Photochemistry of Bromine-Containing Fluorinated Alkenes: Reactivity toward OH and

UV Spectra

Vladimir L. Orkin,* Florent Louis,# Robert E. Huie, and Michael J. Kurylo

Physical and Chemical Properties DiVision, National Institute of Standards and Technology,

Gaithersburg, Maryland 20899

ReceiVed: December 5, 2001; In Final Form: July 30, 2002

The rate constants for the reactions of OH radicals with the fluorinated alkenes containing one Br atom

(CFBrdCF2, CHBrdCF2, CH2dCBr-CF3, CH2dCBr-CF2-CF3, and CH2dCH-CF2-CF2Br), as well as

CF2dCF2, were measured using the flash photolysis resonance fluorescence technique over the temperature

range 250-370 K to give the following Arrhenius expressions: kC2F3Br(T) ) (2.02-0.12

+0.12) × 10-12 exp{(396 (

18)/T} cm3 molecule-1 s-1

; kC2HF2Br(T) ) (1.30-0.18

+0.22) × 10-12 exp{(370 ( 47)/T} cm3 molecule-1 s-1

;

kC3H2F3Br(T) ) (1.36-0.14

+0.17) × 10-12 exp{(317 ( 34)/T} cm3 molecule-1 s-1

; kC4H2F5Br(T) ) (0.98-0.26

+0.35) × 10-12

exp{(369 ( 90)/T} cm3 molecule-1 s-1; kC4H3F4Br(T) ) (0.85-0.12

+0.15) × 10-12 exp{(201 ( 46)/T} cm3

molecule-1 s-1; kC2F4(T) ) (3.39-0.12

+0.22) × 10-12 exp{(323 ( 11)/T} cm3 molecule-1 s-1

. Ultraviolet absorption

spectra of these brominated fluoroalkanes and bromoethene were measured between 164 and 276 nm. On the

basis of these results, the atmospheric lifetimes were estimated to be 1.4, 2.4, 2.8, 3.2, 7.0, and 1.1 days,

respectively. The general pattern of halolalkene reactivity toward OH is discussed.

Introduction

Despite their excellence as fire suppressants, the production

of bromofluorocarbons (Halons) is being phased out because

of the danger they pose to the Earth’s stratospheric ozone layer.

A number of nonbrominated substances have been proposed

and tested, but the effort to find replacements continues to return

to bromine-containing compounds because of the properties of

bromine as a chemically active flame suppressant. One class

of suppressants currently under consideration is the family of

bromofluoroalkenes.1 The presence of a carbon-carbon double

bond is expected to render these substances highly reactive

toward the hydroxyl radical, OH, resulting in an extremely short

tropospheric lifetime, thereby limiting their delivery of bromine

to the stratosphere where it can participate in ozone-destroying

catalytic reactions. To quantify their atmospheric lifetimes, we

have investigated the reactivity toward OH of five singly

brominated fluoroalkenes, chosen to represent a variety of

possible relative placements of bromine, fluorine, and hydrogen

in relation to the double bond: CFBrdCF2, CHBrdCF2, CH2d

CBr-CF3, CH2dCBr-CF2-CF3, and CH2dCH-CF2-CF2Br.

Rate constants were measured over the temperature range 250

to 370 K. We also investigated the reactivity of CF2dCF2 toward

OH over this temperature range. In addition, we have measured

the ultraviolet absorption spectra of all of these compounds and

bromoethene, CHBrdCH2 down to 164 nm.

