- Email: [email protected]
Atmospheric chemistry of methoxyflurane (CH3OCF2CHCl2): Products and mechanisms
Journal Pre-proofs Research paper Atmospheric Chemistry of Methoxyflurane (CH3OCF2CHCl2): Products and Mechanisms S.A. Hass, M.P. Sulbaek Andersen, O.J. Nielsen PII: DOI: Reference:
S0009-2614(19)31033-4 https://doi.org/10.1016/j.cplett.2019.137052 CPLETT 137052
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
7 August 2019 18 December 2019 20 December 2019
Please cite this article as: S.A. Hass, M.P. Sulbaek Andersen, O.J. Nielsen, Atmospheric Chemistry of Methoxyflurane (CH3OCF2CHCl2): Products and Mechanisms, Chemical Physics Letters (2019), doi: https:// doi.org/10.1016/j.cplett.2019.137052
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Atmospheric Chemistry of Methoxyflurane (CH3OCF2CHCl2): Products and Mechanisms
S. A. Hassa, M. P. Sulbaek Andersena,b,*, and O. J. Nielsena
a
Copenhagen Center for Atmospheric Research, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark
b
Department of Chemistry and Biochemistry, California State University Northridge, Northridge, CA, 91330-8262
*Corresponding Author: M. P. Sulbaek Andersen ([email protected])
Abstract Long pathlength/FTIR smog chamber techniques were used to determine the Cl atom initiated atmospheric oxidation mechanisms for methoxyflurane (CH3OCF2CHCl2) in 700 Torr of air or O2 diluent at 296K. The oxidation of CH3OCF2CHCl2 in both air and O2 diluent gave one primary carbonyl-containing product in approximately 100% yield and one secondary bi-carbonyl product in addition to COF2 and COCl2, the latter two both with initial molar yields of 2 ± 1%. A mechanism is proposed for the oxidation of methoxyflurane which explains the observed product distribution. IR spectra for HC(O)OCF2CHCl2 and HC(O)OCF2C(O)Cl are calculated and reported.
1
1. Introduction Halogenated ethers have been widely used worldwide for decades in various industry sectors including as inhalational anesthetics involving compounds such as sevoflurane (CH2FOCH(CF3)2), isoflurane (CH2FOCHClCF3), desflurane (CHF2OCHFCF3), enflurane (CHFClCF2OCF2H) and methoxyflurane (CH3OCF2CHCl2). In comparison to hydrofluorocarbons (HFCs), halogenated ethers generally have lower atmospheric lifetimes and Global Warming Potentials (GWPs) due to the etherlinkage [1], e.g. HFC-143a (CF3CH3) has a lifetime of ~51 years [2] whereas HFE-143 (CF3OCH3) has a lifetime of ~4.7 years [3]. In a recent study [4], OH, Cl and O3 rate constants for methoxyflurane were determined as: k(CH3OCF2CHCl2 + OH) = (2.18 ± 0.38) × 10-13, k(CH3OCF2CHCl2 + Cl) = (5.13 ± 0.66) × 10-13 and k(CH3OCF2CHCl2 + O3) < 1.5 × 10-22 cm3 molecule-1 s-1 in 700 Torr of N2/air diluent, providing the first atmospheric lifetime estimate for CH3OCF2CHCl2 of ~54 days and a 100-year GWP of 4. The OH radical reaction is likely to be the dominant sink of CH3OCF2CHCl2 in the atmosphere. In the present study, FTIR smog chamber techniques were used to determine the products formed in the atmospheric oxidation of CH3OCF2CHCl2. Even though the OH radical is the dominant oxidative species in the atmosphere, Cl atoms is used in this study due to complications employing the OH radical precursor used in our laboratory: CH3ONO. Three complications arise in our experiments using CH3ONO as the OH radical precursor. Firstly, the rate constant for the reaction of OH radicals with methyl nitrite [5], (3.0 ± 1.0) × 10-13 cm3 molecule-1 s-1, is similar to that of methoxyflurane [4], (2.18 ± 0.38) × 10-13 cm3 molecule-1 s-1, thus the consumption of CH3OCF2CHCl2 is effectively limited by the reaction of the OH radicals with CH3ONO. Secondly, as the products formed from the reaction of OH radicals with methyl nitrite display IR absorption bands overlapping with those from the expected methoxyflurane-OH reaction products, positive identification of the latter is compromised. Finally, because the spectral bands from methyl nitrite (i.e., photolysis product of CH3ONO) quickly become oversaturated, these regions are rendered unusable to us for identification of possible oxidation products from CH3OCF2CHCl2. The Cl atom and OH initiated oxidation mechanisms are expected to be similar, with the caveat, that the relative reactivity of the two types of H-atoms in the methoxy group and the CHCl2 group in CH3OCF2CHCl2 could be slightly different in the Cl atom versus OH radical mediated abstraction. As part of this work, theoretical frequency calculations were performed for compounds where experimental IR spectra could not be found.
2
2. Methods Experiments were conducted using the photoreactor at the Copenhagen Center for Atmospheric Research, at University of Copenhagen. The photoreactor consists of a 101 L quartz cylinder interfaced with a Bruker IFS 66v/s FTIR spectrometer. Reactions were initiated using Waldmann F85/100W UV6 lamps (wavelength region at 280-360 nm) surrounding the reactor. The concentrations of reactants and products were monitored using an analytical pathlength of 47.73 m (internal White-cell optics). For further details on the experimental setup see Nilsson et al.[6]. All experiments were performed in 700 Torr of air or O2 diluent at 296 ± 2K. Cl atoms were produced by the photolysis of Cl2, as seen in reaction (1). Cl2 + hν → 2 Cl
(1)
OH radicals were produced by photolysis of methyl nitrite, CH3ONO, in the presence of NO: CH3ONO + hν → CH3O + NO
(2)
CH3O + O2 → CH2O + HO2
(3)
HO2 + NO → OH + NO2
(4)
All reagents used in the experiments, except methyl nitrite, were obtained from commercial sources with purities >97%. Methyl nitrite was prepared by dropwise addition of sulfuric acid, H2SO4, to a saturated solution of sodium nitrite, NaNO2, in methanol and water [7]. The methyl nitrite samples prepared were devoid of any detectable impurities by analysis with FTIR. Theoretical IR-spectra of potential oxidation products were calculated using the Gaussian 09 program[8]. The vibrational frequencies were calculated at the B3LYP/6-311G(d) level of theory and multiplied by a scaling factor of 0.966 [9] to better match experimental frequencies. It should be noted that the calculations do not include the rotational structures of the absorption bands, there is therefore the possibility that a single molecule can expand over a larger area of wavenumbers. The calculated spectra are only used as an aid in interpreting the experimental product spectra. The peak positions are not expected to exactly match the experimental values but are utilized to show approximate absorption band positions.
