Branching Ratios for the Reaction of Selected Carbonyl-Containing

Apr 6, 2012 - Atmospheric Chemistry Division, National Center for Atmospheric Research, P.O. Box 3000, Boulder, Colorado 80307,. United States...
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Branching Ratios for the Reaction of Selected Carbonyl-Containing Peroxy Radicals with Hydroperoxy Radicals Alam S. Hasson,*,† Geoffrey S. Tyndall,‡ John J. Orlando,‡ Sukhdeep Singh,†,§ Samuel Q. Hernandez,†,∥ Sean Campbell,†,⊥ and Yesenia Ibarra† †

Department of Chemistry, 2555 East San Ramon Avenue M/S SB70, California State University, Fresno, Fresno, California 93740, United States ‡ Atmospheric Chemistry Division, National Center for Atmospheric Research, P.O. Box 3000, Boulder, Colorado 80307, United States S Supporting Information *

ABSTRACT: An important chemical sink for organic peroxy radicals (RO2) in the troposphere is reaction with hydroperoxy radicals (HO2). Although this reaction is typically assumed to form hydroperoxides as the major products (R1a), acetyl peroxy radicals and acetonyl peroxy radicals have been shown to undergo other reactions (R1b) and (R1c) with substantial branching ratios: RO2 + HO2 → ROOH + O2 (R1a), RO2 + HO2 → ROH + O3 (R1b), RO2 + HO2 → RO + OH + O2 (R1c). Theoretical work suggests that reactions (R1b) and (R1c) may be a general feature of acyl peroxy and α-carbonyl peroxy radicals. In this work, branching ratios for R1a−R1c were derived for six carbonyl-containing peroxy radicals: C2H5C(O)O2, C3H7C(O)O2, CH3C(O)CH2O2, CH3C(O)CH(O2)CH3, CH2ClCH(O2)C(O)CH3, and CH2ClC(CH3)(O2)CHO. Branching ratios for reactions of Cl-atoms with butanal, butanone, methacrolein, and methyl vinyl ketone were also measured as a part of this work. Product yields were determined using a combination of long path Fourier transform infrared spectroscopy, high performance liquid chromatography with fluorescence detection, gas chromatography with flame ionization detection, and gas chromatography−mass spectrometry. The following branching ratios were determined: C2H5C(O)O2, YR1a = 0.35 ± 0.1, YR1b = 0.25 ± 0.1, and YR1c = 0.4 ± 0.1; C3H7C(O)O2, YR1a = 0.24 ± 0.15, YR1b = 0.29 ± 0.1, and YR1c = 0.47 ± 0.15; CH3C(O)CH2O2, YR1a = 0.75 ± 0.13, YR1b = 0, and YR1c = 0.25 ± 0.13; CH3C(O)CH(O2)CH3, YR1a = 0.42 ± 0.1, YR1b = 0, and YR1c = 0.58 ± 0.1; CH2ClC(CH3)(O2)CHO, YR1a = 0.2 ± 0.2, YR1b = 0, and YR1c = 0.8 ± 0.2; and CH2ClCH(O2)C(O)CH3, YR1a = 0.2 ± 0.1, YR1b = 0, and YR1c = 0.8 ± 0.2. The results give insights into possible mechanisms for cycling of OH radicals in the atmosphere.

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INTRODUCTION Organic peroxy radicals (RO2) are generated during the photooxidation of the vast majority of organic pollutants in the troposphere.1 The principal chemical sinks are reaction with nitrogen oxides (NOx = NO + NO2) and hydroperoxy radicals (HO2). The reaction between organic peroxy and hydroperoxy radicals is generally considered to be a chain terminating process, resulting in the formation of an organic hydroperoxide2−7 (ROOH, reaction R1a). RO2 + HO2 → ROOH + O2 (R1a)

where R = CH3C(O)) with relative branching ratios for YR1a:YR1b of 3:1.9−13 RO2 + HO2 → ROH + O3

More recently, convincing evidence has emerged that acetyl peroxy and acetonyl peroxy radicals form radical products (reaction R1c) with substantial yields.14−17 RO2 + HO2 → RO + OH + O2

(R1c)

A theoretical study provided supporting evidence for the experimental results.18 Structural calculations with Master Equation