Experimental Section

Detailed descriptions of the apparatuses and the experimental

methods used to measure the rate constants for the reactions of

the various fluorinated alkenes with OH and the alkene

absorption spectra are given in previous papers.2-4 Therefore,

only brief descriptions are given here.23

OH Reaction Rate Constant Measurements. The principal

component of the flash photolysis-resonance fluorescence

(FPRF) apparatus is a Pyrex reactor (of approximately 50 cm3

internal volume) thermostated with water or ethanol circulated

through its outer jacket. Reactions were studied in argon carrier

gas (99.9995% purity) at a total pressure of 13.33 kPa (100.0

Torr). Flows of dry argon, argon bubbled through water

thermostated at 276 K, and fluoroalkene mixtures (containing

0.01%-0.1% of the reactant diluted with argon) were premixed

and flowed through the reactor at a total flow rate between 0.6

and 1.4 cm3 s-1, STP. The concentrations of the gases in the

reactor were determined by measuring the mass flow rates and

the total pressure with a MKS Baratron manometer. Flow rates

of argon, the H2O/argon mixture, and reactant/inert gas mixtures

were measured by calibrated Tylan mass flow meters. Hydroxyl

radicals were produced by the pulsed photolysis (1-4 Hz

repetition rate) of H2O (introduced via the 276 K argon/H2O

bubbler) by a xenon flash lamp focused into the reactor. The

radicals were then monitored by their resonance fluorescence

near 308 nm, excited by a microwave-discharge resonance lamp

(330 Pa or 2.5 Torr of a ca. 2% mixture of H2O in UHP helium)

focused into the reactor center. The resonance fluorescence

signal was recorded on a computer-based multichannel scanner

(channel width 100 μs) as a summation of 1000-5000 consecutive flashes. The radical decay signal at each reactant concentration was analyzed as described by Orkin et al.

3 to obtain the

first-order decay rate coefficient due to the reaction under study.

At each temperature, the rate constant was determined from the

slope of the decay rate versus haloalkene concentration plot

using at least seven different concentrations.

All samples, except tetrafluoroethene, were used as supplied

after several freeze/pump/thaw cycles. Bromoethene (CHBrCH2,

* To whom correspondence should be addressed. Also associated with

the Institute of Energy Problems of Chemical Physics, Russian Academy

of Sciences, Moscow 117829, Russia. # Present address: Laboratoire de Physicochimie des Processus de

Combustion, UMR CNRS 8522, Universite des Sciences et Technologies

de Lille, 59655 Villeneuve d'Ascq Cedex, France.

J. Phys. Chem. A 2002, 106, 10195-10199 10195

10.1021/jp014436s This article not subject to U.S. Copyright. Published 2002 by the American Chemical Society

Published on Web 10/04/2002
99.5% purity, Matheson Gas Products), 1-bromo-2,2-difluoroethene (CHBrCF2, 99.5+% purity, Ozone, St. Petersburg,

Russia), and bromotrifluoroethene (CFBrCF2, 99.5+% purity,

Ozone, St. Petersburg, Russia) were used without any further

purification. Tetrafluoroethene (CF2CF2, Union Carbide Corp.)

was ca. 98% purity stabilized with ca. 1% of R-pinene with

hexafluorocyclopropane (1%) and CO2 (0.2%) as the main

impurities. 2-Bromo-3,3,3-trifluoropropene (CH2CBrCF3) and

2-bromo-3,3,4,4,4-pentafluoro-1-butene (CH2CBrCF2CF3) were

at least 98% purity with corresponding bromine-free fluorinated

alkenes as the main impurities (SynQuest Labs., Inc.). They

could also contain corresponding 1-bromoalkenes and 1,2-

dibromoalkanes as minor impurities. 4-Bromo-3,3,4,4-tetrafluoro1-butene (CH2CHCF2CF2Br, SynQuest Labs., Inc.) was at least

98% purity with CH2CHCF2CF2Cl and 2-methoxyethanol

(CH3-O-CH2-CH2OH) as possible impurities. The presence

of the above-mentioned halogenated hydrocarbon impurities

could not result in a noticeable overestimation of the measured

rate constant because they are less reactive than the target

compound. There are two impurities that cause concern:

R-pinene and CH3-O-CH2-CH2OH. Their OH reaction rate

constants at room temperature are ca. 6 × 10-11 and 1.1 × 10-11

cm3 molecule-1 s-1, respectively,5 and exceed the measured

reaction rate constants of the target compounds, C2F4 and CH2-

CHCF2CF2Br. A sample of CF2CF2 was purified by passing

through a -100 °C cold trap to decrease the R-pinene

concentration from ca. 1% to below 0.03% (determined by UV

absorption analysis near 220 nm). GC analysis of a CH2CHCF2-

CF2Br sample for the presence of 2-methoxyethanol indicated

less than ca. 0.1% of this impurity. Thus, these impurity levels

are low enough to result in no noticeable overestimation of the

measured rate constants (less than 0.2% and 1% for C2F4 and

CH2CHCF2CF2Br, respectively).