3
3. Results and Discussion 3.1 Cl Atom Product Study: The Fate of CH2OCF2CHCl2 To investigate the mechanism of Cl atom-initiated oxidation of CH3OCF2CHCl2, mixtures containing 7.32-7.39 mTorr CH3OCF2CHCl2 and approximately 40 mTorr Cl2 in 700 Torr of air or O2 diluent were introduced into the reaction chamber and irradiated with UV black lamps. All experiments were conducted at a temperature of 296 ± 2 K. Figure 1 shows spectra acquired before (Panel A), after 2 minutes (Panel B) and after 25 minutes (Panel C) of UV irradiation of the gas mixture in 700 Torr of air. The experiments revealed the formation of multiple products with carbonyl bands in the range 1760-1850 cm-1 and a band with an approximate center at 1944 cm-1. The two product spectra after 2 minutes and 25 minutes of irradiation (Panels B and C) predominantly show the primary and secondary products, respectively. Closer inspection of the carbonyl region, 1700-2000 cm-1, reveals several possible products formed in the oxidation. Unfortunately, the product bands below 1300 cm-1 are difficult to differentiate. Figure 2 shows an expanded view of the carbonyl region for the primary and secondary products and the 3 smaller carbonyl products readily identified: COCl2, COF2 and COFCl (Panel C). As the experiments have been performed in the presence of O2, it can be expected that most of the products will contain one or more carbonyl groups. As no fluorine or chlorine atoms are attached to a single carbon atom in methoxyflurane, we believe that the COFCl (in Panel C) is formed due to uncharacterized surface reactions on the reaction chamber walls and is therefore not a direct product of the gas phase oxidation reaction of CH3OCF2CHCl2. To determine the yields and the reactivates of the products, the formation of products following UV radiation of the gas mixtures versus the loss of CH3OCF2CHCl2 normalized to the initial CH3OCF2CHCl2 concentration is shown in Figure 3. Multiple trends are visible in Figure 3: A single primary product carbonyl is formed with a distinct feature at 1783 cm-1 (Figure 3, Panel A), which, upon further reaction, appear to yield a secondary carbonyl product with IR bands at 1853-64 cm-1 (Panel B). Phosgene (COCl2) is formed together with carbonyl fluoride (COF2) already in early stages of the experiments (Figure 3, Panel C). Carbon monoxide, CO, also appear as a primary product. Unfortunately, neither CO or CO2 are good markers in our system, and while CO appeared to be produced as a primary product (~6%), a significant initial presence after just 5 seconds of irradiation suggests an additional, and perhaps heterogenous sources. We suggest that CO may be a primary product, but due to background surface reactions in the chamber the initial yield is an upper limit.
4
The reaction mechanism is expected to proceed through hydrogen abstraction, possibly at two different positions: at the methoxy group, CH3O-, and at the chlorine substituted carbon, -CHCl2. CH3OCF2CHCl2 + Cl → CH2OCF2CHCl2 + HCl → CH3OCF2CCl2 + HCl
(5a) (5b)
The two radicals generated in reaction (5) will first add molecular oxygen, reaction (6) and (7), and then undergo reaction with other peroxy radicals in the photoreactor (or NO in the atmosphere) to form alkoxy radicals, reaction (8) and (9). CH2OCF2CHCl2 + O2 + M → OOCH2OCF2CHCl2 + M
(6)
CH3OCF2CCl2 + O2 + M → CH3OCF2C(OO)Cl2 + M
(7)
OOCH2OCF2CHCl2 + NO/RO2 → OCH2OCF2CHCl2 + NO2/RO + O2
(8)
CH3OCF2C(OO)Cl2 + NO/RO2 → CH3OCF2COCl2 + NO2/RO + O2
(9)
The alkoxy radicals formed in reactions (8) and (9) can then produce two possible primary carbonyl products as shown in reaction (10) through reaction of OCH2OCF2CHCl2 with O2 or in reaction (11) with thermal decomposition of CH3OCF2COCl2. OCH2OCF2CHCl2 + O2 + M→ HC(O)OCF2CHCl2 + HO2 + M OCH2CF2CHCl2 CH3OCF2COCl2 CH3OCF2COCl2
∆
∆
∆
(10a)
HCOH + CF2CHCl2
(10b)
CH3OCF2C(O)Cl + Cl
(11a)
CH3OCF2 + COCl2
(11b)
The radical fragments, CF2CHCl2 and CH3OCF2, would rapidly undergo further reaction with O2/NO/RO2 to yield COF2 and COCl2, and COF2 and HCOH, respectively. If formed, the carbonyl products generated in reaction (10) and (11) would be reactive towards Cl atoms and could undergo hydrogen abstraction as shown in reaction (12) and (13). HC(O)OCF2CHCl2 + Cl → HC(O)OCF2CCl2 + HCl
(12)
CH3OCF2C(O)Cl + Cl → CH2OCF2C(O)Cl + HCl
(13)
5
Reactions of the alkyl radicals generated in reaction (12) and (13) would then proceed by a mechanism analogous to reactions (6)-(11), producing carbonyl products, including a product with two carbonyl groups: HC(O)OCF2C(O)Cl. HC(O)OCF2CCl2 + O2/NO/RO2/M → HC(O)OCF2CCl2O + NO2/RO/M (14) CH2OCF2C(O)Cl + O2/NO/RO2/M → OCH2OCF2C(O)Cl + NO2/RO/M (15) HC(O)OCF2CCl2O
∆
HC(O)OCF2C(O)Cl + Cl
OCH2OCF2C(O)Cl + O2 + M→ H(O)COCF2C(O)Cl + HO2 + M
(16) (17)
The possible secondary carbonyl products from reaction (16) and (17) would also be reactive towards Cl atoms and degrade to form CO2, COF2 and C(O)Cl, e.g., reaction (18-22). HC(O)OCF2C(O)Cl + Cl → C(O)OCF2C(O)Cl + HCl C(O)OCF2C(O)Cl
∆
CF2C(O)Cl + CO2
(18) (19)
CF2C(O)Cl + O2 → OOCF2C(O)Cl
(20)
OOCF2C(O)Cl + NO/RO2 → OCF2C(O)Cl + O2
(21)
OCF2C(O)Cl → COF2 + C(O)Cl
(22)
Formation of COF2 and COCl2 is also possible though decomposition HC(O)OCF2CCl2O (generated in reaction (14) yielding COCl2 and an initial radical fragment, HC(O)OCF2: HC(O)OCF2CCl2O
∆
HC(O)OCF2 + COCl2
(23)
HC(O)OCF2 will react further with O2/NO/RO2 followed by decomposition to yield COF2 and CO2. Figure 4 displays the IR absorption bands of the primary product (Panel A) and of the secondary products stripped for H2O, COCl2, COF2 and COFCl (Panel B). Calculated spectra for HC(O)OCF2CHCl2 and HC(O)OCF2C(O)Cl are shown in Panels C and D, respectively. The presence of two bands in the carbonyl stretch region for the perfluoro alkyl formates is caused by syn- and anticonformations. Ab initio calculations have shown that the low- and high-frequency bands can be assigned to the syn- and anti-conformers, respectively. Both HC(O)OCF2CHCl2 and HC(O)OCF2C(O)Cl have free rotation around all single bonds, causing multiple conformers, which explains the broad bands and the multiple peaks as illustrated by the calculated spectra in Panel C and
6