Because organic hydroperoxides are relatively stable in the troposphere,8 reaction R1a is a radical sink. Experimental and theoretical studies have demonstrated that reactions between carbonyl-containing organic peroxy radicals and hydroperoxy radicals are more complex. Acetyl peroxy radicals have been shown to generate acetic acid (reaction R1b, © 2012 American Chemical Society

(R1b)

Special Issue: A. R. Ravishankara Festschrift Received: December 7, 2011 Revised: March 8, 2012 Published: April 6, 2012 6264

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and products were calibrated using commercially available standards unless stated otherwise. The NCAR reactor is a 47 L stainless steel chamber coupled to a Bomem DA3.01 FTIR spectrometer. Photochemistry is initiated using a filtered xenon arc lamp with radiation in the wavelength range 235−400 nm. The Fresno State chamber is a 142 L Teflon-lined reactor coupled to a Nicolet Nexus-470 FTIR spectrometer. Internally mounted blacklight lamps, generating radiation in the wavelength range 300−400 nm, are used as the photolysis source. Hanst-type optics are used to produce IR pathlengths of 32.6 and 50 m for the NCAR and Fresno State chambers, respectively. IR spectra were collected at 1 cm−1 resolution by averaging 200 scans following irradiation times ranging from 30 s to 15 min, depending on the reaction mixture. This process was typically repeated 4−5 times over the course of the experiment, resulting in total irradiation times ranging from 2−75 min. Ports mounted at the end of each chamber were used to extract samples for analysis by HPLC and GC. Additional experiments were carried out at Fresno State using 20 L Tedlar bags surrounded by blacklight lamps within a custom-built chamber. Samples were withdrawn from the chamber for analysis by GC-FID and HPLC at a flow rate of approximately 200 mL/min through unheated Teflon tubing. HPLC samples were bubbled through an impinger containing 15 mL of 18 MΩ water containing 1 mM sulfuric acid and 0.1 mM Na2EDTA. The volume of air sampled from the chamber was typically in the range 0.5−9 L. Samples were subsequently analyzed as described previously14 using an HPLC with postcolumn derivatization and fluorescence detection technique first described by Hellpointner and Gäb.20 GC-FID samples either were collected in a gastight syringe and injected into the GC inlet or were flowed into the sample loop of a six-port valve connected directly to the GC. The GCFID instrument at Fresno State (Agilent 6850) was fitted with a 30 m, 0.25 mm i.d., 0.5 μm film thickness HP5-MS column and used helium as the carrier gas at a flow rate of 1 mL min−1. The oven was held at −10 °C with liquid CO2 for 1 min and then ramped at 5 °C min−1 to 30 °C and then at 30 °C min−1 to a final temperature of 200 °C, which was held for 1 min. The NCAR GC-FID instrument (Agilent 6980N) used the same column, carrier gas, and flow rates as the Fresno State instrument and a similar oven temperature program (−20 °C for 1 min; ramped at 5 °C min−1 to 30 °C and then at 30 °C min−1 to a final temperature of 200 °C, held for 1 min). Additional experiments were carried out at Fresno State to measure carboxylic acid concentrations within the chamber using solid phase microextraction (SPME) fibers coupled with GC−MS, as first described by Orzechowska and co-workers.21 The fibers were conditioned prior to their first use by heating them to 220 °C for 30 min in the inlet of a GC/MS (HP 5890/ 5973). Samples were then pumped from the chamber at a flow rate of 1 L/min into a 1 L pyrex sampling chamber fitted with a septum to allow for the introduction of the SPME fiber. The 70 μm Carbowax/divinylbenzene SPME fiber (Supelco) was then exposed to the sample flow for 1 min. Samples were thermally desorbed from the SPME fiber into the 0.75 mm internal diameter inlet linear of the GC (HP 5890) at 200 °C. To ensure complete desorption of the analytes, the fibers were left in the GC inlet for the duration of the run. A 30 m Stabilwax DA column (Restek) was used to separate the compounds present using helium as the carrier gas at a flow rate of 1 mL min−1.

analysis identified a reaction mechanism that could potentially account for the large observed branching ratios for (R1c). In this mechanism, the HO2 and RO2 radicals form a hydrotetroxide intermediate that can subsequently decompose to form the radical products. The intermediate is internally stabilized by hydrogen bonding between the OH and carbonyl groups, and it is this interaction that enables the RO and OH products to be formed. This suggests that reaction R1c may be a general feature for all acylperoxy and α-carbonyl peroxy radicals. A significant branching ratio for reaction R1c could substantially affect the predicted photochemistry in atmospheric models because this channel results in radical recycling as opposed to removal, which would be the case for reaction R1a. However, the impact of these reactions is clearly dependent on how ubiquitous reaction R1c is for carbonylcontaining peroxy radicals. In this work, branching ratios for reactions R1a−R1c are reported for six carbonyl-containing peroxy radicals. These data are used to evaluate the relationship between the branching ratios and the radical structure.