UV Absorption Cross Section Measurements. The absorption spectra of the bromofluoroalkenes were measured over

the wavelength range of 160-280 nm using a single beam

apparatus consisting of a 1-m vacuum monochromator equipped

with a 600 lines/mm grating. The radiation source was a

Hamamatsu L1385 deuterium lamp, and the detector was a

Hamamatsu R166 photomultiplier. Spectra were recorded at

increments of 0.5 nm at a spectral slit width of 0.5 nm. The

pressure inside the 16.9 ( 0.05 cm absorption cell was measured

by a MKS Baratron manometer at T ) 295 ( 1 K. Absorption spectra of the evacuated cell and of the cell filled with

a gas sample were alternately recorded several times, and the

absorption cross sections at the wavelength λ were calculated as

where [HA] is the concentration of haloalkene in the absorption

cell with the optical path length L. I0(λ) and I[HA](λ) are the

radiation intensities measured after the absorption cell when the

haloalkene concentration was zero and [HA], respectively. The

complete spectrum of each compound was constructed from data

taken over several overlapping wavelength ranges. Data over

each spectral range were obtained at several pressures to verify

adherence to the Beer-Lambert absorption law. The coincidence

of the contiguous parts of spectra taken over overlapping

wavelength ranges was usually better than ca. 1%. The overall

instrumental error associated with uncertainties in the path

length, pressure, temperature stability, and measured absorbance

was estimated to be less than 2% over most of the wavelength

range, increasing to ca. 10%-20% at the long-wavelength ends

of the spectra because of increased uncertainty in measuring

low absorbance values. Different manometrically prepared

mixtures containing 2%, 10%, and 100% of haloalkenes were

used at total pressures in the cell ranging from 4 Pa to 120 kPa

(0.03-900 Torr). There was no difference in the absorption

cross sections obtained with different mixtures, and their use

allowed a wider dynamic range in absorption measurements.

Results and Discussion

Kinetics. Rate constants measured for the reactions of

hydroxyl radicals with the five bromofluoroalkenes and CF2d

CF2 at different temperatures are listed in Table 1. The data

are presented graphically in Figure 1, and the derived Arrhenius

parameters are listed in Table 2, along with available data from

the literature. To the best of our knowledge, there are no other

results available on the reactivity of the bromofluoroalkenes

studied in the present work. The CF2dCF2 reaction was studied

at room temperature in two previous investigations6,7 (see Table

2). A previous measurement from our laboratory6 coincides with

the present result. Acerboni et al.7 measured the reaction by a

relative rate technique with two different reference reactions.

While their reported average value of the rate constant exceeds

the present results, the quoted uncertainty is much larger than

the disagreement.

The effect of fluorination on alkene reactivity is quite

complicated. Nevertheless, one can speculate on the correlation

between reactivity and structure of these halogenated alkenes.

The room-temperature rate constants for the reactions of these

and some other bromine/fluorine-substituted alkenes with the

hydroxyl radical are listed in Table 3. Asymmetric fluorination

of the olefinic carbon atom tends to lessen reactivity, whereas

TABLE 1: Rate Constants Measured for the Reactions of OH with Haloalkenes

k(T),a 10-12 cm3 molecule-1 s-1 (haloalkene concentration range, 1013 molecule/cm3)

T, K CF2dCF2 CFBrdCF2 CHBrdCF2 CH2dCBr-CF3 CH2dCBr-CF2-CF3 CH2dCH-CF2-CF2Br

250 12.3 ( 0.20

(0.39-2.96)

9.79 ( 0.22

(0.39-2.96)

5.79 ( 0.15

(0.77-6.60)

4.87 ( 0.08

(0.78-4.52)

4.23 ( 0.10

(1.56-5.40)

1.92 ( 0.06

(3.12-15.2)

272 8.67 ( 0.21

(0.36-2.72)

5.05 ( 0.10

(1.43-5.44)

4.33 ( 0.15

(1.43-5.44)

3.87 ( 0.10

(0.35-4.48)

298 10.0 ( 0.15

(0.65-2.48)

7.59 ( 0.15

(0.33-3.05)

4.43 ( 0.08

(0.65-6.09)

