Atmospheric Chemistry of n-CH2= CH (CH2) xCN (x= 0-4): Kinetics

The Journal of Physical Chemistry. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14 ... the reaction of n-CH2=CH(CH2)xCN (x = 0-4) with Cl atoms, OH radi...
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Atmospheric Chemistry of n-CH=CH(CH)CN (x = 0-4): Kinetics and Mechanisms Simone Thirstrup Andersen, Sofie Askjar Hass, Louise Boge Frederickson, and Ole John Nielsen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b04547 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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Atmospheric Chemistry of n-CH2=CH(CH2)xCN (x = 0-4): Kinetics and Mechanisms Simone Thirstrup Andersen, Sofie Askjær Hass, Louise Bøge Frederickson, and Ole John Nielsen* Copenhagen Center for Atmospheric Research, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark *Corresponding author: [email protected]

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1 Abstract Smog chamber/Fourier transform infrared (FTIR) techniques were used to measure the kinetics of the reaction of n-CH2=CH(CH2)xCN (x = 0-4) with Cl atoms, OH radicals and O3: k(CH2=CHCN + Cl) = (1.03 ± 0.13) × 10-10, k(CH2=CHCH2CN + Cl) = (2.02 ± 0.35) × 10-10, k(CH2=CH(CH2)2CN + Cl) = (2.75 ± 0.45) × 10-10, k(CH2=CHCN + OH) = (4.21 ± 0.95) × 10-12, k(CH2=CHCH2CN + OH) = (1.55 ± 0.34) × 10-11, k(CH2=CH(CH2)2CN + OH) = (2.98 ± 0.64) × 10-11, k(CH2=CH(CH2)3CN + OH) = (3.34 ± 0.64) × 10-11, k(CH2=CH(CH2)4CN + OH) = (3.61 ± 0.85) × 10-11, k(CH2=CHCN + O3) = (2.55 ± 0.28) × 10-20, k(CH2=CHCH2CN + O3) = (1.17 ± 0.24) × 10-18, k(CH2=CH(CH2)2CN + O3) = (3.35 ± 0.69) × 10-18, k(CH2=CH(CH2)3CN + O3) = (4.07 ± 0.82) × 10-18, and k(CH2=CH(CH2)4CN + O3) = (7.13 ± 1.49) × 10-18 cm3 molecule-1 s-1 at a total pressure of 700 Torr of air or N2 diluents at 296 ± 2 K. CH2ClC(O)CN, HC(O)CN, HC(O)Cl, HCN, NCC(O)OONO2, and ClC(O)OONO2 were identified as products from the Cl initiated oxidation of CH2=CHCN. The product spectra were compared to experimental and theoretically calculated IR spectra. No products could be determined from the oxidation of n-CH2=CH(CH2)xCN (x = 1-4). Using the determined OH rate constants the atmospheric lifetimes for n-CH2=CH(CH2)xCN (x = 0-4) were estimated to be 66, 18, 9.3, 8.3 and 7.7 hours, respectively. It was found that these unsaturated nitriles have no significant atmospheric environmental impact.

2 Introduction The atmospheric chemistry of nitriles has been investigated over the last decades. The interest in nitriles in general comes from their potential future uses and their possible atmospheric implications. CH3CN has been shown to react to give hydrogen cyanide, HCN, under atmospheric conditions1, however, the atmospheric chemistry of unsaturated nitriles has not been investigated in any detail. A few kinetic studies of acrylonitrile2-5 and one study of (E)-3-pentenenitrile and 4-pentenenitrile6 have been published as well as a product study with OH radicals of acrylonitrile and allylcyanide7. However, the kinetics as well as the products of a series of unsaturated nitriles need to be investigated to identify possible trends.

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Unsaturated nitriles are used to form copolymers, and especially the shortest unsaturated nitrile, acrylonitrile (CH2=CHCN), is used for multiple purposes in industry, e.g. in polymer production, acrylic fibers in apparel and home furnishing, plastic, nitrile rubbers, nitrile barrier resins, adiponitrile and acrylamide8. Allylcyanide, CH2=CHCH2CN, does not have as many applications as acrylonitrile, but in the recent years new uses have been presented. It can be used as a film-forming additive in propylene carbonate-based electrolytes for graphite anode in lithium-ion batteries9. The wide use of unsaturated nitriles in industry means that they can be emitted to the atmosphere from multiple different reactions. This study investigates the atmospheric chemistry of a series of unsaturated nitriles, nCH2=CH(CH2)xCN (x = 0-4). Rate constants for the reactions with Cl atoms, OH radicals and O3 are determined, and the reaction products for the Cl + CH2=CHCN reaction have been identified. The atmospheric impact for each nitrile is evaluated based on the results in this study.

