Atmospheric Chemistry of 1 H-Heptafluorocyclopentene (cyc

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Atmospheric Chemistry of 1H-Heptafluorocyclopentene (cycCFCFCFCF=CH–): Rate Constant, Products and Mechanism of Gas-Phase Reactions with OH Radicals, IR Absorption Spectrum, Photochemical Ozone Creation Potential, and Global Warming Potential 2

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Dongpeng Liu, Sheng Qin, Wei Li, Di Zhang, and Zhikai Guo J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b10348 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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The Journal of Physical Chemistry

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Atmospheric Chemistry of 1H-Heptafluorocyclopentene

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(cyc-CF2CF2CF2CF=CH–): Rate Constant, Products and Mechanism of Gas-phase

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Reactions with OH Radicals, IR Absorption Spectrum, Photochemical Ozone

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Creation Potential, and Global Warming Potential

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Dongpeng Liu†,*, Sheng Qin‡, Wei Li‡, Di Zhang‡, Zhikai Guo‡

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†School of Chemical Engineering and Technology, Xi'an Jiaotong University, No.28,

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Xianning West Road, Xi'an, Shaanxi, 710049, P. R. China.

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‡ Zhejiang Research Institute of Chemical Industry, No.387, Tianmushan Road,

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Hangzhou, 310023, P. R. China

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Corresponding Author

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*D.P.L.: E-mail: [email protected]. Tel: +86 29 83748935. Fax: + 86 29 83748935.

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Abstract: The rate constant for gas-phase reactions of OH radicals with

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1H-heptafluorocyclopentene (cyc-CF2CF2CF2CF=CH–) was measured using a relative

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rate method at 298 K: (5.20 ± 0.09) × 10−14 cm3 molecule–1 s–1. The quoted uncertainty

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includes two standard deviations from the least-squares regression, the systematic error

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ACS Paragon Plus Environment


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from the GC analysis, and the uncertainties of the rate constants of the reference

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compounds. The OH radical-initiated oxidation of cyc-CF2CF2CF2CF=CH– gives the

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main products COF2, CO, and CO2, leading to negligible environmental impact. For

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consumptions of cyc-CF2CF2CF2CF=CH– of less than 54%, the yield of the formation

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of ([COF2]+[CO]+[CO2])/5 (based on the conservation of carbon) was 0.99±0.02, which

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is very close to 100%. A possible degradation mechanism was proposed. The radiative

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efficiency (RE) of cyc-CF2CF2CF2CF=CH– measured at room temperature was 0.215

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W m-2 ppb-1. The atmospheric lifetime of cyc-CF2CF2CF2CF=CH– was calculated as

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0.61 years, and the photochemical ozone creation potential (POCP) was negligible. The

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20-, 100-, and 500-year time horizon global warming potentials (GWPs) were estimated

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as 153, 42, and 12, respectively.

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1. Introduction

Since Molina and Rowland discovered that the Earth’s stratospheric ozone was

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being depleted by chlorofluorocarbons (CFCs),1 the production and use of CFCs have

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been phased out. However, all halogen-containing hydrocarbons have strong

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absorptions in the atmospheric transparency window region (800–1200 cm–1)2, which 2


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could lead to greenhouse warming. Hence, attempts have been made worldwide to

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develop high-performance, environmentally acceptable alternatives, and these products

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have already progressed through several generations: CFCs →

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hydrochlorofluorocarbons (HCFCs) → hydrofluorocarbons (HFCs) →hydrofluoroethers

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(HFEs).

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Hydrofluoroolefins (HFOs) are considered a new-generation alternative with no

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stratospheric ozone depletion potential (ODPs) because of their non-chlorine molecular

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structure. They are believed to be removed from the atmosphere primarily by reactions

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with OH radicals in the troposphere rather than by ultraviolet (UV) photolysis in the

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stratosphere;3 other gas-phase oxidation reactions in the atmosphere toward O3, Cl

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atoms, and NO3 radicals are of limited significance.4 Their global warming potentials

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(GWPs) are generally lower because the >C=C< double bonds could enhance the

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reactivity of these substances toward OH radicals and thus shortening their atmospheric

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lifetimes. However, they are volatile and may be released into the atmosphere during

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use, storage, recovery, and facility maintenance; therefore, their atmospheric chemistry

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and, hence, environmental impact must be evaluated prior to their large-scale industrial

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use.