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EXPERIMENTAL SECTION The majority of the experiments were performed using the photochemical reactor located at the National Center for Atmospheric Research (NCAR). Additional experiments with propanal and butanal were carried out using the chamber at California State University Fresno (Fresno State). Experiments were carried out at a total pressure of 800 Torr and at room temperature (295 K). Reaction mixtures containing (3.5−7) × 1014 molecules cm−3 of an organic peroxy radical precursor (aldehyde or ketone), (0, 0.35−3.5) × 1015 molecules cm−3 hydroperoxy radical precursor (an alcohol), (1.4−3.5) × 1015 molecules cm−3 chlorine, nitrogen, and oxygen were introduced into the chamber. Experiments carried out using acetone as the organic peroxy radical precursor were performed with samples labeled with 13C at the carbonyl carbon atom to enable CO2 formation from acetone to be distinguished from other sources. Some additional experiments were carried out in which NO or NO2 (7−10) × 1014 molecules cm−3 were added to the reaction mixture to help determine the branching ratio for Cl-atom attack. The chlorine photodissociates in the presence of UV light to produce chlorine atoms (reaction R2) which then react with the carbonyl compound and alcohol to produce an organic peroxy radical and hydroperoxy radical in reactions R3−R6, respectively. Cl 2 + hν → 2Cl

(R2)

Cl + R−H → R + HCl

(R3)

R + O2 + M → RO2 + M

(R4)

R1R 2CHOH + Cl → R1R 2COH + HCl

(R5)

R1R 2COH + O2 → R1R 2CO + HO2

(R6)

The relative concentrations of RO2 and HO2 radicals within the chamber were varied by adjusting the initial concentrations of the aldehyde/ketone and alcohol. Changes in the concentrations of reactants and products were measured using a combination of Fourier transform infrared (FTIR) spectroscopy, high performance liquid chromatography (HPLC), gas chromatography with flame ionization detection (GC-FID) and gas chromatography−mass spectrometry (GC−MS). Reactants 6265

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Table 1. Principal Chemical Reactions Used in Acuchem Simulations rate coefficient/s−1 or cm3 molecule−1 s−1 b

reactiona

1 × 10−4

Cl 2 = Cl + Cl HO2 Precursors

Cl + CH3OH( +O2 ) = CH 2O + HCl + HO2

6 × 10−11

Cl + CH3C(O)CH 2OH(+ O2 ) = HCl + CH3C(O)CHO + HO2

6 × 10−11

Cl + CH3CH(OH)CH3(+ O2 ) = HCl + CH3C(O)CH3 + HO2

8.4 × 10−11

Propanal Reactions

Cl + C2H5CHO( +O2 ) = HCl + C2H5C(O)O2

1 × 10−10 c

Cl + C2H5CHO( +O2 ) = HCl + RO2

1 × 10−11 c

C2H5C(O)O2 + HO2 = OH + C2H5C(O)O + O2

YR1c × 2.2 × 10−11 (see text for details)

C2H5C(O)O2 + HO2 = C2H5C(O)OH + O3

YR1b × 2.2 × 10−11 (see text for details)

C2H5C(O)O2 + HO2 = C2H5C(O)O2 H + O2

YR1a × 2.2 × 10−11 (see text for details)