3 Methods The experiments were performed using the photoreactor at Copenhagen Center for Atmospheric Research. It consists of a 101 L quartz cylinder surrounded by UV lamps (Waldmann F85/100 UV6 lamps, wavelength region 280-360 nm and Philips TUV 55W HO, wavelength peak at 253.7 nm). Experiments were followed using a Bruker IFS 66v/s FTIR spectrometer with an analytical path length of 40.03m or 43.45 m. IR spectra were obtained by averaging 32 interferograms with a spectral resolution of 0.25 cm−1. See Nilsson et al.10 for further details. The experiments in this study were conducted in 700 Torr air, N2 or O2 diluent at 296 ± 2 K and with no addition of NOx. Cl atoms, OH radicals or O3 were used to initiate the reactions. The Cl atoms were produced from the photolysis of Cl2: Cl2 + hν → 2 Cl

(1)

OH radicals were produced by photolysis of CH3ONO in air: CH3ONO + hν → CH3O + NO

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CH3O + O2 → HCHO + HO2

(3)

HO2 + NO → OH + NO2

(4)

O3 was produced using a commercial ozone generator from O3-Technology (Dielectric barrier discharge; model AC-20). The O3 was pre-concentrated on a silica gel trap to reduce the amount of O2 introduced to the chamber. References and Cl2 were purchased with purities of >99 % from commercial sources. Reactants were purchased with purities of >99, 98, 97, 95, and 98% for liquid CH2=CH(CH2)xCN (x = 0-4), respectively. CH3ONO was synthesized using a well-established procedure11: H2SO4 was added dropwise to a saturated solution of NaNO2 in methanol and water, and was devoid of any detectable impurities using FTIR analysis. The kinetics for the five nitriles with respect to Cl atoms and OH radicals were determined using the relative rate method (equation (I)),12: Cl/OH/O3 + reactant → products

(5)

Cl/OH/O3 + reference → products

(6)

 

ln     =





 

ln    

(I)

where [reactant]0, [reactant]t, [reference]0, and [reference]t are the concentrations of the nitrile and the reference compounds at times 0 and t. kreactant and kreference are the rate constants for the reaction with Cl atoms, OH radicals or O3 for the reactant and the reference, respectively. A linear least-squares fit to the data, when plotting ln([reactant]0/[reactant]t) as a function of ln([reference]0/[reference]t), gives the slope of kreactant/kreference. Uncertainties quoted are two standard deviations of the linear least squares analysis and a 5% uncertainty range associated with the analysis of the IR bands. The kinetics for the reaction of CH2=CHCN with O3 was determined using the absolute rate method (equation (II)),:

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ln    = − 

! "#

× t = −k '( × O( × t

(II)

whereas the rate constant for reaction of O3 with the four longer nitriles were determined using the relative rate method. The pseudo first order rate constants (kpseudo 1st) are obtained from plotting the loss of reactant as a function of time for different O3 concentrations. They are then plotted as a function of O3 concentration and the slope of that plot gives the rate constant for the reaction. The loss of reactants and references were monitored using the following absorption bands: CH2=CHCN: 682 and 1873-1936 cm-1, CH2=CHCH2CN: 931 cm-1, CH2=CH(CH2)2CN: 893-960 cm-1, CH2=CH (CH2)3CN: 893-955 cm-1, CH2=CH (CH2)4CN: 893-949 cm-1 C3H6: 912 cm-1, and C2H4: 950 cm-1. Unwanted loss in the chamber of reactants, references and products can be caused by photolysis, heterogenous reactions and reactions occurring in the dark. The possibility of photolysis of the compounds was investigated by subjecting the reactants to UV radiation in the absence of oxidant precursors Cl2, OH or O3. Heterogenous reactions and reactions happening in the dark was investigated by leaving the reaction mixtures in the dark in the chamber for 30 minutes. None of these control experiments showed evidence of loss for the nitriles. Hence, unwanted loss is unlikely to be a complication for the determination of nitrile rate constants. Theoretical IR spectra used in the product study were calculated using Gaussian0913. The spectra, and their corresponding frequencies, were calculated at the B3LYP/6-31G(d’) level of theory. Optimized geometries and calculated spectra can be found in the supporting information.