D. These two different conformers of the same molecule were utilized for the optimization and frequency calculations. We believe that the primary product spectrum, shown in Panel A of Figure 4, is due to a single molecule: HC(O)OCF2CHCl2, and the secondary product spectrum, shown in Panel B of Figure 4, is due to a single, secondary, by-carbonyl product: HC(O)OCF2C(O)Cl. No significant formation of CH3OCF2C(O)Cl was observed, suggesting that reaction (11a) is likely of minor importance. This does not completely rule out reaction (5b) in being of some importance if any CH3OCF2COCl2 formed in reactions (5b)-(9) actually undergo C-C bond cleavage through reaction (11b) to yield COCl2, COF2 and HCOH in small primary yields (1-2%). If reaction (10b) is important and occurs in competition with reaction (10a), there should be a significant, observable O2 dependence on the yield of HC(O)OCF2CHCl2 and of COF2 and COCl2. No apparent O2 dependence was observed. In contrast, reactions (11a) and (11b) do not involve an oxygen dependent competition. Based on the initial, and O2 partial pressure independent yield of COF2 and COCl2, both 2 ± 1%, we arrive at an upper limit for the reaction of reaction (5b) of 2 %. We do not have a genuine sample of the primary product, HC(O)OCF2CHCl2. Based on the discussion above, the initial yield of COF2 and COCl2 of both 2 ± 1%, and the absence of any other primary product IR absorption bands, we equate the HC(O)OCF2CHCl2 yield to ~98% (estimated base e cross section at 1783 cm-1 of 1.3× 10-18 cm2 molecule-1). It is possible that HOCH2OCF2CHCl2 could be formed in peroxy radical self-reactions, though no absorption features (O-H stretch) could be confirmed in the product spectra. Fluorinated alcohols are in general very reactive towards Cl atoms with rate constants of the order of 10-11 – 1012
cm3 molecule-1 s-1 so that if HOCH2OCF2CHCl2 was formed, it would react with Cl atoms in the
chamber and be converted into HC(O)OCF2CHCl2. As Figure 2 Panel A shows significant loss of HC(O)OCF2CHCl2 via secondary reactions at high (> 60 %) consumptions of CH3OCF2CHCl2, we can use its concentration profile to derive information on its reactivity, relative to that of CH3OCF2CHCl2. The concentration profile can be described by the expression (I)[12]:
( 𝛼(1 ― 𝑥) (1 ― 𝑥) [HC(O)OCF2CHCl2 ]𝑡 [CH3OCF2CHCl2]𝑡
{
=
)
𝑘12 ―1 𝑘5
1 ― (𝑘12/𝑘5)
}
(I)
where x = 1- ([CH3OCF2CHCl2]t/[ CH3OCF2CHCl2]to) is the fractional consumption of CH3OCF2CHCl2, and is the yield of HC(O)OCF2CHCl2 (fixed at 0.98) from reaction of Cl atoms with CH3OCF2CHCl2 (reaction (5a)). The nonlinear fits to the HC(O)OCF2CHCl2 data shown in Figure 2 Panel A (700 Torr air and 700 Torr O2), is a fit to expression (I) yielding and k12/k5 =
7
(0.069 ± 0.012). Using k5 = (5.13 ± 0.66) × 10-13 from Hass et al.[4] to put k12 on an absolute scale gives k12= (3.54 ± 0.77) × 10-14 cm3 molecule-1 s-1. The value for k12 determined in this work appears in good agreement with expectations based on the rate constant for the reaction of Cl atoms with similar compounds, e.g., k(Cl + HC(O)OCHCF2) = (2.00 ± 0.17) × 10-14 cm3 molecule-1 s-1 reported by Wallington et al.[13] Although formates are moderately reactive species, HC(O)OCF2CHCl2 is a factor of 14 less reactive towards Cl atoms than CH3OCF2CHCl2. All other identified products show significant positive curvature in their yield plots for high levels
of
consumption
of
CH3OCF2CHCl2.
The
dominant
secondary
product,
identified
as
HC(O)OCF2C(O)Cl, is also formed with no observable oxygen dependency and appears only to be moderately reactive towards Cl atoms. The solid trace in Figure 3, Panel B shows the difference between the observed and the calculated initial yield ( of HC(O)OCF2CHCl2, scaled with an (arbitrary) factor of 0.15. The excellent agreement between the solid trace and the stars in Panel B strongly suggests that HC(O)OCF2C(O)Cl indeed is a direct oxidation product of HC(O)OCF2CHCl2 If we assume that HC(O)OCF2C(O)Cl is the sole oxidation product of HC(O)OCF2CHCl2, we estimate the absorption cross section (base e) for HC(O)OCF2C(O)Cl at 1863 cm-1 at approximately 5 × 10-19 cm2 molecule-1. The secondary formation of COF2 and COCl2 can have multiple sources, e.g., the HC(O)OCF2 alkyl radical formed in reaction (23) can degrade further forming COF2 and HCOH. Carbon monoxide, CO, which, as discussed above, was observed in the product spectra, can be formed by photolysis of formaldehyde, HCOH, in reactions (24a/24b)-(25). Threshold photolysis energies given by Ernest et al. [10] for reaction (24a) and (24b) are 360.75 nm and 329.72 nm, respectively. HCOH + hν → H2 + CO
(24a)
HCOH + hν → HCO + H
(24b)
HCO + O2 → HO2 + CO
(25)
Formaldehyde was not observed in the product spectra (see Figures 1 and 2). In addition to the photolysis processes mentioned above, HCOH is a factor of 140 times more reactive towards Cl atoms than CH3OCF2CHCl2 (k(Cl+HCOH) = 7 ×10-11 cm3 molecule-1 s-1) [11] rendering detection of any HCOH formed in our system difficult.
4. Atmospheric Impact
8
The atmospheric life time of methoxyflurane, CH3OCF2CHCl2, has previously been determined by Hass et al.[4] as ~54 days, who also reported a 100-year global warming potential of 4. Although the kinetics of the reactions of OH radicals and Cl atoms with CH3OCF2CHCl2 differs significantly, the products, obtained in the present work using Cl atoms as the oxidant, will likely resemble those obtained in the reaction with OH radicals. The experiments conducted in the present work show that the reaction of Cl atoms with CH3OCF2CHCl2 in air and O2 diluent forms one major primary product, which we assign as HC(O)OCF2CHCl2 (~98%), followed by the formation of one major secondary product HC(O)OCF2C(O)Cl. Based on our observations, hydrogen abstraction at the chlorine substituted carbon site, -CHCl2, is believed to be of minor importance (upper limit of 2%) for CH3OCF2CHCl2. Alkoxy radicals formed in the reaction of peroxy radicals with NO are produced with significant chemical activation whereas alkoxy radicals formed by the self-reaction of peroxy radicals are formed with little, or no, activation. Some chemically activated alkoxy radicals are known to undergo prompt decomposition [14]. The atmospheric fate of the alkoxy radicals formed in reactions (8)-(9) and (14)-(15) may therefore be different from that observe in the present study in the absence of NOx. Besides the two major carbonyl containing products mentioned above, several other smaller carbonyl products were observed, including COF2, COCl2 and CO, where the CO is believed to originate from the photolysis of formaldehyde. The simplest mechanism explaining our observations for the oxidation of methoxyflurane is shown in Figure 5 (excluding secondary photolysis of HCOH). The smaller oxidation products of methoxyflurane; HCOH, COF2 and COCl2, all have rapid sinks in the Earth’s atmosphere. Formaldehyde is a short lived gas in the atmosphere with a lifetime of only a few hours due to photolysis [15]. The atmospheric fate of the oxidation products COF2 and COCl2 is uptake into rain, cloud and ocean water on a timescale of 10-15 days [16–19].
9
Figure 1: Infrared Spectra obtained for the product study in air. Panel A: the starting material methoxyflurane (CH3OCF2CHCl2), Panel B: Primary products formed after 2 min of irradiation with methoxyflurane subtracted, Panel C: Secondary products formed after 25 min of irradiation (all the initial CH3OCF2CHCl2 has been consumed).
10
Figure 2: Infrared spectra obtained for the product formations in the carbonyl region. Panel A: Primary product after 2 min irradiation. Panel B: Secondary products after 25 min irradiation, and Panel C: Reference spectra of small carbonyl compounds: COCl2, COFCl and COF2.
