Atmospheric chemistry of C2F5CH2OCH 3 (HFE-365mcf)

Article abstract of DOI:10.1039/c0cp01609h

FTIR smog chamber techniques were used to measure k(Cl + C ₂F₅CH₂OCH₃) = (2.52 ± 0.37) × 10^-11 and k(OH + C₂F₅CH ₂OCH₃) = (5.78 ± 1.02) × 10^-13 cm³ molecule^-1 s^-1 in 700 Torr of air diluent at 296 ± 1 K. The atmospheric lifetime of C₂F ₅CH₂OCH₃ is estimated to be 20 days. Reaction of chlorine atoms with C₂F₅CH₂OCH₃ proceeds 18 ± 2% at the -CH₂- group and 82 ± 2% at the -CH₃ group. Reaction of OH radicals with C₂F ₅CH₂OCH₃ proceeds 44 ± 5% at the -CH₂- group and 56 ± 5% at the -CH₃ group. The atmospheric fate of C₂F₅CH₂OCH₂O radicals is reaction with O₂ to give C₂F ₅CH₂OCHO. The atmospheric fate of C₂F ₅CH(O)OCH₃ radicals is C-C bond-cleavage to give C ₂F₅ radicals and CH₃OCHO (methyl formate). The infrared spectrum was recorded and used to estimate a global warming potential of 6 (100 year time horizon) for C₂F₅CH ₂OCH₃.

Full text of DOI:10.1039/c0cp01609h

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PAPER
Atmospheric chemistry of C F CH OCH (HFE-365mcf)
2
2
5
3
a
a
a
b
D. L. Thomsen, V. F. Andersen, O. J. Nielsen* and T. J. Wallington*
Received 26th August 2010, Accepted 11th November 2010
DOI: 10.1039/c0cp01609h
2 5 2 3
FTIR smog chamber techniques were used to measure k(Cl + C F CH OCH ) =
ꢂ11
ꢂ13
3
ꢂ1 ꢂ1
s
(
2.52 ꢀ 0.37) ꢁ 10
and k(OH + C
2
F
5
CH
2
OCH
3
) = (5.78 ꢀ 1.02) ꢁ 10
CH
to be 20 days. Reaction of chlorine atoms with C F CH OCH proceeds 18 ꢀ 2% at the –CH –
cm molecule
in 700 Torr of air diluent at 296 ꢀ 1 K. The atmospheric lifetime of C
2
F
5
2
OCH is estimated
3
2
5
2
3
2
group and 82 ꢀ 2% at the –CH group. Reaction of OH radicals with C F CH OCH proceeds
3
2
5
2
3
4
4 ꢀ 5% at the –CH – group and 56 ꢀ 5% at the –CH group. The atmospheric fate of
2
3
C
C
2
F CH
5
F
2
OCH
2
O radicals is reaction with O
2
to give C
2
F
5
CH
2
OCHO. The atmospheric fate of
radicals and CH OCHO (methyl
2
5
CH(O)OCH
3
radicals is C–C bond-cleavage to give C
2
F
5
3
formate). The infrared spectrum was recorded and used to estimate a global warming potential of
6
2 5 2 3
(100 year time horizon) for C F CH OCH .
1
. Introduction
2. Experimental section
Recognition of the detrimental environmental impact of the
release of chlorofluorocarbons (CFCs) and Halons into the
All experiments were performed in a 140 litre Pyrex reactor
4
,5
interfaced to a Mattson Sirus 100 FTIR spectrometer. The
reactor was surrounded by 22 fluorescent black lamps
(GE F40BLB) which were used to photochemically initiate
the experiments. Chlorine atoms were generated by photolysis
1
,2
atmosphere has led to an international effort to replace these
compounds with environmentally acceptable alternatives.
