A(T)[M]), where A(T) = (1.6(+2.4)(-1.0) x 10(-20)) exp((1800 +/- 400)/T) cm(3) molecule(-1). The temperature dependent rate constant of the methoxymethyl peroxy radical self-reaction is calculated from the kinetics of the formaldehyde and methyl formate product yields, k(4) = (3.0 +/- 2.1) x 10(-13) exp((700 +/- 250)/T) cm(3) molecule(-1) s(-1). The experimental and kinetics modeling results support a strong preference for the thermal decomposition of alkoxy radicals versus their reaction with O(2) under our laboratory conditions. These characteristics of DME oxidation with respect to temperature and pressure might provide insight into optimizing solid oxide fuel cell operating conditions with DME in the presence of O(2) to maximize power outputs.

The influence of reaction conditions on the photooxidation of diisopropyl ether

The hydroxyl radical initiated oxidation of diisopropyl ether has been studied in the large-volume outdoor European Photoreactor (EUPHORE) and in a small, laboratory-based reactor system. The product distributions determined from the experiments were found to be significantly dependent on the reaction conditions and provide strong evidence for the existence of three distinct regimes within the reaction system. In the presence of NO x , the peroxy radicals react with NO to produce chemically activated (CH 3) 2CHOC(O)(CH 3) 2 alkoxy radicals which undergo decomposition by C C bond scission to yield isopropyl acetate and formaldehyde as the major products. Under conditions where the self-reaction of peroxy radicals dominates, thermoneutral (CH 3) 2CHOC(O)(CH 3) 2 radicals are produced, which appear to undergo two reaction pathways; C C bond scission to yield isopropyl acetate and formaldehyde and isomerisation to form acetone, acetic acid and formaldehyde. Under conditions where the reaction between peroxy and hydroperoxy radicals dominates, unstable hydroperoxides are produced which decompose to yield acetone as the only major reaction product. The results of our study are used to construct chemical mechanisms for the gas-phase photooxidation of diisopropyl ether under various tropospheric conditions.

A semi-empirical correlation for the rate coefficients for cross- and self-reactions of peroxy radicals in the gas-phase

Abstract Peroxy radicals play an important role as reaction intermediates in the atmospheric oxidation of volatile organic compounds (VOCs). The rate coefficients for the self-reactions vary by up to six orders of magnitude (e.g. 3.0×10 −17 cm 3 molecule −1 s −1 for t -C 4 H 9 O 2 and 1.5×10 −11 cm 3 molecule −1 s −1 for CH 3 C(O)O 2 ) with, up to now, no clear pattern. This work represents the first rationalisation of peroxy radical self-reaction trends in reactivity. A correlation between the logarithm of the rate coefficient of peroxy radical self-reactions with the stabilisation energy, the difference in the enthalpy of formation of peroxy radicals with that of the tetraoxide adduct {Δ H f (RO 4 R)−2Δ H f (RO 2 )}, is presented. The equation, log 10 k =−{Δ H f (RO 4 R)−2Δ H f (RO 2 )+235.95}/13.32 (where energy is in kJ mol −1 ) is given to predict the room temperature rate coefficients of peroxy radical self-reactions. The correlation was extended to predict rate coefficients for the cross-reactions of peroxy radicals with CH 3 O 2 . Furthermore, favourable comparisons are made between the predicted rate coefficients and very recent studies of complex peroxy radical systems.