11
Figure 3: Product formation in the Cl atom reactions versus methoxyflurane consumption in air (black) and O2 (white) diluent. Panel A: Primary product (1783 cm-1, hexagons). Panel B: Secondary product (identified at 938 cm-1 and 1853-64 cm-1, stars), and Panel C: Smaller carbonyl products: COF2 (1944 cm-1, upward triangles) and COCl2 (850 cm-1, circles). Solid traces (Panel A) are analytical fits to the data (see main text for details). The dashed traces (Panel C) are a simple 3parameter log-exponent fit to the data to aid the eye in observing the trends.
12
Figure 4: IR spectra obtained in the product study compared to calculated spectra for possible products. Panel A: Primary products obtained after 2 min of irradiation, Panel B: Secondary products obtained after 25 min irradiation with the small carbonyl products subtracted, Panel C: calculated spectra for two possible conformers of HC(O)OCF2CHCl2, Panel D: calculated spectra for two possible conformers of HC(O)OCF2C(O)Cl. The calculated spectra are obtained at the B3LYP/6311G(d) level of theory.
13
Figure 5: Suggested mechanism for the Cl atom-initiated oxidation. Boxes indicate identified products.
14
References [1]
W.T. Tsai, Environmental risk assessment of hydrofluoroethers (HFEs), J Hazard. Mater. 119 (2005) 69–78. doi:10.1016/j.jhazmat.2004.12.018.
[2]
V.L. Orkin, R.E. Huie, M.J. Kurylo, Atmospheric Lifetimes of HFC-143a and HFC-245fa: Flash Photolysis Resonance Fluorescence Measurements of the OH Reaction Rate Constants, J. Phys. Chem. 100 (1996) 8907–8912. doi:10.1021/jp9601882.
[3]
L.K. Christensen, T.J. Wallington, A. Guschin, M.D. Hurley, Atmospheric Degradation Mechanism of CF3OCH3, J. Phys. Chem. A. 103 (2002) 4202–4208. doi:10.1021/jp984455a.
[4]
S.A. Hass, S.T. Andersen, M.P. Sulbaek Andersen, O.J. Nielsen, Atmospheric Chemistry of Methoxyflurane (CH3OCF2CHCl2): Kinetics of the Gas-Phase Reactions with OH Radicals, Cl Atoms and O3, Chem. Phys. Lett. 722 (2019) 119–123. doi:10.1016/j.cplett.2019.02.041.
[5]
O.J. Nielsen, H.W. Sidebottom, M. Donlon, J. Treacy, Rate Constants for the Gas-Phase Reactions of OH Radicals and Cl Atoms with n-Alkyl Nitrites at Atmospheric Pressure and 298 K, Int. J. Chem. Kinet. 23 (1991) 1095–1109. doi:10.1002/chin.199209099.
[6]
E.J.K. Nilsson, C. Eskebjerg, M.S. Johnson, A Photochemical Reactor for Studies of Atmospheric
Chemistry,
Atmos.
Environ.
43
(2009)
3029–3033.
doi:10.1016/j.atmosenv.2009.02.034. [7]
W.D. Taylor, T.D. Allston, M.J. Moscato, G.B. Fazekas, R. Kozlowski, G.A. Takacs, Atmospheric Photodissociation Lifetimes for Nitromethane, Methyl Nitrite, and Methyl Nitrate, Int. J. Chem. Kinet. 12 (1980) 231–240. doi:10.1002/kin.550120404.
[8]
M.J. Frisch, Q.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyrev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J. V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, (2013).
15
[9]
Vibrational Frequency Scaling Factors, Computational Chemistry Comparison and Benchmark
Database
(CCCDCB),
Natl.
Institue
Stand.
Technol.
(n.d.).
https://cccbdb.nist.gov/vibscale2.asp?method=8&basis=7. [10]
C.T. Ernest, D. Bauer, A.J. Hynes, Radical Quantum Yields from Formaldehyde Photolysis in the 30400-32890cm-1 (304-329 nm) Spectral Region: Detection of Radical Photoproducts Using Pulsed Laser Photolysis - Pulsed Laser Induced Fluorescence, J. Phys. Chem. A. 116 (2012) 6983–6995. doi:10.1021/jp2117399.
[11]
J.G. Calvert, A. Mellouki, J.J. Orlando, M.J. Pilling, T.J. Wallington, The Mechanisms of Atmospheric Oxidation of the Oxygenates, Oxford University Press, 2011.
[12]
R.J. Meagher, M.E. McIntosh, M.D. Hurley, T.J. Wallington, A Kinetic Study of the Reaction of Chlorine and Fluorine Atoms with HC(O)F at 295 ± 2 K, Int. J. Chem. Kinet. 29 (1997) 619–625. doi:10.1002/chin.199401097.
[13]
T.J. Wallington, M.D. Hurley, V. Fedotov, C. Morrell, G. Hancock, Atmospheric chemistry of CF3CH2OCHF2 and CF3CHClOCHF2: Kinetics and mechanisms of reaction with Cl atoms and OH radicals and atmospheric fate of CF3C(O·)HOCHF2 and CF3C(O·)ClOCHF2 radicals, J. Phys. Chem. A. 106 (2002) 8391–8398. doi:10.1021/jp020017z.
[14]
T.J. Wallington, M.D. Hurley, J.J. Orlando, G.S. Tyndall, J. Sehested, T.E. Møgelberg, O.J. Nielsen, Role of Excited CF3CFHO Radicals in Atmospheric Chemistry of HFC-134a, J. Phys. Chem. 100 (1996) 18116–18122.
[15]
N.B. Jones, K. Riedel, W. Allan, S. Wood, P.I. Palmer, K. Chance, J. Notholt, Long-term tropospheric formaldehyde concentrations deduced from ground-based fourier transform solar infrared measurements, Atmos. Chem. Phys. 9 (2009) 7131–7142. doi:10.5194/acp-9-71312009.
[16]
D. Fu, C.D. Boone, P.F. Bernath, K.A. Walker, R. Nassar, G.L. Manney, S.D. McLeod, Global phosgene observations from the Atmospheric Chemistry Experiment (ACE) mission, Geophys. Res. Lett. 34 (2007) 2001–2005. doi:10.1029/2007GL029942.
[17]
H.B.
Singh,
Phosgene
in
the
Ambient
Air,
Nature.
264
(1976)
619–621.
doi:10.1038/373357a0. [18]
J.J. Harrison, M.P. Chipperfield, A. Dudhia, S. Cai, S. Dhomse, C.D. Boone, P.F. Bernath, Satellite observations of stratospheric carbonyl fluoride, Atmos. Chem. Phys. 14 (2014) 11915–11933. doi:10.5194/acp-14-11915-2014.
16
[19]
R. Zander, C.P. Rinsland, E. Mahieu, M.R. Gunson, C.B. Farmer, M.C. Abrams, M.K.W. Ko, Increase of Carbonyl Fluoride (COF2) in the Stratosphere and Its Contribution to the 1992 Budget of Inorganic Fluorine in the Upper Stratosphere, J. Geophys. Res. - Atmos. 99 (1994) 16737–16743. doi:10.1016/j.socscimed.2017.04.050.