Hydrofluoroethers (HFEs) are
a class of compounds
which have been developed as potential CFC and Halon
replacements in applications such as the cleaning of electronic
equipment, heat transfer, lubricant deposition, and fire
suppression. HFEs do not contain chlorine and therefore do
not contribute to chlorine based catalytic destruction of
stratospheric ozone. A detailed knowledge of the atmospheric
chemistry of HFEs is needed to assess the environmental
impact of these compounds prior to their large scale industrial
use. To improve our understanding of the atmospheric
chemistry of HFEs in general, and C F CH OCH in
2
of Cl :
Cl
2
+ hn - 2Cl
(1)
OH radicals were generated by UV irradiation of CH
NO/air mixtures:
3
ONO/
CH
CH
HO + NO - OH + NO
3
ONO + hn - CH
3
O + NO
(2)
(3)
(4)
3
O + O - HCHO + HO
2
2
2
5
2
3
particular, the chlorine atom and OH radical initiated
oxidation of C CH OCH was studied in the smog chamber
2
2
2
F
5
2
3
facility at Ford Motor Company. Experiments were
performed to determine the: (i) kinetics and mechanism
of reaction with chlorine atoms, (ii) kinetics and mechanism
of reaction with hydroxyl radicals, (iii) atmospheric
degradation products, and (iv) IR spectrum and global
warming potential. The results from these experiments
Reactant and product concentrations were monitored using
in situ Fourier transform infrared spectroscopy. IR spectra
were derived from 32 co-added interferograms with a spectral
ꢂ1
resolution of 0.25 cm and an analytical path length of 26 m.
Reference spectra were acquired by expanding known volumes
of reference compounds into the chamber. Experiments were
3
compliment and extend a recent study by Oyaro et al. of
performed at 296 ꢀ 1 K in 700 Torr of N
All reagents except CH ONO were obtained from commercial
sources at purities >99%. C CH OCH was supplied
by Fluorochem. Ultra high purity nitrogen, oxygen, and
synthetic air diluent gases were used as received. CH ONO
2 2
/O diluent.
the chemistry of HFEs.
3
2
F
5
2
3
a
Department of Chemistry, University of Copenhagen,
3
Universitetsparken 5, DK-2100 Copenhagen, Denmark.
E-mail: ojn@kiku.dk
Systems Analytics and Environmental Sciences Department,
Ford Motor Company, Mail Drop RIC-2122, Dearborn,
Michigan 48121-2053, USA. E-mail: twalling@ford.com
was prepared by the drop-wise addition of concentrated
H SO to a saturated solution of NaNO2 in methanol
b
2
4
and was devoid of any detectable impurities using FTIR
analysis.
2
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In smog chamber experiments unwanted loss of reactants,
reference compounds, and products via photolysis and
heterogeneous reactions need to be considered. The reactant
and reference compounds used in the present work
3. Results and discussion
3
2 5 2 3
.1 Relative rate study of k(Cl+ C F CH OCH )
The rate of reaction (5) was measured relative to reactions
(6)–(8):
(
C F CH OCH , C H , C H , and C H OH) do not
2 5 2 3 2 4 2 2 2 5
absorb at wavelengths above the Pyrex cut-off (>300 nm).
Neither these compounds nor the expected fluorinated ester
Cl + C
2
F
5
CH
2
OCH
3
2
- Products
- Products
(5)
(6)
(7)
(8)
oxidation products of
2 5 2 3
C F CH OCH are expected to
Cl + C
2
H
be photolyzed in our reactor. To test for the presence of
heterogeneous reactions product mixtures obtained after
UV irradiation were allowed to stand in the dark in the
chamber for 30 min. There was no observable (o2%) loss
of reactants or products, suggesting that heterogeneous
reactions are not a significant complication in the present
experiments. Analysis of the IR spectra was achieved through
a process of spectral stripping in which small fractions of a
reference spectrum were subtracted incrementally from the
sample spectrum.
Cl + C H - Products
2
4
2 5
Cl + C H OH - Products
The initial mixtures consisted of 3.67–8.96 mTorr
CH OCH , 83.7–88.7 mTorr Cl , and 2.94–10.3 mTorr
of the reference compound in 700 Torr of air diluent. The
observed loss of C CH OCH is plotted versus the reference
compounds in Fig. 1.