4 Results and Discussion 4.1 Relative Rate Determination of Cl Atom Kinetics All five nitriles were left in the dark chamber with molecular chlorine for at least 30 minutes to investigate if possible dark reactions between the nitriles and Cl2 occur. CH2=CH(CH2)3CN and CH2=CH(CH2)4CN react with molecular chlorine in our reaction chamber to a degree that makes it impossible to determine the rate constant of the reaction with Cl atoms. No reaction is observed for the 5 ACS Paragon Plus Environment

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shortest three nitriles, meaning if the reaction occurs it would be negligible when determining the rate constants with Cl atoms. The rate of reactions (7-9) was measured relative to reactions (10) and (11). The initial reaction concentration of the reactants were 10.32-35.5 mTorr CH2=CH(CH2)xCN (x=0-2). Initial reaction concentrations of the references and reagent, Cl2, were 8.0-14.0 mTorr C3H6, 4.4-5.4 mTorr C2H4, and 37.0-70.1 mTorr Cl2 in 700 Torr air or N2. The total photolysis time varied between 35 seconds and 190 seconds. CH2=CHCN + Cl → Products

(7)

CH2=CHCH2CN + Cl → Products

(8)

CH2=CH(CH2)2CN + Cl → Products

(9)

C3H6 + Cl → Products

(10)

C2H4 + Cl → Products

(11)

Figure 1 shows the loss of CH2=CHCN versus the loss of the reference compounds. Using linear least squares analyses the following rate constant ratios k7/k10 = 0.391 ± 0.040 and k7/k11 = 1.11 ± 0.068 are obtained. Using k10 = (2.64 ± 0.21) × 10-10 14 and k11 = (9.29 ± 0.51) × 10-11 15 the obtained values of k7 were (1.03 ± 0.13) × 10-10 and (1.03 ± 0.09) × 10-10 cm3 molecule-1 s-1, respectively. A similar approach is used to determine rate constants for CH2=CHCH2CN and CH2=CH(CH2)2CN. Table 1 shows a summary of all the rate constant ratios and individual Cl atom rate constant determinations from this work. For each nitrile, the individual rate constants are identical within the ranges of uncertainty. We report final values of k7, k8, and k9 as the averages of the two determinations with uncertainties that encompass the extremes of the individual determinations. Hence, k7 = (1.03 ± 0.13) × 10-10, k8 = (2.02 ± 0.35) × 10-10, and k9 = (2.75 ± 0.45) × 10-10 cm3 molecule-1 s-1. The rate constant, k7, determined in the present work is in agreement with the previously determined constant by Teruel et al.2 of (1.11 ± 0.23) × 10-10 cm3 molecule-1 s-1. However, a study by Aranda et al.3 determined the

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constant to be 1.75 × 10-12 cm3 molecule-1 s-1, which show a large discrepancy with the number determined in the present work and that reported by Teruel et al. This, however has been determined at a pressure of 0.5-3 Torr, meaning that the discrepancy could be due to a pressure dependence. Previously, k9 has been determined by Colomer et al.6 as (2.56 ± 0.26) × 10-10 cm3 molecule-1 s-1 which is in agreement with the value determined in the present work. To the best of our knowledge we present here the first determination of the rate constant, k8.

4.2 Relative Rate Determination of OH Radical Kinetics The rate of reactions (12-16) was measured relative to reactions (17) and (18). The initial reaction concentrations of the reactants were 5.2-37.0 mTorr CH2=CH(CH2)xCN (x=0-4). Initial reaction concentrations of the references and OH radical precursor, CH3ONO, were 5.4-14.59 mTorr C3H6, 4.37.1 mTorr C2H4, and 75.8-144 mTorr CH3ONO in 700 Torr air diluent. The total photolysis time varied between 3.5 and 15 minutes. CH2=CHCN + OH → Products

(12)

CH2=CHCH2CN + OH → Products

(13)

CH2=CH(CH2)2CN + OH → Products

(14)

CH2=CH(CH2)3CN + OH → Products

(15)

CH2=CH(CH2)4CN + OH → Products

(16)

C3H6 + OH → Products

(17)

C2H4 + OH → Products

(18)

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The loss of CH2=CHCN versus the loss of the reference compounds is shown in Figure 2. Using linear least squares analyses the following rate constant ratios, k12/k17 = 0.135 ± 0.013 and k12/k18 = 0.513 ± 0.050, are obtained. Using k17 = (2.99 ± 0.52) × 10-11 16 and k18 = (8.52 ± 1.28) × 10-12 17 give k12 = (4.04 ± 0.79) × 10-12 and (4.37 ± 0.78) × 10-12 cm3 molecule-1 s-1, respectively. A similar approach is used to determine rate constants for CH2=CH(CH2)xCN for (x = 1-4). A summary of all the rate constant ratios and individual rate constant determinations with OH radicals from this work can be found in Table 2. The individual determined rate constants for each nitrile are identical within the ranges of uncertainties. The final values of k12, k13, k14, k15, and k16 are the averages of the two determinations with uncertainties that encompass the extremes of the individual determinations. Hence, k12 = (4.21 ± 0.95) × 10-12, k13 = (1.55 ± 0.34) × 10-11, k14 = (2.98 ± 0.64) × 10-11, k15 = (3.34 ± 0.64) × 10-11, and k16 = (3.61 ± 0.85) × 10-11 cm3 molecule-1 s-1. The rate constant k12 determined in the present work is in agreement with that previously determined by Harris et al.4 of (4.06 ± 0.41) × 10-12 cm3 molecule-1 s-1. k12 has also been reported by Teruel et al.2 i.e., (1.11 ± 0.33) × 10-11 cm3 molecule-1 s-1. The latter value is in disagreement with the value determined in the present study and the one reported by Harris et al. The rate constant, k14, has previously been determined experimentally by Colomer et al.6 to be (2.90 ± 0.64) × 10-11 cm3 molecule-1 s-1, which is in agreement with the value determined in the present study.