C2H5C(O)O = CO2 + C2H5O2

1 × 105

C2H5O2 + C2H5C(O)O2 = C2H5C(O)O + C2H5O + O2

9 × 10−12

C2H5O2 + C2H5C(O)O2 = C2H5C(O)OH + CH3CHO + O2

2 × 10−12

C2H5O2 + C2H5O2 = C2H5O + C2H5O + O2

4 × 10−14

C2H5O2 + C2H5O2 = C2H5OH + CH3CHO + O2

1.7 × 10−14

C2H5O2 + HO2 = C2H5O2 H + O2

7.8 × 10−12

C2H5C(O)O2 + C2H5C(O)O2 = C2H5C(O)O + C2H5C(O)O + O2

1.4 × 10−11

Butanal Reactions

Cl + C3H 7CHO(+ O2 ) = C3H 7C(O)O2 + HCl

YR12a × 1.6 × 10−10 (see text for details)c

Cl + C3H 7CHO( +O2 ) = RO2 + HCl

(1 − YR12a) × 1.6 × 10−10 (see text for details)c

C3H 7C(O)O2 + C3H 7C(O)O2 = C3H 7C(O)O + C3H 7C(O)O + O2

1.4 × 10−11

C3H 7C(O)O2 + RO2 = C3H 7C(O)O + RO + O2

9 × 10−12

C3H 7C(O)O2 + RO2 = C3H 7C(O)OH + other products

2 × 10−12

RO2 + RO2 = products

4 × 10−12

C3H 7C(O)O2 + HO2 = C3H 7C(O)O2 H + O2

YR1a × 2.2 × 10−11 (see text for details)

C3H 7C(O)O2 + HO2 = C3H 7C(O)OH + O3

YR1b × 2.2 × 10−11 (see text for details)

C3H 7C(O)O2 + HO2 = C3H 7C(O)O + OH + O2

YR1c × 2.2 × 10−11 (see text for details)

RO2 + HO2 = ROOH + O2

1.6 × 10−11

C3H 7C(O)O = CO2 + other products

1 × 106 Acetone Reactions

Cl + CH3C(O)CH3 = CH3C(O)CH 2 + HCl

2.1 × 10−12

CH3C(O)CH 2 + O2 = CH3C(O)CH 2O2

5 × 10−12

2CH3C(O)CH 2O2 = 2CH3C(O)CH 2O + O2

6 × 10−12

2CH3C(O)CH 2O2 = CH3C(O)CHO + CH3C(O)CH 2OH + O2

2 × 10−12

CH3C(O)CH 2O = CH3C(O)O2 + CH 2O

6 × 107

CH3C(O)CH 2O + O2 = HO2 + CH3C(O)CHO

1 × 10−14

CH3C(O)CH 2O2 + CH3C(O)O2 = CH3C(O)CH 2O + CH3CO2 + O2

1 × 10−11

CH3C(O)CH 2O2 + CH3C(O)O2 = CH3C(O)CHO + CH3C(O)OH + O2

1 × 10−12

CH3C(O)CH 2O2 + CH3O2 = CH3C(O)CH 2O + CH3O + O2

1.1 × 10−12

CH3C(O)CH 2O2 + CH3O2 = CH3C(O)CHO + CH3OH + O2

1.4 × 10−12

CH3C(O)CH 2O2 + CH3O2 = CH3C(O)CH 2OH + CH 2O + O2

1.4 × 10−12

CH3C(O)CH 2O2 + HO2 = CH3C(O)CH 2OOH + O2

YR1a × 9 × 10−12 (see text for details)

CH3C(O)CH 2O2 + HO2 = CH3C(O)CH 2O + OH + O2

YR1c × 9 × 10−12 (see text for details)

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Table 1. continued rate coefficient/s−1 or cm3 molecule−1 s−1 b

reactiona Butanone Reactions

Cl + CH3C(O)CH 2CH3(+ O2 ) = CH3C(O)CH(O2 )CH3 + HCl

YR19a × 4.1 × 10−11 (see text for details)c

Cl + CH3C(O)CH 2CH3( +O2 ) = RO2 + HCl

(1 − YR19a) × 4.1 × 10−11 (see text for details)c

2CH3C(O)CH(O2 )CH3 = 2CH3C(O)CH(O)CH3 + O2

YR34a × 4 × 10−12 (see text for details)

2CH3C(O)CH(O2 )CH3 = other products

(1 − YR34a) × 4 × 10−12 (see text for details)