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1H-heptafluorocyclopentene (cyc-CF2CF2CF2CF=CH–) is a new potential HFO

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alternative that has been proposed for use in the semiconductor device production field

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and as a monomer for fluorine-containing polymer synthesis, as an intermediate of

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fluorine-containing pharmaceuticals, and as a solvent.5 To expand and improve our

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knowledge of the atmospheric chemistry and environmental impact of this compound,

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the rate constant for the reaction between cyc-CF2CF2CF2CF=CH– and OH

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cyc-CF2CF2CF2CF=CH– + OH → Products

k1

(1)

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has been studied using a smog chamber-gas chromatography (GC) (flame ionization

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detection [FID]) technique6 in He diluent with an initial total pressure of 400 Torr at

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298 K. This temperature is the standard room temperature used in evaluations and

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presentations of rate constants and in estimations of the atmospheric lifetimes of

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short-lived species.4 The atmospheric lifetime was calculated. The products and

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mechanism of the OH radical-initiated oxidation reactions were investigated under

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an initial pressure of 700 Torr in He at 298 K. The infrared (IR) absorption spectrum

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and radiative efficiency (RE) of the compound were obtained at room temperature,

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and the photochemical ozone creation potential (POCP) and GWP were also

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estimated. 4


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2. Experimental Section

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2.1 OH Reaction Rate Constant Measurements

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2.1.1 Apparatus and the Reliability Test

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The measurements of the OH reaction rate constant were performed using a

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double-walled cylindrical quartz chamber (internal volume: approximately 8.3 dm3)

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surrounded by ten UV fluorescent lamps (254 ± 4 nm; Haining Hansen Lighting Co.,

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Ltd., Jiaxing, Zhejiang, China) and interfaced to a GC equipped with a FID (GC-2014

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TF/SPL; Shimadzu, Tokyo, Japan). The schematic diagram of this apparatus is provided

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in Figure 1.

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Figure 1. Schematic diagram of the main experimental apparatus

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The spectral irradiance E(254nm) of ten UV fluorescent lamps was determined as

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(2.83 ± 0.03) × 1015 photons cm–2 s–1 nm–1 in He under an initial total pressure of

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375–675 Torr (shown in the Supporting Information). The temperature of the chamber 5



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was controlled by circulating water/ethylene glycol solution between the outer walls and

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was monitored using two side-attached thermocouples with an accuracy of ±1 K. The

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pressure in the chamber was measured by an electronic vacuum gauge DVR 2

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(Vacuubrand, Germany). A stainless steel greaseless vacuum line equipped with an

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electric circulating pump was connected to the chamber for sample preparation and gas

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mixing. The chamber can be operated at temperatures between 243–323K and pressures

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between 0.7–760 Torr. Periodically, the concentrations of the gases in the chamber were

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tested using the GC-FID equipped with an automatic sampling system, which revealed a

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0.20% mass decrease for each run.

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The OH radicals were generated by the UV photolysis of O3 in the presence of

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water vapor. O3 was produced from O2 via silent electrical discharge using a

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commercial O3 ozonizer (JR-S-10, Nanjing JinRen Environmental Science and

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Technology Co., Ltd., China). A gas mixture of 2% O3/O2 was introduced into the

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chamber continuously, and the flow rate was controlled by a mass flowmeter (D07-11C,

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Beijing Sevenstar Electronics Co., Ltd., China).

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O3 + hν → O (1D) + O2

(2)

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O (1D) + H2O → 2OH

(3) 6


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In relative rate experiments, the following reactions occurred.

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Reactant + OH → Products

ks

(4)

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Reference + OH → Products

kr

(5)

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The OH reaction rate constant ks of the sample reactant was derived by monitoring

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its loss relative to that of a reference compound. The decays of the reactant and

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reference were then plotted using equation (I),

(I)

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where [Reactant]t, and [Reference]t are the concentrations of the sample reactant and

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reference compound at times t0 and t, respectively, and kr is the rate constant for the

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reaction of OH radicals with the reference. The 0.20% mass decrease of the mixture

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gases caused by the GC-FID sampling was deducted from the collected data.