4.3 Rate determination of O3 with nitriles The rate constant of reaction (19) was determined using the absolute rate method. The decrease in CH2=CHCN was followed as a function of time at different excess concentrations of O3. The initial reaction concentrations were 31.2-33.0 mTorr CH2=CHCN and 0.65-1.90 Torr O3 in 700 Torr air diluent. CH2=CHCN + O3 → Products

(19)

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Figure 3 shows pseudo-first-order constants for the reaction of CH2=CHCN with O3 as a function of the various O3 concentrations, where the former was obtained from the inset in figure 3, and the concentration of CH2=CHCN was followed as a function of time. The slope in figure 3 gives the rate constant for the reaction of CH2=CHCN with O3: k19 = (2.55 ± 0.28) × 10-20 cm3 molecule-1 s-1. The uncertainty on the rate constant is two standard deviations on the slope combined with ±5% uncertainty on the analysis. k19 has previously been determined by Munchi et al.5 to be 1.38 × 10-19 cm3 molecule-1 s-1, which is 5.4 times higher than what is determined in this study. There are no obvious reasons for this discrepancy.

The rate constants for the four longer nitriles with O3 were determined using the relative rate method since they react too fast in excess O3 to be able to follow the loss of nitrile as a function of time. The rate of reactions (20-23) were measured relative to reactions (24) and (25). The initial reaction concentration of the reactants were 4.6-17.4 mTorr CH2=CH(CH2)xCN (x=1-4). Initial reaction concentrations of the references and reagent, O3, were 9.4-10.6 mTorr C3H6, 5.1-6.6 mTorr C2H4, and 10.8-36 mTorr O3 in 700 Torr air. CH2=CHCH2CN + O3 → Products

(20)

CH2=CH(CH2)2CN + O3 → Products

(21)

CH2=CH(CH2)3CN + O3 → Products

(22)

CH2=CH(CH2)4CN + O3 → Products

(23)

C3H6 + O3 → Products

(24)

C2H4 + O3 → Products

(25)

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The loss of CH2=CHCH2CN versus the loss of the reference compounds is shown in Figure 4. Using linear least squares analyses the following rate constant ratios, k20/k24 = 0.106 ± 0.007 and k20/k25 = 0.804 ± 0.057, are obtained. Using k24 = (1.00 ± 0.10) × 10-17 18 and k25 = (1.60 ± 0.10) × 10-18 18 give k20 = (1.06 ± 0.13) × 10-19 and (1.29 ± 0.12) × 10-19 cm3 molecule-1 s-1, respectively. A similar approach is used to determine rate constants for CH2=CH(CH2)xCN for x = 2-4. A summary of all the rate constant ratios and individual rate constant determinations with O3 from this work can be found in Table 3. The individual determined rate constants for each nitrile are identical within the ranges of uncertainties. The final values of k20, k21, k22, and k23 are the averages of the two determinations with uncertainties that encompass the extremes of the individual determinations. Hence, k20 = (1.17 ± 0.24) × 10-18, k21 = (3.35 ± 0.69) × 10-18, k22 = (4.07 ± 0.82) × 10-18, and k23 = (7.13 ± 1.49) × 10-18 cm3 molecule-1 s-1, respectively. All experiments were performed without OH radical scavenger.

4.4 IR spectra Calibrated IR spectra for CH2=CH(CH2)xCN for (x = 0-4) can be found in the supporting information (S10-S14). The integrated absorption cross sections (600-2000 cm-1) for the five nitriles have been determined to be (1.36 ± 0.08) x 10-18, (1.80 ± 0.14) x 10-18, (1.87 ± 0.11) x 10-18, (1.94 ± 0.13) x 10-18, and (1.14 ± 0.08) x 10-18 cm2 molecule-1 in the order of x = 0 to x = 4, respectively. It should be noted that the sample of CH2=CH(CH2)2CN contained ethyl acetate, CH3C(O)OCH2CH3, which has been subtracted from the spectrum meaning the integrated absorption cross section should be taken as a best estimate. Optimized geometries and theoretically calculated spectra of CH2=CH(CH2)xCN for (x = 0-4) can be found in table S1-S5 and figure S15-S19. The theoretically calculated spectra contain the same peaks as the experimental spectra, however, they are slightly shifted and the C≡N stretching mode has lower intensities in the experimental spectra than in the calculated ones. Both differences could be due to the level of theory used in the calculations of the spectra.