3.88 ( 0.15

(1.31-7.59)

3.51 ( 0.07

(0.66-5.23)

1.65 ( 0.04

(2.62-21.9)

330 6.63 ( 0.13

(0.59-2.51)

4.06 ( 0.05

(1.19-5.52)

3.55 ( 0.10

(0.59-5.50)

2.92 ( 0.06

(3.36-11.0)

370 8.09 ( 0.10

(0.26-2.45)

5.89 ( 0.37

(0.26-1.53)

3.55 ( 0.05

(1.06-4.92)

3.22 ( 0.08

(2.11-9.82)

2.68 ( 0.08

(2.11-9.82)

1.48 ( 0.03

(2.63-17.7)

a Uncertainties represent statistical levels of confidence of 95% and do not include an estimated uncertainty of 4% associated with possible

systematic errors.

σ(λ) ) ln{I0(λ)/I[HA](λ)}

[HA]L

10196 J. Phys. Chem. A, Vol. 106, No. 43, 2002 Orkin et al.
further fluorination (yielding more symmetric substitution)

compensates for the original deactivation. This can be seen from

the reactivities of CH2CH2, CH2CF2, CHFCH2, and CF2CF2.

This suggests that the alkene reactivity toward OH depends

mainly on the geometrical localization of π-electron density.

The π-electron localization in the center of the CdC bond

(CH2dCH2, CF2dCF2) promotes the OH addition reaction,

while the shift of the electron density toward the carbon atom

(due to the high electron affinity of the F atom) obstructs it. It

is interesting that the -CF3 group adjacent to the double bond

causes an even more pronounced deactivating effect than the

F-atom despite the smaller electron affinity of -CF3 and longer

distance from the CdC double bond. This suggests the

importance of a geometrical factor, as well as the electron

affinity of the substitute, on the delocalization of π-electrons.

Bromination of the olefinic carbon located between the double

bond and a fluorinated alkyl group increases the reactivity of

fluorinated Cng3 alkenes, while Br on a remote carbon has

essentially no effect. This suggests that the large electron rich

Br atom shields the deactivating adjacent -CF3 (-C2F5) group.

Ultraviolet Spectra. The ultraviolet absorption spectra of the

halogenated alkenes obtained in this work are presented in

Figures 2 and 3. The spectral data are provided in the Supporting

Information and at www.nist.gov/kinetics/spectra/index.htm.

These UV spectra reflect the interaction between π-electrons

of the CdC double bond and the electrons of the bromine atom.

The CH2dCBr- group has a very strong absorption with a

maximum at 190-200 nm (see spectra for CH2CHBr, CH2-

CBrCF3, and CH2CBrCF2CF3), which suppresses the distinctive

shorter wavelength absorption of CH2dCH- group (see spectrum for CH2CHCF3 in Figure 2). The absorption in the

TABLE 2: Arrhenius Parameters for OH Reactions of Br-Containing Fluoroalkenes and Their Estimated Atmospheric

Lifetimesa

molecule A × 1012,

cm3 molecule-1 s-1

E/R ( ∆E/R,

K

k(298) × 1012

cm3 molecule-1 s-1

atmospheric

lifetime, daysb

CF2dCF2 3.39 ( 0.22 -323 ( 11 9.98 ( 0.15 1.1

10.2 ( 0.56

11.3 ( 3.37

CFBrdCF2 2.02 ( 0.12 -396 ( 18 7.62 ( 0.15 1.4

CHBrdCF2 1.30-0.18

+0.22 -370 ( 47 4.53 ( 0.10 2.4

CH2dCBr-CF3 1.36-0.14

+0.17 -317 ( 34 3.94 ( 0.15 2.8

CH2dCBr-CF2-CF3 0.98-0.26

+0.35 -369 ( 90 3.39 ( 0.14 3.2

CH2dCH-CF2-CF2Br 0.85-0.12

+0.15 -201 ( 46 1.68 ( 0.05 7.0

a Uncertainties represent statistical levels of confidence of 95% and do not include estimated systematic errors. b Atmospheric lifetimes were

estimated in the manner typically used for long-lived compounds. Such estimations assume a uniform tropospheric distribution that is not correct

for short-lived compounds.

Figure 1. Arrhenius plot showing the average rate constants for the

reactions of OH with halogenated alkenes obtained at each temperature.