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As a test for the experimental system, the gas-phase reactions of HFC-134a

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(CF3CH2F) with OH radicals has been studied at 298K using CH4 as the reference

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compound. The flow rates of 2% O3/O2 were 2, 5, 9, and 11 cm3 min–1, and the OH

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radical concentrations were calculated to be 7.52 × 1010, 1.07 × 1011, 1.23 × 1011, and

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1.47 × 1011 radicals cm−3, respectively. Comparison of the kOH(CF3CH2F) / kOH(CH4)

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values obtained at different O3/O2 flow rates indicated that the flow rates did not

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interfere the measurement results. The final value of kOH(CF3CH2F) / kOH(CH4) reported 7



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as the average of the individual determinations was 0.596 ± 0.008, which was in good

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agreement with the value of 0.698 (+0.156, –0.127) derived from the JPL 10-67 within

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experimental uncertainty (The experimental details could be found in the Supporting

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Information).

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2.1.2 Measurements of cyc-CF2CF2CF2CF=CH–

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A gaseous sample of cyc-CF2CF2CF2CF=CH– was prepared in a 1-L glass bottle.

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The sample reactant, a reference compound, and H2O were introduced into the chamber

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using a Hamilton syringe, diluted by He gas with a total pressure of 400 Torr, and

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mixed through the stainless steel greaseless vacuum line for approximately 40 min.

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Then, a gas mixture of 2% O3/O2 was produced and continuously introduced into the

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chamber at a rate of 1.5–4.0 cm3 min–1 at standard temperature and pressure (STP),

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leading to an increase of 2–12.3 Torr during the measurements. Four UV lamps were

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used to generate OH radicals and to start the reactions. Automatic sampling and GC

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data collection were conducted using a time interval of 7.5 min. After 37.5 – 80 min, the

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reactions were stopped, and the chamber was cleaned by vacuumizing.

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The initial concentrations of the cyc-CF2CF2CF2CF=CH– sample and the reference

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compounds CF3CHFCH2F, CH3CH2CF3, and H2O in the experiments were ranged from

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3.13 – 5.90 × 1014, 3.05 – 4.14 × 1014, 3.48 – 4.14 × 1014, and ～1.19 × 1018 molecules 8


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cm–3, respectively. The decays of cyc-CF2CF2CF2CF=CH–, CF3CHFCH2F, and

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CH3CH2CF3 were 82–90%, 48–52%, and 79–88% at the end of the reactions,

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respectively.

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The operating parameters of the GC-FID instrument were as follows: capillary

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column, TG-Bond Q (length: 30 m; i.d.: 0.53 mm; Thermo Fisher Scientific, Inc.,

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USA); column temperature, 170 ℃ (reference: CF3CHFCH2F) and 145℃ (reference:

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CH3CH2CF3); detector temperature, 220℃; and injection port temperature, 150℃.

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2.2 OH Radical-initiated Degradation Products and Mechanism Study

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By changing the interface to a Nicolet iS50 Fourier transform IR (FTIR)

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spectrometer, the apparatus described in section 2.1.1 could be used to study the OH

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radical-initiated degradation products and the mechanism of the sample reactant.

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Certain amounts of sample reactant and water were introduced into the chamber and

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mixed in He diluent with a total pressure of 700 Torr at 298±1 K for 40 min. Then, a

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gas mixture of 2% O3/O2 was produced and continuously introduced into the chamber at

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a rate of 4.0 cm3 min–1, at STP; and six UV lamps were turned on for the irradiation.

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The loss of sample reactant and the formation of products were monitored by FTIR

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spectroscopy with a mercury cadmium telluride (MCT) detector at a time interval of 3 9



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min. A PIKE multiple-reflection IR gas cell with a 2200-cm3 volume and a 10-m optical

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path length was used in this study. The spectrometer was operated at a resolution of 0.5

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cm-1.

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The products were identified by a process of spectral stripping in which scaled

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reference spectra were subtracted from the sample spectrum and quantified using

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integrated absorption features. Reference spectra were obtained by expanding known

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volumes of the reference compounds into the reaction chambers or from the published

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literature.