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4.5 Product Study of the Reactions of Cl Atoms with Nitriles. The atmospheric degradation of CH2=CHCN, CH2=CHCH2CN, and CH2=CH(CH2)2CN were investigated using their reactions with Cl atoms. As it was discovered that CH2=CH(CH2)3CN and CH2=CH(CH2)4CN react with molecular chlorine, they have not been investigated as it would give a combination of products from the reaction with Cl atoms and with molecular chlorine. Cl atoms are used to initiate the oxidation even though OH radical reactions are expected to be the most dominating in the atmosphere, because Cl atoms afford a much more accessible analysis and the reaction mechanism is expected to be similar to that of the OH reaction. Studies were performed under similar conditions with 10.74-31.3 mTorr CH2=CH(CH2)xCN, x= 0-2 and 62-107 mTorr Cl2 in 700 Torr air/O2 diluent. It is expected that the Cl atoms add to the double bond, though hydrogen abstraction is also expected to become more important when going from 0 to 2 methylene groups. Figure 5 shows the product study of the Cl initiated oxidation of CH2=CHCN. Panel A shows the spectrum of CH2=CHCN before photolysis. Panel B shows the spectrum of the reaction mixture after 100 seconds of photolysis. Panel C shows the products of the reaction where the remaining peaks from CH2=CHCN has been subtracted. From panel C, HCN, CO, HC(O)CN can be observed at 713 cm-1, 2040-2220 cm-1, 913 and 1716 cm-1, respectively. Panel D is a reference spectrum of HC(O)Cl, which can also be observed in the product spectrum. The peak observed at 2230 cm-1 is similar to the peak observed for CH3C(O)CN (Figure 6) meaning that another product most likely contains a -C(O)CN group. The most likely product containing -C(O)CN is CH2ClC(O)CN. However, we do not have access to a reference spectrum of CH2ClC(O)CN, therefore, panel E shows a calculated spectrum of CH2ClC(O)CN instead. The peaks in the calculated spectrum can be slightly shifted from what is observed in an experimental IR spectrum, which can be seen for CH3C(O)CN in figure 6, where an experimental and a calculated spectrum are displayed. Spectra of two other possible products, HC(O)CHClCN and CH2ClC(O)H, have been calculated to exclude them as possible products; the spectra can be found in figure S21-S22 and their optimized geometries can be found in table S7-S8. Cl atoms adds to the terminal carbon creating the most stable radical, which reacts with oxygen to give a peroxy radical: CH2=CHCN + Cl → CH2ClCHCN

(26)

CH2ClCHCN + O2 → CH2ClCHO2CN

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The peroxy radical reacts with NO, RO2 or itself to give an alkoxy radical. The alkoxy radical can either react with oxygen to give CH2ClC(O)CN or undergo C-C bond cleavage into CH2Cl and HC(O)CN: CH2ClCHO2CN + NO/RO2 → CH2ClCHOCN + NO2/RO + O2

(28)

CH2ClCHOCN + O2 → CH2ClC(O)CN + HO2

(29)

CH2ClCHOCN → CH2Cl + HC(O)CN

(30)

CH2Cl radicals reacts with oxygen to give a peroxy radical, which reacts with NO or RO2 to give an alkoxy radical, where a hydrogen can be abstracted by oxygen to give HC(O)Cl19: *+/-+.

CH2Cl + O2 /00001 CH2ClO + NO2/RO + O2

(31)

CH2ClO + O2 → HC(O)Cl + HO2

(32)

HC(O)CN have previously been observed to react through hydrogen abstraction followed by addition of O2 to give peroxy radicals1. HC(O)Cl is expected to react through the same mechanism in our chamber. The peroxy radicals either reacts heterogeneously with NO2 from the walls of the chamber to give NCC(O)OONO2 and ClC(O)OONO2, respectively, or they react with NO or RO2 to give alkoxy radicals, which can abstract a hydrogen to give NCC(O)OH and ClC(O)OH, respectively1. The two acids are unstable and dissociate into HCN and CO2 or HCl and CO2, respectively1, 20: HC(O)CN + Cl → C(O)CN + HCl

(33)

C(O)CN + O2 → NCC(O)OO

(34)

NCC(O)OO + NO2 → NCC(O)OONO2

(35)

NCC(O)OO + NO/RO2 → NCC(O)O + NO2/RO + O2

(36)

NCC(O)O + RH → NCC(O)OH + R

(37)

NCC(O)OH → HCN + CO2

(38)

HC(O)Cl + Cl → C(O)Cl + HCl

(39)

C(O)Cl + O2 → ClC(O)OO

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ClC(O)OO + NO2 → ClC(O)OONO2

(41)