17
Graphical abstract
18
Highlights
First study of the atmospheric oxidation mechanism for methoxyflurane (CH3OCF2CHCl2) Methoxyflurane yields one major primary oxidation product, HC(O)OCF2CHCl2 Secondary products include HC(O)OCF2C(O)Cl, COF2 and COCl2
19
Author credit statement
S. A. Hass: Investigation, Writing- Original draft preparation M. P. Sulbaek Andersen: Investigation, Writing- Reviewing and Editing, Supervision O. J. Nielsen: Conceptualization, Resources, Supervision
20
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
21
S0009-2614(19)31033-4 https://doi.org/10.1016/j.cplett.2019.137052 CPLETT 137052
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
7 August 2019 18 December 2019 20 December 2019
Please cite this article as: S.A. Hass, M.P. Sulbaek Andersen, O.J. Nielsen, Atmospheric Chemistry of Methoxyflurane (CH3OCF2CHCl2): Products and Mechanisms, Chemical Physics Letters (2019), doi: https:// doi.org/10.1016/j.cplett.2019.137052
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Atmospheric Chemistry of Methoxyflurane (CH3OCF2CHCl2): Products and Mechanisms
S. A. Hassa, M. P. Sulbaek Andersena,b,*, and O. J. Nielsena
a
Copenhagen Center for Atmospheric Research, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark
b
Department of Chemistry and Biochemistry, California State University Northridge, Northridge, CA, 91330-8262
*Corresponding Author: M. P. Sulbaek Andersen ([email protected])
Abstract Long pathlength/FTIR smog chamber techniques were used to determine the Cl atom initiated atmospheric oxidation mechanisms for methoxyflurane (CH3OCF2CHCl2) in 700 Torr of air or O2 diluent at 296K. The oxidation of CH3OCF2CHCl2 in both air and O2 diluent gave one primary carbonyl-containing product in approximately 100% yield and one secondary bi-carbonyl product in addition to COF2 and COCl2, the latter two both with initial molar yields of 2 ± 1%. A mechanism is proposed for the oxidation of methoxyflurane which explains the observed product distribution. IR spectra for HC(O)OCF2CHCl2 and HC(O)OCF2C(O)Cl are calculated and reported.
1
1. Introduction Halogenated ethers have been widely used worldwide for decades in various industry sectors including as inhalational anesthetics involving compounds such as sevoflurane (CH2FOCH(CF3)2), isoflurane (CH2FOCHClCF3), desflurane (CHF2OCHFCF3), enflurane (CHFClCF2OCF2H) and methoxyflurane (CH3OCF2CHCl2). In comparison to hydrofluorocarbons (HFCs), halogenated ethers generally have lower atmospheric lifetimes and Global Warming Potentials (GWPs) due to the etherlinkage [1], e.g. HFC-143a (CF3CH3) has a lifetime of ~51 years [2] whereas HFE-143 (CF3OCH3) has a lifetime of ~4.7 years [3]. In a recent study [4], OH, Cl and O3 rate constants for methoxyflurane were determined as: k(CH3OCF2CHCl2 + OH) = (2.18 ± 0.38) × 10-13, k(CH3OCF2CHCl2 + Cl) = (5.13 ± 0.66) × 10-13 and k(CH3OCF2CHCl2 + O3) < 1.5 × 10-22 cm3 molecule-1 s-1 in 700 Torr of N2/air diluent, providing the first atmospheric lifetime estimate for CH3OCF2CHCl2 of ~54 days and a 100-year GWP of 4. The OH radical reaction is likely to be the dominant sink of CH3OCF2CHCl2 in the atmosphere. In the present study, FTIR smog chamber techniques were used to determine the products formed in the atmospheric oxidation of CH3OCF2CHCl2. Even though the OH radical is the dominant oxidative species in the atmosphere, Cl atoms is used in this study due to complications employing the OH radical precursor used in our laboratory: CH3ONO. Three complications arise in our experiments using CH3ONO as the OH radical precursor. Firstly, the rate constant for the reaction of OH radicals with methyl nitrite [5], (3.0 ± 1.0) × 10-13 cm3 molecule-1 s-1, is similar to that of methoxyflurane [4], (2.18 ± 0.38) × 10-13 cm3 molecule-1 s-1, thus the consumption of CH3OCF2CHCl2 is effectively limited by the reaction of the OH radicals with CH3ONO. Secondly, as the products formed from the reaction of OH radicals with methyl nitrite display IR absorption bands overlapping with those from the expected methoxyflurane-OH reaction products, positive identification of the latter is compromised. Finally, because the spectral bands from methyl nitrite (i.e., photolysis product of CH3ONO) quickly become oversaturated, these regions are rendered unusable to us for identification of possible oxidation products from CH3OCF2CHCl2. The Cl atom and OH initiated oxidation mechanisms are expected to be similar, with the caveat, that the relative reactivity of the two types of H-atoms in the methoxy group and the CHCl2 group in CH3OCF2CHCl2 could be slightly different in the Cl atom versus OH radical mediated abstraction. As part of this work, theoretical frequency calculations were performed for compounds where experimental IR spectra could not be found.
2
2. Methods Experiments were conducted using the photoreactor at the Copenhagen Center for Atmospheric Research, at University of Copenhagen. The photoreactor consists of a 101 L quartz cylinder interfaced with a Bruker IFS 66v/s FTIR spectrometer. Reactions were initiated using Waldmann F85/100W UV6 lamps (wavelength region at 280-360 nm) surrounding the reactor. The concentrations of reactants and products were monitored using an analytical pathlength of 47.73 m (internal White-cell optics). For further details on the experimental setup see Nilsson et al.[6]. All experiments were performed in 700 Torr of air or O2 diluent at 296 ± 2K. Cl atoms were produced by the photolysis of Cl2, as seen in reaction (1). Cl2 + hν → 2 Cl
(1)
OH radicals were produced by photolysis of methyl nitrite, CH3ONO, in the presence of NO: CH3ONO + hν → CH3O + NO
(2)
CH3O + O2 → CH2O + HO2
(3)
HO2 + NO → OH + NO2
(4)
All reagents used in the experiments, except methyl nitrite, were obtained from commercial sources with purities >97%. Methyl nitrite was prepared by dropwise addition of sulfuric acid, H2SO4, to a saturated solution of sodium nitrite, NaNO2, in methanol and water [7]. The methyl nitrite samples prepared were devoid of any detectable impurities by analysis with FTIR. Theoretical IR-spectra of potential oxidation products were calculated using the Gaussian 09 program[8]. The vibrational frequencies were calculated at the B3LYP/6-311G(d) level of theory and multiplied by a scaling factor of 0.966 [9] to better match experimental frequencies. It should be noted that the calculations do not include the rotational structures of the absorption bands, there is therefore the possibility that a single molecule can expand over a larger area of wavenumbers. The calculated spectra are only used as an aid in interpreting the experimental product spectra. The peak positions are not expected to exactly match the experimental values but are utilized to show approximate absorption band positions.