The solid lines are the least-squares fits to the data points with the

Arrhenius parameters presented in Table 2.

TABLE 3: Hydroxyl Radical Reactivity toward Haloalkenes

molecule

k(298) × 1012,

cm3 molecule-1 s-1 ref

CH2dCH2 9.0 20

CH2dCHF 5.5 21

CH2dCF2 2.0 22

CF2dCF2 10. this work

CHBrdCH2 7.0 21

CFBrdCF2 7.6 this work

CHBrdCF2 4.5 this work

CH2dCH-CH3 30. 20

CH2dCH-CF3 1.5 6

CH2dCF-CF3 1.1 6

CF2dCF-CF3 2.2 6

CH2dCBr-CF3 3.9 this work

CH2dCBr-CF2-CF3 3.4 this work

CH2dCH-CF2-CF2Br 1.7 this work

Figure 2. Ultraviolet absorption spectra of several CH2dCBrcontaining haloalkenes and CH2dCH-CF2-CF2Br at T ) 295 K.

Absorption spectra of CH3Br,8 CF3-CF2Br,9 and CH2dCH-CF3

6 are

shown for comparison purposes.

Photochemistry of Br-Containing Fluorinated Alkenes J. Phys. Chem. A, Vol. 106, No. 43, 2002 10197
maximum is about 1.5 orders of magnitude higher than that of

a bromoalkane such as CH3Br8 and is the strongest absorption

exhibited by any brominated hydrocarbon in the region of the

stratospheric transparency window near 200 nm. In contrast,

the spectrum of CH2dCH-CF2-CF2Br appears to be a

superposition of absorption by a fluoroalkene6 (such as CH2-

CHCF3) and a bromofluoroalkane (such as CF3CF2Br9). The

absorption spectra of Br-containing fluorinated ethenes (CHBrd

CF2 and CFBrdCF2) are presented in Figure 3. In contrast with

the CH2dCBr- group, they exhibit a less-pronounced absorption at 190-200 nm while showing much stronger absorption

at the shorter wavelengths. Their longest wavelength absorption

bands are of slightly higher intensity than those of bromofluoroethanes9,10 and are about 20-30 nm red-shifted (see Figure

3).

Atmospheric Implications. The atmospheric lifetimes of the

alkenes under study can be estimated on the basis of the

photochemical properties obtained in the present work. All of

the compounds are very reactive toward OH, while their

absorption cross sections for tropospheric UV radiation (above

290 nm) are negligible. Therefore, the reactions with hydroxyl

radicals in the troposphere dictate their atmospheric lifetimes.

A simple scaling procedure has proven to be valid for relatively

long-lived compounds that are well-mixed throughout the

troposphere. For such chemicals, lifetimes can be estimated

using the equation11

where τi

OH and τMCOH are the atmospheric lifetimes of the

compound of interest and methyl chloroform (MC), respectively,

due to reactions with hydroxyl radicals in the troposphere only

and ki(270) and kMC(270) ) 5.78 × 10-15 cm3 molecule-1 s-1

(ref 8) are the rate constants for the reactions of OH with these

substances at T ) 270 K. The value of τMC

OH was obtained

following the procedure used by Prinn et al.12 from the measured

lifetime of MC, τMC ) 4.8 years, when an ocean loss of 85

years and a stratospheric loss of 37 years are taken into account.

Applying this method to the alkenes of this study yields the

estimated atmospheric lifetimes listed in Table 2. As can be

seen, the lifetimes derived here are very short, much shorter

than the characteristic time of mixing processes in the troposphere, and hence are only crude estimates. The correct residence

time of the compounds in the atmosphere will depend on the

emission location and season, as well as local atmospheric

conditions. Nevertheless, these estimations give a useful scaling

among these alkenes and demonstrate that they are extremely

short-lived in the atmosphere.

Note that these calculated lifetimes reflect the globally

averaged sink for a compound due to its reaction with OH in

the troposphere and, therefore, lie between the shortest local

lifetimes in the tropical region and longest ones at high latitudes.