ClC(O)OO + NO/RO2 → ClC(O)O + NO2/RO + O2

(42)

ClC(O)O + RH → ClC(O)OH + R

(43)

ClC(O)OH → HCl + CO2

(44)

NCC(O)OONO2 (790, 1121, 1298, 1764, and 1827 cm-1)1, ClC(O)OONO2 (790, 1000, 1298, 1764, and 1827 cm-1)21, and HCN (713 cm-1) can be observed as secondary products in figure 7 (HCl and CO2 can also be observed), where panel A is a spectrum of the primary products as seen in Panel C in figure 5. Panel B displays the spectrum after additional 240 seconds of photolysis. Panel C displays the secondary products, where the peaks for HC(O)CN can be observed as downwards peaks as HC(O)CN reacts faster than CH2ClC(O)CN and HC(O)Cl. CH2ClC(O)CN can react through hydrogen abstraction followed by addition of oxygen to give CHClOOC(O)CN: CH2ClC(O)CN + Cl → CHClC(O)CN + HCl

(45)

CHClC(O)CN + O2 → CHClOOC(O)CN

(46)

CHClOOC(O)CN reacts with either NO, RO2 or itself to give the alkoxy radical, which can then undergo C-C cleavage into HC(O)Cl and C(O)CN: CHClOOC(O)CN + NO/RO2 → CHClOC(O)CN + NO2/RO + O2

(47)

CHClOC(O)CN → HC(O)Cl + C(O)CN

(48)

HC(O)Cl reacts as described in reactions (39-44) and C(O)CN radicals reacts as described in reactions (34-38). The Cl initiated oxidation of CH2=CHCN is summarized in figure 8. The mechanism in figure 8 is supplemented by the estimated yields in figure 9, where the yields of the identified products are plotted versus the loss of CH2=CHCN. When CH2=CHCN has reacted, the primary products react further. Figure 10 shows the estimated yields of the identified products versus the loss of the primary product CH2ClC(O)CN. The yields of HC(O)Cl, HCN and CO are estimated using calibrated reference spectra. HC(O)CN is estimated using an absorption cross section σ = 3.7 × 10-19 cm2 molecule-1 at 921 cm-1 1, 22. CH2ClC(O)CN is estimated using a calibrated reference spectrum of CH3C(O)CN and the relationship between the peak intensity observed in the theoretically calculated 13 ACS Paragon Plus Environment

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spectra of CH2ClC(O)CN (figure S20 or panel E in figure 5) and CH3C(O)CN (figure S23 or figure 6) at 2230 cm-1. NCC(O)OONO2 and ClC(O)OONO2 are estimated using an absorption cross section σ = 1.0 × 10-18 cm2 molecule-1 at 1840 cm-1 23 of CH3C(O)OONO2. The yield of CH2ClC(O)CN is estimated to be 72 ± 5% in air and 80 ± 5% in 700 Torr O2 showing a [O2] dependence, which is consistent with the split between reaction (29) and (30). Another confirmation of the pathway ratio can be found when estimating the yields of HC(O)Cl and HC(O)CN in air and O2. The yields of HC(O)Cl and HC(O)CN are estimated to be 15 ± 2% and 13 ± 1% in air and 4 ± 0.5% and 5 ± 0.5% in 700 Torr O2, respectively. HC(O)Cl and HC(O)CN are formed in a 1:1 ratio from the oxidation of CH2=CHCN in air and in O2, which would be expected from the proposed mechanism. From figure 10 it can be observed that HC(O)CN decays rapidly during continued Cl oxidation where HC(O)Cl decreases more slowly, which is also consistent with the proposed mechanism. HC(O)CN decays rapidly because it reacts with Cl atoms (reaction 33) and it is not formed from any other reactions. The loss of HC(O)CN is not analyzed through the entire experiment since the analysis becomes difficult due to interfering peaks from other products. HC(O)Cl decreases slowly since it reacts with Cl atoms (reaction 39), but it is also formed when CH2ClC(O)CN is oxidized (reactions 45-48). HCN and CO are both formed in less than 2% yield when looking at consumption of CH2=CHCN less than 80%. The concentration of both CO and HCN increases rapidly towards the end of the oxidation of CH2=CHCN and in the beginning of the oxidation of the other formed products due to the rapid decrease in HC(O)CN. CO increases more than HCN in the beginning as it can also be formed from HC(O)Cl reacting, however the CO concentration decreases towards the end as it is oxidized to CO2. NCC(O)OONO2 and ClC(O)OONO2 are both not observed before HC(O)CN, HC(O)Cl and CH2ClC(O)CN reacts in figure 10, which again is consistent with the proposed mechanism.