RO2 + RO2 = RO + RO + O2

2.9 × 10−12

RO2 + RO2 = products

1.9 × 10−12

CH3C(O)CH(O2 )CH3 + RO2 = CH3C(O)CH(O)CH3 + RO + O2

2.4 × 10−12

CH3C(O)CH(O2 )CH3 + RO2 = products

1.6 × 10−12

CH3C(O)CH(O)CH3(+ O2 ) = CH3CHO + CH3C(O)O2

1 × 107 c

CH3C(O)CH(O2 )CH3 + HO2 = CH3C(O)CH(O2 H)CH3 + O2

YR1a × 1 × 10−11 (see text for details)

CH3C(O)CH(O2 )CH3 + HO2 = CH3C(O)CH(O)CH3 + OH + O2

YR1c × 1 × 10−11 (see text for details)

MAC Reactions

Cl + CH3C(CH 2)CHO(+ O2 ) = CH3C(O2 )(CH 2Cl)CHO

(1 − YR21a) × 3 × 10−10 (see text for details)c

Cl + CH3C(CH 2)CHO(+ O2 ) = RO2 + HCl

YR21a × 3 × 10−10 (see text for details)c

2CH3C(O2 )(CH 2Cl)CHO = 2CH3C(O)(CH 2Cl)CHO + O2

2.4 × 10−12

RO2 + RO2 = RO + RO + O2

1.0 × 10−11

CH3C(O2 )(CH 2Cl)CHO + RO2 = CH3C(O)(CH 2Cl)CHO + RO + O2

2.4 × 10−12

CH3C(O)(CH 2Cl)CHO( +O2 ) = CH3C(O)CH 2Cl + HO2 + CO

1 × 107 c

CH3C(O2 )(CH 2Cl)CHO + HO2 = CH3C(O2 H)(CH 2Cl)CHO + + O2

YR1a × 1 × 10−11 (see text for details)

CH3C(O2 )(CH 2Cl)CHO + HO2 = CH3C(O)(CH 2Cl)CHO + + OH + O2

YR1c × 1 × 10−11 (see text for details)

OH + CH3C(CH 2)CHO(+ O2 ) = CH3C(O2 )(CH 2OH)CHO

1.5 × 10−11 (see text for details)c

OH + CH3C(CH 2)CHO(+ O2 ) = RO2 + H 2O

1.5 × 10−11 (see text for details)c

CH3C(O2 )(CH 2OH)CHO + HO2 = CH3C(O2 H)(CH 2OH)CHO + + O2

YR1a × 1 × 10−11 (see text for details)

CH3C(O2 )(CH 2OH)CHO + HO2

YR1c × 1 × 10−11 (see text for details)

= CH3C(O)(CH 2OH)CHO + +OH + O2

RO2 + HO2 = ROOH + O2

YR1a × 2.2 × 10−11 (see text for details)

RO2 + HO2 = ROH + O3

YR1b × 2.2 × 10−11 (see text for details)

RO2 + HO2 = RO + OH + O2

YR1c × 2.2 × 10−11 (see text for details) MVK Reactions

Cl + CH 2CHC(O)CH3 + O2 = CH 2ClCH(O2 )C(O)CH3

YR27a × 2 × 10−10 (see text for details)

Cl + CH 2CHC(O)CH3 + O2 = RO2

(1 − YR27a) × 2 × 10−10 (see text for details)

2CH 2ClCH(O2 )C(O)CH3 = 2CH 2ClCH(O)C(O)CH3 + O2

YR34a × 4 × 10−12 (see text for details)

2CH 2ClCH(O2 )C(O)CH3 = products

(1 − YR34a) × 4 × 10−12 (see text for details)

RO2 + RO2 = RO + RO + O2

2.9 × 10−12

RO2 + RO2 = products

1.9 × 10−12

CH 2ClCH(O2 )C(O)CH3 + RO2 = CH 2ClCH(O)C(O)CH3 + RO + O2

2.4 × 10−12

CH 2ClCH(O2 )C(O)CH3 + RO2 = products

1.6 × 10−12 c

CH 2ClCH(O)C(O)CH3(+ O2 ) = CH 2ClCHO + CH3C(O)O2

1 × 107

CH 2ClCH(O2 )C(O)CH3 + HO2 = CH 2ClCH(O2 H)C(O)CH3 + O2

YR1a × 1 × 10−11 (see text for details)

CH 2ClCH(O2 )C(O)CH3 + HO2 = CH 2ClCH(O)C(O)CH3 + OH + O2

YR1c × 1 × 10−11 (see text for details)