Reactivity of the CF3CFHO Radical: Thermal Decomposition and Reaction with O2

The kinetics of CF 3 CFHO radical reactions was studied at 20 and 35 Torr and from 259 to 297 K by flash photolysis with time-resolved mass spectrometry. The CF 3 CFHO 2 radical was generated by flash photolysis of Cl 2 in the presence of CF 3 CFH 2 and O 2 , and CF 3 CFHO was formed by the reaction of CF 3 CFHO 2 with Cl and by the peroxy radical self-reaction. The rate coefficient ratio for (1) CF 3 CFHO + O 2 → CF 3 C(O)F + HO 2 and (2) CF 3 CFHO → HC(O)F + CF 3 was determined by observing the formation of HC(O)F as a function of the partial pressure of O 2 . At 20 Torr, k 1 /k 2 = (2.4 ′ 0.8) × 10 - 2 5 e ( 4 2 4 8 ′ 5 5 0 ) / T cm 3 molecule - 1 , and at 35 Torr, k 1 /k 2 = (9.1 ′ 3.3) x 10 - 2 6 e ( 4 3 7 0 ′ 5 0 0 ) / T cm 3 molecule - 1 . Ab initio molecular orbital calculations for optimized geometries, vibrational frequencies, and total energies of CF 3 CFHO, the reaction products, and the C-C bond-breaking transition state were made. Energies were calculated by the G2 and G2(MP2) methods. An RRKM model for reaction 2 was based on geometries and energies from the ab initio calculations. RRKM calculated values of k 2 were combined with the experimentally determined k 1 /k 2 to estimate k 1 . Comparisons were made with estimates made with two other RRKM models (Schneider, W. F.; Wallington, T. J.; Barker, J. R.; Stahlberg, E. A. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1850; Somnitz, H.; Zellner, R. Phys. Chem. Chem. Phys. 2001, 3, 2352). The ab initio calculations predict the 298 K enthalpy of formation of CF 3 CFHO to be -203.0 kcal mol - 1 (G2MP2) and -203.4 kcal mol - 1 (G2).

Kinetic Study of the Self-Reactions of the BrCH2CH2O2 and BrCH(CH3)CH(CH3)O2 Radicals between 275 and 373 K

A conventional flash photolysis technique was used to measure the self-reaction rate constants of the primary BrCH 2 CH 2 O 2 (2-bromoethylperoxy) and secondary BrCH(CH 3 )CH(CH 3 )O 2 (2-bromo- 1 -methylpropylperoxy) β-brominated peroxy radicals, at temperatures in the range of 275 -373 K. The absolute UV absorption spectra of BrCH 2 CH 2 O 2 and BrCH(CH 3 )CH(CH 3 )O 2 were also measured and compared to those obtained previously for these radicals. The temperature dependence of the self-reaction rate constants provided the following Arrhenius expressions: k(BrCH 2 CH 2 O 2 + BrCH 2 CH 2 O 2 ) = (6.15 + 5 . 5 1 - 2 . 9 1 ) x 10 - 1 4 exp{(1247 ′ 203) K/T} cm 3 molecule - 1 s - 1 and k(BrCH(CH 3 )CH(CH 3 )O 2 + BrCH(CH3)CH(CH3)O 2 ) = (7.60 + 2 2 . 0 5 - 5 . 6 5 ) x 10 - 1 5 exp{(1305 ′ 428) K/T} cm 3 molecule - 1 s - 1 , where the uncertainties represent 95% confidence limits associated with the statistical fitting procedure and include the contribution for the expanded uncertainties in the individual rate constant. These results confirm the enhancement of the peroxy radical self-reaction reactivity upon β-substitution, which is similar for Br, Cl, or OH substituents. Structure-activity relationships are proposed for self-reactions of β-substituted peroxy radicals.

Atmospheric Oxidation Mechanism of Methyl Formate

Smog chamber/FTIR techniques were used to study the Cl atom initiated oxidation of CH3C(O)OCH3 in 700 Torr of N2/O2 at 296 K. Relative rate techniques were used to measure k(Cl+CH3C(O)OCH3) = (2.2 ± 0.3) × 10-12, k(Cl+CH3C(O)CH3) = (2.2 ± 0.4) × 10-12, k(Cl+CH3C(O)OC(O)H) = (1.0 ± 0.1) × 10-13, and k(Cl+ ClCH2C(O)OCH3) = (8.5 ± 1.9) × 10-13 cm3 molecule-1 s-1. The reaction of Cl+CH3C(O)OCH3 was found to proceed more than 95% via H-abstraction at the −OCH3 site. The fate of the CH3C(O)OCH2O· radical was studied in 700 Torr of N2/O2 diluent at 296 K in the absence and presence of NO. Two loss mechanisms were identified: reaction with O2 to give CH3C(O)OC(O)H and α-ester rearrangement to give CH3C(O)OH and HCO· radicals. It was found that α-ester rearrangement is more likely when CH3C(O)OCH2O· radicals were produced via the CH3C(O)OCH2O2· + NO reaction than when they were produced via the self-reaction of peroxy radicals. In one atmosphere of air ([O2] = 160 Torr) containing NO at 296 K it can be calculated...