3
3. Results and Discussion 3.1 Cl Atom Product Study: The Fate of CH2OCF2CHCl2 To investigate the mechanism of Cl atom-initiated oxidation of CH3OCF2CHCl2, mixtures containing 7.32-7.39 mTorr CH3OCF2CHCl2 and approximately 40 mTorr Cl2 in 700 Torr of air or O2 diluent were introduced into the reaction chamber and irradiated with UV black lamps. All experiments were conducted at a temperature of 296 ± 2 K. Figure 1 shows spectra acquired before (Panel A), after 2 minutes (Panel B) and after 25 minutes (Panel C) of UV irradiation of the gas mixture in 700 Torr of air. The experiments revealed the formation of multiple products with carbonyl bands in the range 1760-1850 cm-1 and a band with an approximate center at 1944 cm-1. The two product spectra after 2 minutes and 25 minutes of irradiation (Panels B and C) predominantly show the primary and secondary products, respectively. Closer inspection of the carbonyl region, 1700-2000 cm-1, reveals several possible products formed in the oxidation. Unfortunately, the product bands below 1300 cm-1 are difficult to differentiate. Figure 2 shows an expanded view of the carbonyl region for the primary and secondary products and the 3 smaller carbonyl products readily identified: COCl2, COF2 and COFCl (Panel C). As the experiments have been performed in the presence of O2, it can be expected that most of the products will contain one or more carbonyl groups. As no fluorine or chlorine atoms are attached to a single carbon atom in methoxyflurane, we believe that the COFCl (in Panel C) is formed due to uncharacterized surface reactions on the reaction chamber walls and is therefore not a direct product of the gas phase oxidation reaction of CH3OCF2CHCl2. To determine the yields and the reactivates of the products, the formation of products following UV radiation of the gas mixtures versus the loss of CH3OCF2CHCl2 normalized to the initial CH3OCF2CHCl2 concentration is shown in Figure 3. Multiple trends are visible in Figure 3: A single primary product carbonyl is formed with a distinct feature at 1783 cm-1 (Figure 3, Panel A), which, upon further reaction, appear to yield a secondary carbonyl product with IR bands at 1853-64 cm-1 (Panel B). Phosgene (COCl2) is formed together with carbonyl fluoride (COF2) already in early stages of the experiments (Figure 3, Panel C). Carbon monoxide, CO, also appear as a primary product. Unfortunately, neither CO or CO2 are good markers in our system, and while CO appeared to be produced as a primary product (~6%), a significant initial presence after just 5 seconds of irradiation suggests an additional, and perhaps heterogenous sources. We suggest that CO may be a primary product, but due to background surface reactions in the chamber the initial yield is an upper limit.
4
The reaction mechanism is expected to proceed through hydrogen abstraction, possibly at two different positions: at the methoxy group, CH3O-, and at the chlorine substituted carbon, -CHCl2. CH3OCF2CHCl2 + Cl → CH2OCF2CHCl2 + HCl → CH3OCF2CCl2 + HCl
(5a) (5b)
The two radicals generated in reaction (5) will first add molecular oxygen, reaction (6) and (7), and then undergo reaction with other peroxy radicals in the photoreactor (or NO in the atmosphere) to form alkoxy radicals, reaction (8) and (9). CH2OCF2CHCl2 + O2 + M → OOCH2OCF2CHCl2 + M
(6)
CH3OCF2CCl2 + O2 + M → CH3OCF2C(OO)Cl2 + M
(7)
OOCH2OCF2CHCl2 + NO/RO2 → OCH2OCF2CHCl2 + NO2/RO + O2
(8)
CH3OCF2C(OO)Cl2 + NO/RO2 → CH3OCF2COCl2 + NO2/RO + O2
(9)
The alkoxy radicals formed in reactions (8) and (9) can then produce two possible primary carbonyl products as shown in reaction (10) through reaction of OCH2OCF2CHCl2 with O2 or in reaction (11) with thermal decomposition of CH3OCF2COCl2. OCH2OCF2CHCl2 + O2 + M→ HC(O)OCF2CHCl2 + HO2 + M OCH2CF2CHCl2 CH3OCF2COCl2 CH3OCF2COCl2
∆
∆
∆
(10a)
HCOH + CF2CHCl2
(10b)
CH3OCF2C(O)Cl + Cl
(11a)
CH3OCF2 + COCl2
(11b)
The radical fragments, CF2CHCl2 and CH3OCF2, would rapidly undergo further reaction with O2/NO/RO2 to yield COF2 and COCl2, and COF2 and HCOH, respectively. If formed, the carbonyl products generated in reaction (10) and (11) would be reactive towards Cl atoms and could undergo hydrogen abstraction as shown in reaction (12) and (13). HC(O)OCF2CHCl2 + Cl → HC(O)OCF2CCl2 + HCl
(12)
CH3OCF2C(O)Cl + Cl → CH2OCF2C(O)Cl + HCl
(13)
5
Reactions of the alkyl radicals generated in reaction (12) and (13) would then proceed by a mechanism analogous to reactions (6)-(11), producing carbonyl products, including a product with two carbonyl groups: HC(O)OCF2C(O)Cl. HC(O)OCF2CCl2 + O2/NO/RO2/M → HC(O)OCF2CCl2O + NO2/RO/M (14) CH2OCF2C(O)Cl + O2/NO/RO2/M → OCH2OCF2C(O)Cl + NO2/RO/M (15) HC(O)OCF2CCl2O
∆
HC(O)OCF2C(O)Cl + Cl
OCH2OCF2C(O)Cl + O2 + M→ H(O)COCF2C(O)Cl + HO2 + M
(16) (17)
The possible secondary carbonyl products from reaction (16) and (17) would also be reactive towards Cl atoms and degrade to form CO2, COF2 and C(O)Cl, e.g., reaction (18-22). HC(O)OCF2C(O)Cl + Cl → C(O)OCF2C(O)Cl + HCl C(O)OCF2C(O)Cl
∆
CF2C(O)Cl + CO2
(18) (19)
CF2C(O)Cl + O2 → OOCF2C(O)Cl
(20)
OOCF2C(O)Cl + NO/RO2 → OCF2C(O)Cl + O2
(21)
OCF2C(O)Cl → COF2 + C(O)Cl
(22)
Formation of COF2 and COCl2 is also possible though decomposition HC(O)OCF2CCl2O (generated in reaction (14) yielding COCl2 and an initial radical fragment, HC(O)OCF2: HC(O)OCF2CCl2O
∆
HC(O)OCF2 + COCl2
(23)
HC(O)OCF2 will react further with O2/NO/RO2 followed by decomposition to yield COF2 and CO2. Figure 4 displays the IR absorption bands of the primary product (Panel A) and of the secondary products stripped for H2O, COCl2, COF2 and COFCl (Panel B). Calculated spectra for HC(O)OCF2CHCl2 and HC(O)OCF2C(O)Cl are shown in Panels C and D, respectively. The presence of two bands in the carbonyl stretch region for the perfluoro alkyl formates is caused by syn- and anticonformations. Ab initio calculations have shown that the low- and high-frequency bands can be assigned to the syn- and anti-conformers, respectively. Both HC(O)OCF2CHCl2 and HC(O)OCF2C(O)Cl have free rotation around all single bonds, causing multiple conformers, which explains the broad bands and the multiple peaks as illustrated by the calculated spectra in Panel C and
6