RO2 + HO2 = ROOH + O2

1.4 × 10−11 Other Reactions

CH3C(O)O2 + CH3C(O)O2 = CH3CO2 + CH3CO2 + O2

1.4 × 10−11

CH3C(O)O2 + CH3O2 = CH3CO2 + CH3O + O2

1.1 × 10−11

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Table 1. continued rate coefficient/s−1 or cm3 molecule−1 s−1 b

reactiona Other Reactions

CH3C(O)O2 + HO2 = CH3C(O)OOH + O2

9 × 10−12

CH3C(O)O2 + HO2 = CH3C(O)OH + O3

4 × 10−12

CH3C(O)O2 + HO2 = CH3CO2 + OH + O2

9 × 10−12

CH3O2 + CH3O2 = CH3OH + CH 2O + O2

2.2 × 10−13

CH3O2 + CH3O2 = CH3O + CH3O + O2

1.3 × 10−13

CH3O2 + HO2 = CH3OOH + O2

5.2 × 10−12

HO2 + HO2 = H 2O2 + O2

3 × 10−12

CH3O + O2 = HO2 + CH 2O

2 × 10−15

Cl + CH 2O = HCO + HCl

7.3 × 10−11

HCO + O2 = HO2 + CO

5.5 × 10−12 c

CH3CO2 (+ O2 ) = CH3O2 + CO2

1 × 108 c

Cl + CH3C(O)CHO( +O2 ) = HCl + CH3C(O)O2 + CO

4.5 × 10−11

Cl + CH3C(O)CH 2O2 = CH3C(O)CH 2O + ClO

1 × 10−10

Cl + CH3C(O)CH 2OOH = HCl + OH + CH3C(O)CHO

1 × 10−10

a Additional secondary reactions with minimal effects on the model fits are not shown. bRate coefficients are taken from refs 22, 26, 39−41. RO2 + RO2 rate coefficients are estimated from these references for reactions where data are unavailable. cThe reaction consists of more than one elementary reaction step. The rate coefficient listed corresponds to the rate limiting step.

The column was initially held at 50 °C for 4 min and was then ramped at 12 °C min−1 to a final temperature of 200 °C. This temperature was held for 3 min, resulting in a total run time of 19.5 min. The mass spectrometer (HP 5973) was operated in electron impact ionization mode (at 70 eV) and signals were collected in the selective ion monitoring (SIM) mode using m/z 59 and m/z 73 for all of the acids.

reactions to provide the best fit to the experimental product yields measured in the absence of an HO2 precursor. Simulations were then run in which the appropriate RO2 + HO2 branching ratios were systematically varied. Best fits and uncertainties are estimated “by eye”. The uncertainties in the reported branching ratios are determined by propagating the estimated uncertainty in the model fit to the experimental data with the uncertainties in the experimental product yields. Branching Ratios for Cl + Carbonyls. Chlorine atoms abstract hydrogen atoms from carbonyl compounds to produce an alkyl radical. However, because chlorine atoms are not highly selective,22 reactions R3 and R4 result in a mixture of organic peroxy radicals and the branching ratios for several of the aldehydes/ketones studied are not well established. Because the product yields predicted by the chemical model are sensitive to these branching ratios (see below), additional experiments were performed to determine the yields of the desired organic peroxy radicals in reactions R3 and R4 for selected organic peroxy radical precursors. The branching ratios determined in these experiments are summarized in Table 2

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RESULTS AND DISCUSSION Product yields, defined as Δ[product]/−Δ[reactant], were determined in each experiment from linear least-squares fits to plots of [product] against −Δ[reactant]. Reported uncertainties in these yields are one standard deviation of the slope of these fits. Branching ratios for the RO2 + HO2 reaction were then determined by fitting the measured product yields to the output from a detailed chemical model (Acuchem19) in which the branching ratios are adjustable parameters. The key reactions in the reaction mechanism used in these simulations are shown in Table 1. Rate data are not available for a number of the RO2 + RO2 reactions of potential importance in these experiments, and were estimated from literature values for structurally similar radicals. Additional secondary reactions were included in the model but were found to have a minor impact on the model fits. The sensitivity of the model output to the rate coefficients used in the chemical mechanism was evaluated by systematically varying the values for the reaction rate constants by up to a factor of 10. For the majority of the reactions included in the model, the predicted concentrations are virtually independent of the rate coefficients used. The model output is, however, highly sensitive to the branching ratios used for the major RO2 + RO2 and RO2 + HO2 branching ratios. In the absence of an HO2 source, the observed product yields are dominated by the RO2 + RO2 reactions for the major RO2 radicals generated in the initial steps of the mechanism. In the presence of a high concentration of an HO2 source, the products are largely determined by the RO2 + HO2 reaction. Model fits were carried out by first adjusting the branching ratios for RO2 + RO2