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2.3 Integrated IR Absorption Cross-section Measurements

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The IR absorption spectrum of cyc-CF2CF2CF2CF=CH– was obtained using a

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Nicolet iS50 FTIR spectrometer with a triglycine sulfate (TGS) detector at a spectral

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resolution of 0.5 cm−1 and recorded with a step size of 0.241 cm−1. A 10-cm glass

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absorption cell fitted with KBr windows was used to obtain the absorption spectra at

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room temperature. Background spectra were collected by filling the cell with 700-Torr

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purified air. The absorption spectra of the sample reactant at different pressures were

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alternately recorded at least five times. The pressure of the sample in the cell was

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monitored using a MKS 974B manometer (1 × 10−8–1500 Torr) and an INFICON 10


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capacitance diaphragm gauge CDG025D (0–10 Torr). The absorption cross-section σ (ν)

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of the sample reactant at wavenumber ν (cm–1) was calculated according to the

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Beer−Lambert absorption law,2 and the integrated cross-section S (ν1,ν2) between

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wavenumbers ν1 and ν2 was then determined as follows:

(II)

5

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2.4 Materials

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A sample of cyc-CF2CF2CF2CF=CH– with a purity of 99.98% was synthesized in

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our laboratory5 and purified by a simple distillation (tower temperature: bottom, 57℃;

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top, 46℃). The impurity levels of cyc-CF2CF2CF2CCl=CF–, cyc-CF2CF2CF2CF=CF–,

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cyc-CF2CF2CHFCF=CF–, and cyc-CF2CHFCF2CF=CF– were determined by GC

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analysis (GC area percentage): 0.011%, 0.003%, 0.003%, and 0.004%, respectively.

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The reference compounds of CF3CHFCH2F (99.0% purity) and CH3CH2CF3 (99.0%

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purity) were purchased from SynQuest Labs, Inc. (USA) The kinetic measurements

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were not affected by the impurities in the samples and the reference compounds because

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the GC peaks of the reactants were separated and monitored independently via the

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relative rate method.

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To identify the degradation products, authentic sample of COF2 (99.9% purity) was 11



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provided by Sinochem Modern Environmental Protection Chemicals (Xi`an) Co., Ltd.

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(China); and the CO2 (99.995% purity) and CO (99.9% purity) was purchased from

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Hangzhou New Century Mixed Gas co., Ltd. (China).

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Helium (99.999% purity; BOC Gases (SuZhou) Co., Ltd., China) was used as a

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carrier gas in the GC analysis and as a diluent gas in all kinetic experiments due to its

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low O(1D) quenching efficiency. O2 (99.999% purity; ChengGong Gases Co., Ltd.,

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Deqing, China) was used to generate O3.

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3. Results & Discussion

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3.1 OH Reaction Rate Constant k1 at 298 K

10

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The rate constant k1 at 298 K of reaction (1) was measured relative to those of reactions (6) and (7):

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CF3CHFCH2F + OH → Products

k6

(6)

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CH3CH2CF3 + OH → Products

k7

(7)

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3.0

2.5

0

ln([Reactant]t /[Reactant]t)

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CF3CHFCH2F 2.0

1.5

CH3CH2CF3 1.0

0.5

0 0

0.4

0.8

1.2

1.6

2.0

ln([Reference]t /[Reference]t)

1

0

2

Figure 2. Loss of cyc-CF2CF2CF2CF=CH– versus CF3CHFCH2F (triangles) and

3

CH3CH2CF3 (circles) in the presence of OH radicals under an initial total pressure of

4

400 Torr in He.

5

Figure 2 shows the loss of cyc-CF2CF2CF2CF=CH– plotted versus the loss of the

6

reference compounds. A linear least-squares analysis gives the slopes of k1/k6 = 3.01 ±

7

0.05 and k1/k7 = 1.09 ± 0.01 with zero intercepts. Using k6 = 1.69 × 10–14 (±15%) 7 and

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k7 = 4.88 × 10–14 (±15%) cm3 molecule–1 s–1 7 gives k1 = (5.09 ± 0.08) × 10–14 and (5.31

9

± 0.04) × 10–14 cm3 molecule–1 s–1, respectively. Indistinguishable values of k1 were

10

obtained using the two different references, and the disagreement was less than 5%. We

11

report the final value of (5.20 ± 0.09) × 10–14 cm3 molecule–1 s–1 as the average of the