Linear least squares analyses of the data in Fig. 1 give
/k /k /k
C
F
2
5
2
3
2
2
F
5
2
3
The relative rate method is a well established technique for
measuring the reactivity of Cl atoms and OH radicals with
organic compounds. Kinetic data are derived by monitoring
k
5
6
= 0.51 ꢀ 0.05, k
5
7
= 0.28 ꢀ 0.03, and k
5
8
= 0.24 ꢀ
ꢂ11 6
ꢂ11 6
0.02. Using k
ꢂ10 7
6
= 5.07 ꢁ 10 , k
7
= 9.29 ꢁ 10 , and k
8
=
ꢂ11
1.0 ꢁ 10
gives k
5
= (2.58 ꢀ 0.25) ꢁ 10 , (2.60 ꢀ 0.28) ꢁ
ꢂ11 3 ꢂ1 ꢂ1
ꢂ11
the loss of C
2
F
5
CH
2
OCH
3
relative to two or more reference
10 , and (2.40 ꢀ 0.25) ꢁ 10
cm molecule
choose to quote a final value for k which is the average of the
5
s . We
compounds. The decays of the reactant and reference are then
plotted using the expression:
individual determinations together with uncertainties that
encompass the extremes of the individual determinations,
ꢂ11
3
cm molecule s .
ꢂ1 ꢂ1
hence, k
5
= (2.52 ꢀ 0.37) ꢁ 10
ꢀ½
ꢁ
ꢀ
ꢁ
C F5CH2OCH3ꢃ
kreactant
½referenceꢃ
2
t0
t0
Interestingly, our result is significantly different from that
ln
¼
ln
ðIÞ
½
C F5CH2OCH3ꢃ
kreference
½referenceꢃ
2
t
t
reported by Oyaro et al. who used the relative rate method to
measure k
5
/k
8
= 0.419 ꢀ 0.022 and hence k
5
= (4.0 ꢀ 0.8) ꢁ
. Oyaro et al. studied the reactivity
of chlorine atoms towards both CHF CF CH OCH
ꢂ11
3
ꢂ1 ꢂ1
3
1
0
cm molecule
s
where [C F CH OCH ] , [C F CH OCH ] , [reference] and
5
2
5
2
3 t
2
2
3 t
t0
0
2
2
2
3
[
reference]
t
are the concentrations of C
2
5
F CH
2
OCH
3
and
3 2 2 3
and CF CF CH OCH . The reactivity of chlorine atoms
reference at times ‘‘t
the rate constants for reactions of chlorine atoms or OH
radicals with C CH OCH and the reference. Quoted
0
’’ and ‘‘t’’, and kreactant and kreference are
towards organic molecules typically decreases with increasing
fluorination and it would be expected that CF CF CH OCH
3
3
2
2
2
F
5
2
3
would have a reactivity which was comparable to, or less than,
that of CHF CF CH OCH . Contrary to expectations, Oyaro
uncertainties include two standard deviations from the
linear regressions and uncertainties inherent in analysis
of the IR spectra (typically ꢀ1% of the initial reactant
concentration).
2
2
2
3
3
et al. reported that the rate constant for reaction of chlorine
ꢂ11
atoms with CF
3
CF
2
CH
2
OCH
3
,
(4.0
ꢀ
0.8)
CF
ꢁ
10
,
,
was greater than that towards CHF
ꢂ11
2
2
CH
2
OCH
3
3
cm molecule
ꢂ11
ꢂ1 ꢂ1
(
2.65 ꢀ 0.17) ꢁ 10
s
. Our value of
3
cm molecule s based upon
ꢂ1 ꢂ1
k
5
= (2.52 ꢀ 0.37) ꢁ 10
experiments using three different reference compounds, is
consistent with the rate data reported for CHF CF CH OCH
by Oyaro et al., and we believe should be preferred over the
2
2
2
3
3
3
3
value of k5 reported by Oyaro et al. Oyaro et al. used
GC-MS to monitor the loss of CHF CF CH OCH and
3
2
2
2
CF CF CH OCH in their experiments. This analytical
3
2
2
3
technique should be reliable, and we are not able to offer an
explanation for the apparent internal inconsistency in the
kinetic data for CHF
2 2 2 3 3 2 2 3
CF CH OCH and CF CF CH OCH
3
reported by Oyaro et al.