Table 2. Experimental Branching Ratios for Selected Cl + Carbonyl Reactions reaction

branching ratio

Cl + CH3CH 2CH 2C(O)H

Cl + CH3C(O)CH 2CH3

Cl + MAC

Cl + MVK 6268

YR12a = 0.65, YR12b = 0.08, YR12c = 0.13, YR12d = 0.14 YR12a ≥ 0.42, YR12b < 0.25, YR12c = 0.21, YR12d ≤ 0.12 YR12a = 0.66 ± 0.04 YR19a = 0.64 ± 0.04 YR19a = 0.73 ± 0.09 YR19a = 0.76 ± 0.07, YR19b = 0.031 ± 0.006, YR19c = 0.225 ± 0.025 YR21a = 0.2, YR21b = 0.8 YR21a = 0.25, YR21b = 0.47 YR27a = 0.65

ref this work 27 26 this work 29 30 this work 34 this work

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Figure 1. Reaction pathways leading to the formation of major products from the Cl/OH radical initiated photoxidation of carbonyls in the absence of an HO2 source: (a) butanal + Cl/OH; (b) butanone + Cl; (c) methyl vinyl ketone + Cl; (d) methacrolein + Cl. Some reactants and products are not shown. Labeled reactions are the initial steps in a sequence of reactions leading to the products shown.

oxygen, butanoyl peroxy radicals are produced with a high yield.

and are discussed in detail below. The major reaction pathways and products are summarized in Figure 1. Cl + Butanal. The yield of peroxy butanoyl nitrate (PBN) from the photolysis of Cl2/butanal/NO2/air mixtures was measured by FTIR. Because PBN is not commercially available, a reference spectrum was generated in situ by the photolysis of isopropyl nitrite (IPN)/butanal/ NO/NO 2 /air mixtures. IPN photolyzes to produce isopropoxy radicals and NO (reaction R7), which leads to the formation of OH radicals and acetone via reactions R8 and R9.

CH3CH 2CH 2CHO + OH( +O2 + M) → CH3CH 2CH 2C(O)O2 + H 2O( +M)

These peroxy radicals then react with NO 2 to produce PBN (reaction R11). CH3CH 2CH 2C(O)O2 + NO2 + M → CH3CH 2CH 2C(O)O2 NO2 + M

CH3CH(ONO)CH3 + hv → CH3CH(O)CH3 + NO (R7)

CH3CH(O)CH3 + O2 → CH3C(O)CH3 + HO2

(R8)

HO2 + NO → OH + NO2

(R9)

(R10)

(R11)

In the experiments, PBN is the only reaction product observed in the IR absorption spectrum during the early stages of the reaction. An IR absorption cross section of 7.0 × 10−19 cm−1 at 1835 cm−1 was used to calibrate for PBN, which is consistent with the cross sections of PPN and PiBN measured in this lab. This corresponded to a PBN yield of 85%. Monedero and co-workers24 recently reported a significantly lower cross section for this species (3.6 × 10−19 cm−1 at 1834 cm−1) using NO3 to oxidize butanal to PBN. However, the present study is consistent with the higher cross sections for PANs measured previously in this laboratory.

The OH radicals then react with butanal (reaction R10). It is well established that the reaction between hydroxyl radicals and small linear aldehydes occurs to a large extent by H-atom abstraction from the acyl group (e.g., see ref 23 and references therein), and so in the presence of 6269

dx.doi.org/10.1021/jp211799c | J. Phys. Chem. A 2012, 116, 6264−6281

The Journal of Physical Chemistry A

Article

reduced because of scavenging of propoxy radicals by NO and NO2.

The major pathways leading to the observed reaction products are shown schematically in Figure 1a). Chlorine atoms may abstract a hydrogen atom from any of the four carbon atoms of butanal to produce an alkyl or acyl radical that reacts with oxygen, leading to the formation of four different peroxy radicals (reactions R12a−R12d).