D. These two different conformers of the same molecule were utilized for the optimization and frequency calculations. We believe that the primary product spectrum, shown in Panel A of Figure 4, is due to a single molecule: HC(O)OCF2CHCl2, and the secondary product spectrum, shown in Panel B of Figure 4, is due to a single, secondary, by-carbonyl product: HC(O)OCF2C(O)Cl. No significant formation of CH3OCF2C(O)Cl was observed, suggesting that reaction (11a) is likely of minor importance. This does not completely rule out reaction (5b) in being of some importance if any CH3OCF2COCl2 formed in reactions (5b)-(9) actually undergo C-C bond cleavage through reaction (11b) to yield COCl2, COF2 and HCOH in small primary yields (1-2%). If reaction (10b) is important and occurs in competition with reaction (10a), there should be a significant, observable O2 dependence on the yield of HC(O)OCF2CHCl2 and of COF2 and COCl2. No apparent O2 dependence was observed. In contrast, reactions (11a) and (11b) do not involve an oxygen dependent competition. Based on the initial, and O2 partial pressure independent yield of COF2 and COCl2, both 2 ± 1%, we arrive at an upper limit for the reaction of reaction (5b) of 2 %. We do not have a genuine sample of the primary product, HC(O)OCF2CHCl2. Based on the discussion above, the initial yield of COF2 and COCl2 of both 2 ± 1%, and the absence of any other primary product IR absorption bands, we equate the HC(O)OCF2CHCl2 yield to ~98% (estimated base e cross section at 1783 cm-1 of 1.3× 10-18 cm2 molecule-1). It is possible that HOCH2OCF2CHCl2 could be formed in peroxy radical self-reactions, though no absorption features (O-H stretch) could be confirmed in the product spectra. Fluorinated alcohols are in general very reactive towards Cl atoms with rate constants of the order of 10-11 – 1012
cm3 molecule-1 s-1 so that if HOCH2OCF2CHCl2 was formed, it would react with Cl atoms in the
chamber and be converted into HC(O)OCF2CHCl2. As Figure 2 Panel A shows significant loss of HC(O)OCF2CHCl2 via secondary reactions at high (> 60 %) consumptions of CH3OCF2CHCl2, we can use its concentration profile to derive information on its reactivity, relative to that of CH3OCF2CHCl2. The concentration profile can be described by the expression (I)[12]:
( 𝛼(1 ― 𝑥) (1 ― 𝑥) [HC(O)OCF2CHCl2 ]𝑡 [CH3OCF2CHCl2]𝑡
{
=
)
𝑘12 ―1 𝑘5
1 ― (𝑘12/𝑘5)
}
(I)
where x = 1- ([CH3OCF2CHCl2]t/[ CH3OCF2CHCl2]to) is the fractional consumption of CH3OCF2CHCl2, and is the yield of HC(O)OCF2CHCl2 (fixed at 0.98) from reaction of Cl atoms with CH3OCF2CHCl2 (reaction (5a)). The nonlinear fits to the HC(O)OCF2CHCl2 data shown in Figure 2 Panel A (700 Torr air and 700 Torr O2), is a fit to expression (I) yielding and k12/k5 =
7
(0.069 ± 0.012). Using k5 = (5.13 ± 0.66) × 10-13 from Hass et al.[4] to put k12 on an absolute scale gives k12= (3.54 ± 0.77) × 10-14 cm3 molecule-1 s-1. The value for k12 determined in this work appears in good agreement with expectations based on the rate constant for the reaction of Cl atoms with similar compounds, e.g., k(Cl + HC(O)OCHCF2) = (2.00 ± 0.17) × 10-14 cm3 molecule-1 s-1 reported by Wallington et al.[13] Although formates are moderately reactive species, HC(O)OCF2CHCl2 is a factor of 14 less reactive towards Cl atoms than CH3OCF2CHCl2. All other identified products show significant positive curvature in their yield plots for high levels
of
consumption
of
CH3OCF2CHCl2.
The
dominant
secondary
product,
identified
as
HC(O)OCF2C(O)Cl, is also formed with no observable oxygen dependency and appears only to be moderately reactive towards Cl atoms. The solid trace in Figure 3, Panel B shows the difference between the observed and the calculated initial yield ( of HC(O)OCF2CHCl2, scaled with an (arbitrary) factor of 0.15. The excellent agreement between the solid trace and the stars in Panel B strongly suggests that HC(O)OCF2C(O)Cl indeed is a direct oxidation product of HC(O)OCF2CHCl2 If we assume that HC(O)OCF2C(O)Cl is the sole oxidation product of HC(O)OCF2CHCl2, we estimate the absorption cross section (base e) for HC(O)OCF2C(O)Cl at 1863 cm-1 at approximately 5 × 10-19 cm2 molecule-1. The secondary formation of COF2 and COCl2 can have multiple sources, e.g., the HC(O)OCF2 alkyl radical formed in reaction (23) can degrade further forming COF2 and HCOH. Carbon monoxide, CO, which, as discussed above, was observed in the product spectra, can be formed by photolysis of formaldehyde, HCOH, in reactions (24a/24b)-(25). Threshold photolysis energies given by Ernest et al. [10] for reaction (24a) and (24b) are 360.75 nm and 329.72 nm, respectively. HCOH + hν → H2 + CO
(24a)
HCOH + hν → HCO + H
(24b)
HCO + O2 → HO2 + CO
(25)
Formaldehyde was not observed in the product spectra (see Figures 1 and 2). In addition to the photolysis processes mentioned above, HCOH is a factor of 140 times more reactive towards Cl atoms than CH3OCF2CHCl2 (k(Cl+HCOH) = 7 ×10-11 cm3 molecule-1 s-1) [11] rendering detection of any HCOH formed in our system difficult.
4. Atmospheric Impact
8
The atmospheric life time of methoxyflurane, CH3OCF2CHCl2, has previously been determined by Hass et al.[4] as ~54 days, who also reported a 100-year global warming potential of 4. Although the kinetics of the reactions of OH radicals and Cl atoms with CH3OCF2CHCl2 differs significantly, the products, obtained in the present work using Cl atoms as the oxidant, will likely resemble those obtained in the reaction with OH radicals. The experiments conducted in the present work show that the reaction of Cl atoms with CH3OCF2CHCl2 in air and O2 diluent forms one major primary product, which we assign as HC(O)OCF2CHCl2 (~98%), followed by the formation of one major secondary product HC(O)OCF2C(O)Cl. Based on our observations, hydrogen abstraction at the chlorine substituted carbon site, -CHCl2, is believed to be of minor importance (upper limit of 2%) for CH3OCF2CHCl2. Alkoxy radicals formed in the reaction of peroxy radicals with NO are produced with significant chemical activation whereas alkoxy radicals formed by the self-reaction of peroxy radicals are formed with little, or no, activation. Some chemically activated alkoxy radicals are known to undergo prompt decomposition [14]. The atmospheric fate of the alkoxy radicals formed in reactions (8)-(9) and (14)-(15) may therefore be different from that observe in the present study in the absence of NOx. Besides the two major carbonyl containing products mentioned above, several other smaller carbonyl products were observed, including COF2, COCl2 and CO, where the CO is believed to originate from the photolysis of formaldehyde. The simplest mechanism explaining our observations for the oxidation of methoxyflurane is shown in Figure 5 (excluding secondary photolysis of HCOH). The smaller oxidation products of methoxyflurane; HCOH, COF2 and COCl2, all have rapid sinks in the Earth’s atmosphere. Formaldehyde is a short lived gas in the atmosphere with a lifetime of only a few hours due to photolysis [15]. The atmospheric fate of the oxidation products COF2 and COCl2 is uptake into rain, cloud and ocean water on a timescale of 10-15 days [16–19].
9
Figure 1: Infrared Spectra obtained for the product study in air. Panel A: the starting material methoxyflurane (CH3OCF2CHCl2), Panel B: Primary products formed after 2 min of irradiation with methoxyflurane subtracted, Panel C: Secondary products formed after 25 min of irradiation (all the initial CH3OCF2CHCl2 has been consumed).
10
Figure 2: Infrared spectra obtained for the product formations in the carbonyl region. Panel A: Primary product after 2 min irradiation. Panel B: Secondary products after 25 min irradiation, and Panel C: Reference spectra of small carbonyl compounds: COCl2, COFCl and COF2.