12

individual determinations with error limits that encompass the extremes of the

13

determinations. 13



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1

In smog chamber experiments, the unwanted losses of sample reactant and

2

reference compounds via heterogeneous reactions, photolysis, and dark chemistry must

3

be considered. The initial gas mixtures of sample reactant and a reference compound

4

diluted by He with a total pressure of 400 Torr, were left to stand in the chamber for 40

5

min at 298±1 K. UV irradiation was provided by four UV lamps for 40 min, and then,

6

the upper limit of H2O or O3 used in this work was independently introduced into the

7

reaction chamber. The mixtures were allowed to stand in the dark in the chamber for 40

8

min. The loss of any of the sample reactant and reference was observed to be < 2%,

9

which does not exceed the inherent instrumental uncertainty caused by the GC-FID

10

analysis (±2%). This finding suggests that heterogeneous reactions, photolysis, and dark

11

chemistry do not significantly affect the present experiments. The total uncertainty

12

includes two standard deviations from the least-squares regression, the systematic error

13

from the GC analysis, and the uncertainty of the rate constant from the reference.

14

To the best of our knowledge, no previous studies have investigated k1; thus, we

15

compared our result with those obtained for several analogous compounds (Table 1). As

16

expected based on the similarity of the molecular structures, the reactivity of OH

17

radicals towards cyc-CF2CF2CF2CF=CH– is similar to that towards

14


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1

cyc-CF2CF2CF=CH–, differing by less than 16%. The weaker reactivity of

2

cyc-CF2CF2CF2CF=CH– could be attributed to its more stable five-membered ring

3

structure. Indeed, cyc-CF2CF2CF2CF=CH– is 1287-fold less reactive than its

4

non-substituted counterpart cyc-C5H8. This observation is consistent with the

5

well-known, reactivity-decreasing effect of electron-withdrawing fluorine substituents

6

towards electrophilic OH radicals.8 Moreover, the kOH of cyc-CF2CF2CF2CF=CH– is

7

approximately 3 times greater than that of cyc-CF2CF2CF2CHFCH2–, indicating that the

8

introduction of a >C=C< double bond to increase the reactivity of organic molecules

9

towards OH radicals4 is also effective for cyclohaloalkanes. Table 1 Rate constants for the reactions of OH with cyc-CF2CF2CF2CF=CH– and analogous compounds measured at approximately 298 K. kOH, Compound

3

T, Ref.

–1 –1

cm molecule s

K

cyc-CF2CF2CF2CF=CH–

(5.20 ± 0.09) × 10–14

298

this work

cyc-CF2CF2CF2CHFCH2–

(1.72 ± 0.05) × 10–14

298

9

cyc-CF2CF2CF=CH–

(6.15 ± 0.16) × 10–14

298

10

cyc-C5H8

6.7 × 10–11

298

11

10

3.2 Products and Mechanism of OH Radical-initiated Oxidation of

11

cyc-CF2CF2CF2CF=CH–

15



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1

The products and the mechanism of the OH radical-initiated oxidation of

2

cyc-CF2CF2CF2CF=CH– were investigated by irradiating mixtures of 3.57 × 1014

3

molecules cm–3 cyc-CF2CF2CF2CF=CH– and 2.38 × 1017 molecules cm–3 H2O in He

4

diluent at an initial total pressure of 700 Torr. A gas mixture of 2% O3/O2 was

5

introduced continuously into the chamber as the OH-radicals-generating agent and

6

oxidizing agent. Figure 3 shows the IR spectra at 810–2300 cm−1 obtained before (A)

7

and after (B) in which the gas mixture was subjected to 36 min of UV irradiation. The

8

consumption of cyc-CF2CF2CF2CF=CH– was 54%. The subtraction of IR features

9

attributable to cyc-CF2CF2CF2CF=CH– from panel (B) gives the product spectrum

10

shown in panel (C). A comparison of the IR features in panel (C) with the reference

11

spectra of COF2 in panel (D) shows the formation of this product. All of these spectra

12

were processed by subtracting the water features.