The Cl initiated oxidation of CH2=CHCH2CN and CH2=CH(CH2)2CN were investigated in the same way as CH2=CHCN though it was not possible to identify any of the products. It was not possible to add enough of the nitriles to the chamber to get proper product spectra as particle formation is observed from baseline shift in the entire IR region. From the experiments C=O bonds could be identified at 1700-1800 cm-1. No IR signals can be attributed to HCN in the oxidation of CH2=CHCH2CN and CH2=CH(CH2)2CN.

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5 Atmospheric Impact The present study serves to expand our knowledge of how unsaturated nitriles react in the atmosphere. Reactions with Cl atoms, OH radicals and O3 are all assumed to primarily react through addition to the double bond. Photolysis and heterogenous reactions were not observed under the conditions of the experiments, however CH2=CH(CH2)3CN and CH2=CH(CH2)4CN were observed to react with Cl2 in the dark. Since the rate constants with respect to OH radicals are all higher than 4 × 10-12 cm3 molecule1 -1

s , wet and dry deposition is not expected to have a major impact on the lifetime of any of the five

nitriles. The atmospheric lifetime is therefore estimated using a global 24 hour-average tropospheric OH concentration of 1.0 × 106 molecules cm-3 24. The lifetimes are estimated to be 66, 18, 9.3, 8.3 and 7.7 hours for CH2=CH(CH2)xCN (x = 0-4), respectively. These lifetimes are only approximations, since the concentration of OH radicals is a global approximation, but in reality, the concentration varies with season and location. Additionally the rate constants used here are determined at 296 K, but it would be more appropriate to use rate constants determined at 272 K 25. However, it is not expected to have a major impact on the lifetimes. As all the lifetimes are less than 3 days no global warming potential has been calculated. Hydrogen cyanide, HCN, was observed as a product from the oxidation of CH2=CHCN, but it was not observed to be formed from any of the other nitriles. Loss of HC(O)Cl in the atmosphere might be due to wet deposition26

6 Associated Content Supporting Information Kinetic plots, calibrated spectra, optimized geometries, calculated spectra.

7 Acknowledgement We thank Mads Peter Sulbaek-Andersen for inspiring discussions and help with the graphics. 15 ACS Paragon Plus Environment

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Table 1: Rate constant ratios, reference rate constant values, and the individual determinations of the rate constants of the reactions of CH2=CH(CH2)xCN for x = 0 - 2 with Cl atoms. kReference kReactant x Reference kReactant/kReference (cm3 molecule-1 s-1) (cm3 molecule-1 s-1) 0 C3H6 0.391 ± 0.040 (2.64 ± 0.21) × 10-10 a (1.03 ± 0.13) × 10-10 -11 b 0 C2H4 1.11 ± 0.068 (9.29 ± 0.51) × 10 (1.03 ± 0.09) × 10-10 1 C3H6 0.717 ± 0.062 (2.64 ± 0.21) × 10-10 a (1.89 ± 0.22) × 10-10 -11 b 1 C2H4 2.302 ± 0.226 (9.29 ± 0.51) × 10 (2.14 ± 0.24) × 10-10 2 C3H6 0.996 ± 0.070 (2.64 ± 0.21) × 10-10 a (2.63 ± 0.28) × 10-10 -11 b 2 C2H4 3.083 ± 0.310 (9.29 ± 0.51) × 10 (2.86 ± 0.33) × 10-10 a Ezell et al.14 b Wallington et al. 15

Table 2: Rate constant ratios, reference rate constant values, and the individual determinations of the rate constants of the reactions of CH2=CH(CH2)xCN for x = 0 - 4 with OH radicals. kReference kReactant x Reference kReactant/kReference 3 -1 -1 3 (cm molecule s ) (cm molecule-1 s-1) 0 C3H6 0.135 ± 0.013 (2.99 ± 0.52) × 10-11 a (4.04 ± 0.79) × 10-12 -12 b 0 C2H4 0.513 ± 0.050 (8.52 ± 1.28) × 10 (4.37 ± 0.78) × 10-12 1 C3H6 0.530 ± 0.045 (2.99 ± 0.52) × 10-11 a (1.58 ± 0.30) × 10-11 -12 b 1 C2H4 1.77 ± 0.173 (8.52 ± 1.28) × 10 (1.51 ± 0.27) × 10-11 2 C3H6 0.978 ± 0.065 (2.99 ± 0.52) × 10-11 a (2.92 ± 0.54) × 10-11 -12 b 2 C2H4 3.572 ± 0.411 (8.52 ± 1.28) × 10 (3.04 ± 0.58) × 10-11 3 C3H6 1.103 ± 0.068 (2.99 ± 0.52) × 10-11 a (3.30 ± 0.60) × 10-11 -12 b 3 C2H4 3.967 ± 0.315 (8.52 ± 1.28) × 10 (3.38 ± 0.57) × 10-11 4 C3H6 1.158 ± 0.081 (2.99 ± 0.52) × 10-11 a (3.46 ± 0.64) × 10-11 -12 b 4 C2H4 4.406 ± 0.501 (8.52 ± 1.28) × 10 (3.75 ± 0.71) × 10-11 a Zellner and Lorentz 16 b Calvert et al. 17