Kinetics of the Self-Reactions of Peroxy Radicals Arising from Chlorine-Initiated Oxidation of Chloroethenes

The kinetics of the self-reactions of CHCl2CHClO2, CHCl2CCl2O2, and CCl3CCl2O2 have been investigated using a conventional flash photolysis/UV absorption setup at room temperature and at atmospheric pressure. The UV absorption spectra of CHCl2CHClO2, CHCl2CCl2O2, and CCl3CCl2O2 were determined between 230 and 290 nm. All spectra present a broad band centered around 250 nm, with a decrease of cross sections going from the less chlorinated to the more chlorinated radicals. Spectra of CCl3C(O)Cl and CHCl2C(O)Cl, formed in the self-reactions of CCl3CCl2O2 and CHCl2CHClO2, respectively, were also measured between 220 and 290 nm. Values of (3.6 ± 0.9) × 10-12, (7.0 ± 1.5) × 10-12, and (5.0 ± 1.5) × 10-12 (units of cm3 molecule-1 s-1, statistical errors 2σ) have been obtained at 298 K for the rate constants of the self-reactions of CHCl2CHClO2, CHCl2CCl2O2, and CCl3CCl2O2 radicals, respectively. Kinetic analyses of self-reactions were very sensitive to the reaction mechanism used and, in particular, to the way chloroethoxy radicals reacted. Our experimental observations are totally consistent with end-product studies reported in the literature and provide a valuable confirmation of the mechanisms. As the determination of rate constants depends on complex reaction systems, a total uncertainty factor of nearly 2 has been estimated for all rate constants measured, and no clear reactivity trend can be extracted from these data. Nevertheless, we propose for the rate constants of chloroethylperoxy radical self-reactions an average value of (5 ± 2) × 10-12 cm3 molecule-1 s-1 at 298 K.

Kinetic studies of the self-reaction and reaction with HO2 of a tertiary ?-brominated peroxy radical between 298 and 393 K

The kinetics of reactions of the tertiary β-brominated peroxy radical BrC(CH3)2C(CH3)2O2 (2-bromo-1,1,2-trimethylpropylperoxy) have been studied using the laser flash photolysis technique, photolysing HBr at 248 nm in the presence of O2 and 2,3-dimethylbut-2-ene. At room temperature, a rate constant of (2.0 ± 0.8) × 10−14 cm3 molecule−1 s−1 was determined for the BrC(CH3)2C(CH3)2O2 self-reaction. The reaction of BrC(CH3)2C(CH3)2O2 with HO2 was investigated in the temperature range 306–393 K, yielding the following Arrhenius expression: k(BrC(CH3)2C(CH3)2O2 + HO2) = (2.04 ± 0.25) × 10−12 exp[(501 ± 36)K/T] cm3 molecule−1 s−1, giving by extrapolation (1.10 ± 0.13) × 10−11 cm3 molecule−1 s−1 at 298 K. These results confirm the enhancement of the peroxy radical self-reaction reactivity upon β-substitution, which is similar for Br and OH substituents. In contrast, no significant effect of substituent has been observed on the rate constant for the reactions of peroxy radicals with HO2. The global uncertainty factors on rate constants are equal to nearly 2 for the self-reaction and to 1.35 for the reaction with HO2. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 33: 41–48, 2001

Peroxy radicals from night-time reaction of N03 with organic compounds

REACTIONS of free radicals are the driving force for most chemical processes in the atmosphere. Although hydroxyl (OH) and nitrate (NO3) radicals are the most reactive, peroxy radicals such as HO2 and CH3O2 have an important role as intermediates in tropospheric ozone generation and as precursors to OH radicals1. Moreover, self-reaction of peroxy radicals is the only known gas-phase source of peroxides to the atmosphere. It has long been known that there exist pathways to the formation of radicals—in particular NO3 (refs 2–5) —that do not require sun-light, yet radical chemistry is usually discussed in terms of photochemistry occurring during the day1. The possibility of night-time production of peroxy radicals has received little attention, except perhaps in highly polluted air6–8. Here we show that recent kinetic investigations9,10 combined with field observations of NO3, organic and other relevant species11,12 suggest that tropospheric high night-time concentrations of peroxy radicals—possibly even exceeding daytime values—may occur under certain atmospheric conditions, for instance in coastal areas or forests and in air masses affected by urban emission or biomass burning. The prerequisite for this occurrence is the simultaneous presence in the same air mass of reactive organic species, oxides of nitrogen and ozone.

Bimolecular self-reaction of peroxy radicals. Oxygen-18 isotope study

Bimolecular self-reaction of peroxy radicals. Oxygen-18 isotope study