16


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0.16 0.12

(A) Before irradiation cyc-CF2CF2CF2CF=CH-

0.08 0.04 0 0.06

(B) After 36 min of irradiation

0.04

Absorbance

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0.02 0 (C) = （B)-(A)

COF2

0.06 0.04 0.02

CO2

CO

0 0.8

(D) COF2

0.6 0.4 0.2 0 2200

1

2000

1800

1600

1400

1200

1000

Wavenumber (cm-1)

2

Figure 3. IR spectra acquired before (A) and after 36-min UV irradiation (B) of a

3

mixture of 3.57 × 1014 molecules cm–3 cyc-CF2CF2CF2CF=CH– and 2.38 × 1017

4

molecules cm–3 H2O in the presence of the O3/O2 gases in He diluent at an initial total

5

pressure of 700 Torr and 298±1 K. Panel (C) shows the residual IR features after

6

subtracting the features attributable to cyc-CF2CF2CF2CF=CH– from panel (B). Panel

7

(D) shows the reference spectrum of COF2.

8

9

COF2, CO, and CO2 were identified as the carbon-containing products of the OH radical-initiated oxidation of cyc-CF2CF2CF2CF=CH–. Figure 4 shows a plot of the 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

observed formation of COF2, CO, and CO2 versus the loss of cyc-CF2CF2CF2CF=CH–.

2

As shown in Figure 4, the formation of COF2 has a linearly relationship with the loss of

3

cyc-CF2CF2CF2CF=CH– when the consumption is 54% (data not shown), the

14

concentration of COF2 decreased, possibly because of heterogeneous reactions on the

15

chamber wall. In addition, the yield of ([COF2] + [CO] + [CO2])/5 was less than 100%,

16

which may be attributed to error from the quantitative analysis of CO2. Therefore, these

17

data were not used in the calculation of the product yields. In the atmosphere, reactions

18

with OH radicals and the photolysis of COF2 are believed to be too slow to be of any 18


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1

significance, and the products are essentially eliminated in water droplets, resulting in

2

HF and CO2 within several days.12 Thus, the environmental impacts of these products

3

will be negligible.

14

8x10

14

-3

△[Product] (molecule cm )

2.0x10

14

7x10

△[Product] (molecule cm-3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6x1014 5x1014

1.5x1014

（[COF2]+[CO]+[CO2])/5

1.0x1014

COF2

13

5.0x10

0 0

5.0x1013

1.0x1014

1.5x1014

2.0x1014

△[cyc-CF 2CF2CF2CF=CH-] (molecule cm-3)

14

4x10

3x1014

CO2

2x1014 14

1x10

CO 0 0

5.0x1013

1.0x1014

1.5x1014

2.0x1014

△[cyc-CF2CF2CF2CF=CH-] (molecule cm-3)

4

5

Figure 4. Formation of COF2 (●, olive), CO (▲, red), and CO2 (▼, blue) versus the loss

6

of cyc-CF2CF2CF2CF=CH- observed following the 36-min UV irradiation of a mixture

7

of 3.57 × 1014 molecules cm–3 cyc-CF2CF2CF2CF=CH– and 2.38 × 1017 molecules cm–3

8

H2O in the presence of the O3/O2 gases in He diluent at an initial total pressure of 700

9

Torr and 298±1 K. The inset shows one fifth of the total concentration of COF2, CO,

10

and CO2 versus the loss of cyc-CF2CF2CF2CF=CH- under the same experimental

11

conditions.

12

13

Analogous to the oxidation mechanisms of alkenes11 and short-chain haloolefins,4 the reaction of cyc-CF2CF2CF2CF=CH– with OH radicals is expected to proceed via 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

addition to the >C=C< double bond, and the mechanism for the observed formation of

2

COF2, CO, and CO2 is shown in Figure 5. The initial attack of OH radicals on

3

cyc-CF2CF2CF2CF=CH– may give two different fluorocycloalkyl

4

radicals—cyc-CF2CF2CF2CF(•)CH(OH)– and cyc-CF2CF2CF2CF(OH)CH(•)–—that are

5

then involved in two different degradation pathways: (a) and (b). Taking pathway (a) as

6

an example, the generated cyc-CF2CF2CF2CF(•)CH(OH)– radicals react with O2 to give

7

the corresponding peroxyl cyc-CF2CF2CF2CF(OO•)CH(OH)– radicals, which are then

8

converted to alkoxy radicals (cyc-CF2CF2CF2CF(O•)CH(OH)–) by reaction with HO2 or