Such lifetimes have clear physical meaning for long-lived

chemicals that are well mixed in the troposphere following their

emission. Such lifetimes can be used for quantitative analysis

of chemical concentrations in the atmosphere. In the case of

short-lived compounds, these calculated parameters should lie

between the atmospheric lifetimes in the tropical and high

latitude areas. Calculation of the entire range of real atmospheric

lifetimes requires detailed 3-D modeling. In two recent papers,

3-D modeling was performed to study the atmospheric fate of

1-bromopropane, a short-lived compound with an estimated

lifetime of 14.4 days using the above-mentioned scaling

procedure. The detailed calculations resulted in lifetime ranges

of 8-24 days13 and 9-19.5 days14 with the shorter values

corresponding to tropical emission and the longer ones to

emission at higher latitudes.

Using these rough lifetime estimates together with the

measured UV absorption spectra, one can calculate approximate

ozone depletion potentials (ODPs) for these chemicals. On the

basis of an ODP for CF3Br of 13 and a lifetime of 65 years,15

ODP values considerably smaller than 0.01 are obtained for the

bromofluoroalkenes studied. The determination of the correct

ODP values requires 3D modeling calculations that consider

local emission conditions. Moreover, based on the shortness of

the lifetimes of the alkenes themselves, correct ODP values must

take into account the lifetimes of any bromine-containing

products resulting from the tropospheric oxidation of the alkenes.

Therefore, it is important to estimate the total lifetime of Br

atoms in the form of volatile degradation products produced in

the oxidation processes initiated by the reactions of OH with

Br-containing fluoroalkenes.

We can speculate on a possible degradation mechanism for

Br-containing fluorinated alkenes in the atmosphere. It is

worthwhile to point out that the production of a relatively longlived bromine-containing degradation product is not likely for

a compound with a brominated olefinic carbon (i.e., all of the

bromoalkenes studied in the present work, except CH2dCHCF2-CF2Br). Because of the relatively weak C-Br bound, the

Br atom can be quickly ejected to form either a carbonyl (Cd

O) or a hydroxyalkene (CdC-OH), depending on which

olefinic carbon acquired OH in the initial addition reaction. Note

that analogous processes are known in the oxidation of

chlorinated compounds. Thus, CdO formation and Cl atom

release are a part of kinetic scheme for the decomposition of

hydrochlorofluoroalkanes (HCFCs) in the atmosphere.16 Along

similar lines, the release of Cl atom following the reaction of

OH with trans-1,2-dichloroethene (CHCldCHCl) has been

directly observed.17 In accordance with the kinetic scheme

shown in Figure 4, the Br atom will probably be released

immediately after the initial reaction between OH and brominated alkene in the lower troposphere (probably within minutes

in the case of the CdO formation channel). Furthermore,

Figure 3. Ultraviolet absorption spectra of CF2dCFBr and CF2dCHBr

at T ) 295 K. Absorption spectra of bromofluoroalkanes9,10 are shown

for comparison purposes.

τi

OH ) kMC(270)

ki(270)

τMC

OH

10198 J. Phys. Chem. A, Vol. 106, No. 43, 2002 Orkin et al.
haloalkylperoxynitrate formation cannot significantly increase

the lifetime of bound bromine and therefore its transport to the

stratosphere because of the short lifetime of such compounds

due to thermal decomposition.18,19

Acknowledgment. This work was supported by the Upper

Atmosphere Research Program of the National Aeronautics and

Space Administration and by the Next Generation Fire Suppression Technology Program, funded by the Department of

Defense Strategic Environmental Research and Development

Program under MIPR Number W74RDV73243630.

Supporting Information Available: The ultraviolet absorption cross sections of halogenated alkenes are presented in Table

4. This material is available free of charge via the Internet at

http://pubs.acs.org.

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(22) Howard, C. J. J. Chem. Phys. 1976, 65, 4771-4777.

(23) Certain commercial equipment, instruments, or materials are

identified in this article to adequately specify the experimental procedure.

Such identification does not imply recognition or endorsement by the

National Institute of Standards and Technology, nor does it imply that the

material or equipment identified is necessarily the best available for the

purpose.

Figure 4. Kinetic scheme for the degradation of bromoalkenes of the

form CdC-Br in the atmosphere.

Photochemistry of Br-Containing Fluorinated Alkenes J. Phys. Chem. A, Vol. 106, No. 43, 2002 10199