11
Figure 3: Product formation in the Cl atom reactions versus methoxyflurane consumption in air (black) and O2 (white) diluent. Panel A: Primary product (1783 cm-1, hexagons). Panel B: Secondary product (identified at 938 cm-1 and 1853-64 cm-1, stars), and Panel C: Smaller carbonyl products: COF2 (1944 cm-1, upward triangles) and COCl2 (850 cm-1, circles). Solid traces (Panel A) are analytical fits to the data (see main text for details). The dashed traces (Panel C) are a simple 3parameter log-exponent fit to the data to aid the eye in observing the trends.
12
Figure 4: IR spectra obtained in the product study compared to calculated spectra for possible products. Panel A: Primary products obtained after 2 min of irradiation, Panel B: Secondary products obtained after 25 min irradiation with the small carbonyl products subtracted, Panel C: calculated spectra for two possible conformers of HC(O)OCF2CHCl2, Panel D: calculated spectra for two possible conformers of HC(O)OCF2C(O)Cl. The calculated spectra are obtained at the B3LYP/6311G(d) level of theory.
13
Figure 5: Suggested mechanism for the Cl atom-initiated oxidation. Boxes indicate identified products.
14
References [1]
W.T. Tsai, Environmental risk assessment of hydrofluoroethers (HFEs), J Hazard. Mater. 119 (2005) 69–78. doi:10.1016/j.jhazmat.2004.12.018.
[2]
V.L. Orkin, R.E. Huie, M.J. Kurylo, Atmospheric Lifetimes of HFC-143a and HFC-245fa: Flash Photolysis Resonance Fluorescence Measurements of the OH Reaction Rate Constants, J. Phys. Chem. 100 (1996) 8907–8912. doi:10.1021/jp9601882.
[3]
L.K. Christensen, T.J. Wallington, A. Guschin, M.D. Hurley, Atmospheric Degradation Mechanism of CF3OCH3, J. Phys. Chem. A. 103 (2002) 4202–4208. doi:10.1021/jp984455a.
[4]
S.A. Hass, S.T. Andersen, M.P. Sulbaek Andersen, O.J. Nielsen, Atmospheric Chemistry of Methoxyflurane (CH3OCF2CHCl2): Kinetics of the Gas-Phase Reactions with OH Radicals, Cl Atoms and O3, Chem. Phys. Lett. 722 (2019) 119–123. doi:10.1016/j.cplett.2019.02.041.
[5]
O.J. Nielsen, H.W. Sidebottom, M. Donlon, J. Treacy, Rate Constants for the Gas-Phase Reactions of OH Radicals and Cl Atoms with n-Alkyl Nitrites at Atmospheric Pressure and 298 K, Int. J. Chem. Kinet. 23 (1991) 1095–1109. doi:10.1002/chin.199209099.
[6]
E.J.K. Nilsson, C. Eskebjerg, M.S. Johnson, A Photochemical Reactor for Studies of Atmospheric
Chemistry,
Atmos.
Environ.
43
(2009)
3029–3033.
doi:10.1016/j.atmosenv.2009.02.034. [7]
W.D. Taylor, T.D. Allston, M.J. Moscato, G.B. Fazekas, R. Kozlowski, G.A. Takacs, Atmospheric Photodissociation Lifetimes for Nitromethane, Methyl Nitrite, and Methyl Nitrate, Int. J. Chem. Kinet. 12 (1980) 231–240. doi:10.1002/kin.550120404.
[8]
M.J. Frisch, Q.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyrev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J. V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, (2013).
15
[9]
Vibrational Frequency Scaling Factors, Computational Chemistry Comparison and Benchmark
Database
(CCCDCB),
Natl.
Institue
Stand.
Technol.
(n.d.).
https://cccbdb.nist.gov/vibscale2.asp?method=8&basis=7. [10]
C.T. Ernest, D. Bauer, A.J. Hynes, Radical Quantum Yields from Formaldehyde Photolysis in the 30400-32890cm-1 (304-329 nm) Spectral Region: Detection of Radical Photoproducts Using Pulsed Laser Photolysis - Pulsed Laser Induced Fluorescence, J. Phys. Chem. A. 116 (2012) 6983–6995. doi:10.1021/jp2117399.
[11]
J.G. Calvert, A. Mellouki, J.J. Orlando, M.J. Pilling, T.J. Wallington, The Mechanisms of Atmospheric Oxidation of the Oxygenates, Oxford University Press, 2011.
[12]
R.J. Meagher, M.E. McIntosh, M.D. Hurley, T.J. Wallington, A Kinetic Study of the Reaction of Chlorine and Fluorine Atoms with HC(O)F at 295 ± 2 K, Int. J. Chem. Kinet. 29 (1997) 619–625. doi:10.1002/chin.199401097.
[13]
T.J. Wallington, M.D. Hurley, V. Fedotov, C. Morrell, G. Hancock, Atmospheric chemistry of CF3CH2OCHF2 and CF3CHClOCHF2: Kinetics and mechanisms of reaction with Cl atoms and OH radicals and atmospheric fate of CF3C(O·)HOCHF2 and CF3C(O·)ClOCHF2 radicals, J. Phys. Chem. A. 106 (2002) 8391–8398. doi:10.1021/jp020017z.
[14]
T.J. Wallington, M.D. Hurley, J.J. Orlando, G.S. Tyndall, J. Sehested, T.E. Møgelberg, O.J. Nielsen, Role of Excited CF3CFHO Radicals in Atmospheric Chemistry of HFC-134a, J. Phys. Chem. 100 (1996) 18116–18122.
[15]
N.B. Jones, K. Riedel, W. Allan, S. Wood, P.I. Palmer, K. Chance, J. Notholt, Long-term tropospheric formaldehyde concentrations deduced from ground-based fourier transform solar infrared measurements, Atmos. Chem. Phys. 9 (2009) 7131–7142. doi:10.5194/acp-9-71312009.
[16]
D. Fu, C.D. Boone, P.F. Bernath, K.A. Walker, R. Nassar, G.L. Manney, S.D. McLeod, Global phosgene observations from the Atmospheric Chemistry Experiment (ACE) mission, Geophys. Res. Lett. 34 (2007) 2001–2005. doi:10.1029/2007GL029942.
[17]
H.B.
Singh,
Phosgene
in
the
Ambient
Air,
Nature.
264
(1976)
619–621.
doi:10.1038/373357a0. [18]
J.J. Harrison, M.P. Chipperfield, A. Dudhia, S. Cai, S. Dhomse, C.D. Boone, P.F. Bernath, Satellite observations of stratospheric carbonyl fluoride, Atmos. Chem. Phys. 14 (2014) 11915–11933. doi:10.5194/acp-14-11915-2014.
16
[19]
R. Zander, C.P. Rinsland, E. Mahieu, M.R. Gunson, C.B. Farmer, M.C. Abrams, M.K.W. Ko, Increase of Carbonyl Fluoride (COF2) in the Stratosphere and Its Contribution to the 1992 Budget of Inorganic Fluorine in the Upper Stratosphere, J. Geophys. Res. - Atmos. 99 (1994) 16737–16743. doi:10.1016/j.socscimed.2017.04.050.
17
Graphical abstract
18
Highlights
First study of the atmospheric oxidation mechanism for methoxyflurane (CH3OCF2CHCl2) Methoxyflurane yields one major primary oxidation product, HC(O)OCF2CHCl2 Secondary products include HC(O)OCF2C(O)Cl, COF2 and COCl2
19
Author credit statement
S. A. Hass: Investigation, Writing- Original draft preparation M. P. Sulbaek Andersen: Investigation, Writing- Reviewing and Editing, Supervision O. J. Nielsen: Conceptualization, Resources, Supervision
20
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
21