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Table 3: Rate constant ratios, reference rate constant values, and the individual determinations of the rate constants of the reactions of CH2=CH(CH2)xCN for x = 1 - 4 with O3. kReference kReactant x Reference kReactant/kReference (cm3 molecule-1 s-1) (cm3 molecule-1 s-1) 1 C3H6 0.106 ± 0.007 (1.00 ± 0.10) × 10-17 a (1.06 ± 0.13) × 10-18 -18 a 1 C2H4 0.804 ± 0.057 (1.60 ± 0.10) × 10 (1.29 ± 0.12) × 10-18 2 C3H6 0.305± 0.019 (1.00 ± 0.10) × 10-17 a (3.05 ± 0.36) × 10-18 -18 a 2 C2H4 2.282 ± 0.195 (1.60 ± 0.10) × 10 (3.65 ± 0.39) × 10-18 3 C3H6 0.368 ± 0.022 (1.00 ± 0.10) × 10-17 a (3.68 ± 0.43) × 10-18 -18 a 3 C2H4 2.787 ± 0.189 (1.60 ± 0.10) × 10 (4.46 ± 0.41) × 10-18 4 C3H6 0.648 ± 0.042 (1.00 ± 0.10) × 10-17 a (6.48 ± 0.77) × 10-18 -18 a 4 C2H4 4.866 ± 0.423 (1.60 ± 0.10) × 10 (7.79 ± 0.83) × 10-18 a Atkinson et al.18

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Figures

Figure 1: Fractional loss of CH2=CHCN versus C2H4 (circles) and C3H6 (squares) in the presence of Cl atoms at 296 ± 1 K. The error bars reflect the uncertainty of ± 5% on the analysis.

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Figure 2: Fractional loss of CH2=CHCN versus C2H4 (circles) and C3H6 (squares) in the presence of OH radicals at 296 ± 1 K. The error bars reflect the uncertainty of ± 5% on the analysis.

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Figure 3: Pseudo first-order rate constants for CH2=CHCN with O3 versus different O3 concentrations. The inset shows the loss of CH2=CHCN as a function of time at different O3 concentrations. Each symbol indicate different O3 concentrations: 0.647 Torr (white triangle), 0.745 Torr (gray triangle), 0.828 Torr (black triangle), 0.932 Torr (white circle), 1.468 Torr (gray circle), and 1.900 Torr (black circle).

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Figure 4: Fractional loss of CH2=CHCH2CN versus C2H4 (circles) and C3H6 (squares) in the presence of O3 at 296 ± 1 K. The error bars reflect the uncertainty of ± 5% on the analysis.

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Figure 5: Primary product study of the Cl initiated oxidation of CH2=CHCN. Panel A shows CH2=CHCN with 107 mTorr Cl2 in 700 Torr air before photolysis. Panel B shows the mixture after 100 seconds of UV. Panel C shows the residual spectrum with the remaining features of CH2=CHCN subtracted. Panel D shows an experimental spectrum of HC(O)Cl (the peak at 2350 cm-1 is from CO2). Panel E shows a calculated spectrum of CH2ClC(O)CN.

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Figure 6: Comparison between a calculated spectrum and an experimental spectrum of CH3C(O)CN. The dashed lines show which peaks in the calculated spectrum corresponds to the peaks in the experimental one.

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Figure 7: Secondary products from the Cl initiated oxidation of CH2=CHCN. Panel A (identical to panel B in figure 5) shows the primary products from the oxidation of CH2=CHCN. Panel B shows the mixture after 240 seconds of photolysis. Panel C shows the residual spectrum where remaining features of the primary products have been subtracted (HC(O)CN is the negative peaks, since it degrades faster than the other primary products).

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Figure 8: Proposed mechanism of the Cl initiated oxidation of CH2=CHCN. The boxes show identified products.

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Figure 9: Panel A) Formation of HC(O)Cl (stars) and HC(O)CN (diamonds) versus the loss of CH2=CHCN in 700 Torr air (black) and O2 (gray). Panel B) Formation of HCN (downward triangles), CO (upward triangles), and CH2ClC(O)CN (circles, right axis) versus the loss of CH2=CHCN in 700 Torr air (black) and O2 (gray).

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Figure 10: Formation/Loss of HC(O)CN (diamonds), HC(O)Cl (stars), HCN (downward triangles), CO (upward triangles), NCC(O)OONO2 (squares), and ClC(O)OONO2 (hexagons) versus the loss of the primary product CH2ClC(O)CN in 700 Torr air (black) and O2 (gray). 27 ACS Paragon Plus Environment

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