CH3CH 2CH 2O2 + NO( +O2 ) → CH3CH 2CHO + NO2 + HO2

Reaction R14c is potentially competitive with the reaction of this alkoxy radical with O2, which would result in the formation of multifunctional products. However, on the basis of the study of a structurally similar radical,25 reaction R14c is expected to be the major pathway, accounting for over 80% of the products. The peroxy radical generated in reaction R14c is expected to form formaldehyde and CO via reaction with NO followed by decomposition and reaction with O2.

CH3CH 2CH 2C(O)H + Cl + O2 ( +M) → CH3CH 2CH 2C(O)O2 + HCl( +M)

(R12a)

→ CH3CH 2CH(O2 )C(O)H + HCl(+ M)

(R12b)

→ CH3CH(O2 )CH 2C(O)H + HCl(+ M)

(R12c)

→ CH 2(O2 )CH 2CH 2C(O)H + HCl(+ M) (R12d)

O2 CH 2C(O)H + NO → OCH 2C(O)H + NO2

In the presence of excess NO2, the butanoyl peroxy radicals react almost exclusively via reaction R11 to produce PBN. The measured yield of PBN in these experiments is therefore equivalent to the branching ratio for reaction R12a, i.e., 0.65 ± 0.1. Mixtures containing Cl2, butanal, NO, and air were then photolyzed, and product yields were determined using a combination of FTIR spectroscopy (for all products) and GC-FID (for carbonyl compounds). The measured yields for CO, CO2, propanal, and acetaldehyde in these experiments are 0.21 ± 0.02, 0.7 ± 0.09, 0.26 ± 0.04, and 0.13 ± 0.02, respectively. The reported propanal and acetaldehyde yields are averages of the values measured by FTIR and GC-FID. The organic peroxy radicals formed in reaction R4 react with NO, when present, to form alkoxy radicals (reactions R13a−R13d).

→ CH 2O + CO + HO2

CH 2(OH)CH 2CH 2C(O)O2 + NO → CH 2(OH)CH 2CH 2C(O)O + NO2 → CH 2(OH)CH 2CH 2 +CO2

(R13a)

(R13b)

(R13c)

CH 2(O2 )CH 2CH 2C(O)H + NO → CH 2(O)CH 2CH 2C(O)H + NO2

(R13d)

These species are then expected to fragment or isomerize (reactions R14a−R14d). CH3CH 2CH 2C(O)O( +O2 ) → CH3CH 2CH 2O2 + CO2 (R14a)

CH3CH 2CH(O)C(O)H( +O2 ) → CH3CH 2CHO + CO + HO2

(R14b)

CH3CH(O)CH 2C(O)H( +O2 ) → CH3CHO + O2 CH 2C(O)H

(R14c)

CH 2(O)CH 2CH 2C(O)H( +O2 ) → CH 2(OH)CH 2CH 2C(O)O2

(R18)

which likely leads to formation of the multifunctional species, HOCH2CH2CHO. Thus in the presence of NO, propanal is produced via channels R12a and R12b, CO2 is produced via channels R12a and R12d, CO is produced via channels R12b and R12c, and acetaldehyde is produced via channel R12c. The yield of acetaldehyde in these experiments is expected to be about 80% of the branching ratio for reaction R12c.25 Because CO is only expected to be formed from R12b and R12c, the branching ratio for reaction R12b can be estimated as YCO − Yacetaldehyde/0.8 (which includes a correction for the 20% of radicals formed in reaction R13c that are expected to react with O2 rather than via reaction R14c). Finally, the branching ratio for reaction R12d can be estimated from 1 − (YR12a + YR12b + YR12c) using the branching ratios for reactions R12a−R12c as determined above. Combining these results together gives the following branching ratios: YR12a = 0.65, YR12b = 0.05, YR12c = 0.16, and YR12d = 0.14. There are two previously published values for YR12a. Singh and co-workers26 measured the yield of butanoyl chloride from the photolysis of butanal/Cl2/N2 mixtures and reported a branching ratio of 0.68 for reaction R12a, in excellent agreement with this work. Wu and Mu measured27 product yields in a series of experiments using butanal/Cl2/NO/air mixtures in a study that is analogous to the work reported here. Branching ratios of ≥0.42,