9

RO2 (R=cyc-CF2CF2CF2CF(•)CH(OH)–, F(O)CCH(OH)(CF2)x=1-3(•),

10

F(O)C(CF2)x=1-3(•), etc.). Via a ring-opening reaction, the alkoxy radicals yield

11

F(O)CCH(OH)CF2CF2CF2(•) and/or F(O)C CF2CF2CF2CH(OH)(•) by two competing

12

pathways: (a1) and (a2). In pathway (a1), F(O)CCH(OH)CF2CF2CF2(•) radicals

13

undergo reactions with O2, RO2, and HO2 and unimolecular dissociation, giving the

14

product COF2 and intermediate radicals F(O)CCH(OH)CF2CF2(•); after repeating a

15

similar reaction sequence, the F(O)CCH(OH)CF2CF2(•) radicals are converted to

16

products COF2 and F(O)CC(O)H. In pathway (a2), F(O)CCF2CF2CF2CH(OH)(•)

17

radicals react with O2 to yield F(O)CCF2CF2CF2C(O)H. Although no atmospheric data

18

related to F(O)CC(O)H and F(O)CCF2CF2CF2C(O)H are available, based on the results 20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

obtained for the analogous compound H(O)CC(O)H, 7,13 they are considered to be

2

primarily degraded via reaction with OH radicals and photolysis in our experimental

3

system. Additionally, their reactivities are expected to be substantially higher than that

4

of the cyc-CF2CF2CF2CF=CH– (kOH,298K(HC(O)C(O)H) = 1.15 × 10–11 (±50%) cm3

5

molecule–1 s–1, 7 kphotolysis(HC(O)C(O)H) = (1.1 ± 0.7)×10-5 s–1) 14; thus, they are not

6

sufficiently stable in the chamber to be observed. The products of F(O)CC(O)H from

7

the OH radical-initiated oxidation and photolysis are speculated to be CO and/or CO2

8

and HF, and for F(O)CCF2CF2CF2C(O)H, they are CO and/or CO2, HF, and COF2. HF

9

was not observed during the experiments, possibly because of heterogeneous reactions

10

on the reactor wall. These inferences are consistent with our presented experimental

11

results and will be verified by future experiments once the two pure compounds of

12

F(O)CC(O)H and F(O)CCF2CF2CF2C(O)H are obtainable. As shown in Figure 5, the

13

degradation products of pathway (b) are the same as those of pathway (a); thus, we do

14

not repeat this mechanism in detail.

15

The competition between the different reaction paths is difficult to determine based

16

on the experimental results presented herein. To this end, a computational study may be

17

helpful, and we suggest that specialists undertake this type of study.

21



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1

2

Figure 5. Proposed mechanism of OH radical-initiated oxidation of

3

cyc-CF2CF2CF2CF=CH-; solid-line boxes indicate the observed products, and

4

dashed-line boxes indicate the unobserved intermediates and products. Solid gray boxes

5

(I) and (II) reflect the proposed degradation mechanisms of F(O)CC(O)H and

6

F(O)CCF2CF2CF2C(O)H, respectively.

7

3.3 IR Absorption Spectrum of cyc-CF2CF2CF2CF=CH−

8

The IR absorption spectrum of cyc-CF2CF2CF2CF=CH− between 400 and 2500 22


Page 22 of 32

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cm−1 was derived from an average of five spectra of 1.44–2.48-Torr

2

cyc-CF2CF2CF2CF=CH– in 700-Torr purified air and is shown in Figure 6. This

3

spectrum is also available in the Supporting Information with a spectral resolution of

4

0.5 cm−1. The integrated IR absorption cross-section is (2.37 ± 0.03) × 10–16 cm

5

molecule–1. We are unaware of any other IR absorption data for this compound.

3.0

2.5

2.0

-18

2

-1

cm molecule ,base e)

1

Cross-section (10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

1.0

0.5

0 500

1000

1500

2000

2500

Wavenumber (cm-1)

6

7

Figure 6. IR spectrum (base e) of cyc-CF2CF2CF2CF=CH– measured using a FTIR

8

spectrometer at a resolution of 0.5 cm–1 at 298 ±1 K.

9

The overall error for our IR cross-section test arising from uncertainties in the

10

pressure measurements (

Atmospheric Chemistry of 1 H-Heptafluorocyclopentene (cyc

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