3
.2 Products of Cl + C F CH OCH in 700 Torr of N /O
2 5 2 3 2 2
To investigate the atmospheric oxidation mechanism of
CH OCH experiments were performed using mixtures
of 5.88–10.9 mTorr C CH OCH , 39.7–88.2 mTorr Cl , and
5–700 Torr O2 made up to 700 Torr total pressure as
C
2
F
5
2
3
2
F
5
2
3
2
1
Fig.
1
Decay of
C
2
F
5
2 3 2 2 2 4
CH OCH versus C H (circles), C H
(
squares), and C
2
H
5
OH (triangles) in the presence of chlorine atoms
appropriate with N . Fig. 2 shows spectra before (A) and after
2
in 700 Torr air diluent.
(B) irradiation of a mixture of 5.88 mTorr C F CH OCH and
5
2
2
3
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spectrum of COF
2
. Comparison of panel C with panel B
2
shows the formation of COF in the system. Methyl formate
was also identified as a product by virtue of its characteristic
ꢂ1
absorption features at 1210 and 1755 cm . The spectrum
shown in panel D in Fig. 2 was obtained by subtracting
features attributable to C F CH OCH , COF , and CH OCHO
2
5
2
3
2
3
from panel B.
Fig. 3 shows plots of the formation of CH
COF versus the fractional loss of C CH
through the COF data in Fig. 3 is a linear least squares fit
which gives a molar yield of COF
of 20 ꢀ 3%. For low
consumptions of C CH OCH the yields of CH OCHO and
COF were indistinguishable. The simplest explanation
for the formation of CH OCHO and COF is that these
3
OCHO and
2
2
F
5
2 3
OCH . The line
2
2
F
2 5
2
3
3
2
3
2
compounds are formed following the decomposition of the
C F CH(O)OCH alkoxy radical by the following reactions
2
5
3
(
RO represents an alkyl peroxy radical):
2
Cl + C F CH OCH - C F CHOCH + HCl (5a)
2
5
2
3
2
2
2
5
3
C
F
2 5
CHOCH
3
+ O
- C
- C
2
F
5
CH(OO)OCH
3
(9)
2
C F
5
CH(OO)OCH
3
+ RO
2
F
5
CH(O)OCH
3
+ RO + O
10)
2
(
C
2
F
5
CH(O)OCH
3
2
- CF
- C
3
CF
2
+ CH
3
OCHO (11)
C
2
F
5
CH(O)OCH
CF CF
CF
3
+ O
+ O
2
F
5
C(O)OCH
3
+ HO
2
(12)
(13)
(14)
(15)
3
2
2
2
- CF
- CF
3
3
CF
2
OO
O + RO + O
2
CF
3
2
OO + RO
CF CF
CF
2
3
2
O - CF
3
+ COF
2
Fig. 2 IR spectra obtained before (A) and after (B) a 15 second
irradiation of mixture of 5.88 mTorr CH OCH and
8.2 mTorr Cl in 700 Torr of air diluent. Panel C is a reference
spectrum of COF . Panel D shows the residual spectrum (attributed to
CH OCHO) after subtraction of features due to C CH OCH
COF and CH OCHO from panel B.
a
C
2
F
5
2
3
8
2
As seen from Fig. 3, variation of [O ] over the range
2
2
1
5–700 Torr had no discernable effect on the yields of COF2
or CH OCHO. We conclude that reaction (12) does not
compete with reaction (11) for the available C CH(O)OCH
radicals. The dotted line through the CH OCHO data in Fig. 3
C
2
F
5
2
2
F
5
2
3
,
3
2
3
2
F
5
3
3
is a fit of a second order polynomial and is provided to
aid visual inspection of the data trend. The curvature
3
of the CH OCHO yield plot indicates that there is one, or
more, loss mechanisms for this species in the system. The
reaction of chlorine atoms with CH OCHO is too slow,
3
ꢂ12
3
cm molecule s , to
ꢂ1 ꢂ1 8
k(Cl + CH OCHO) = 1.4 ꢁ 10
3
account for the observed curvature. The CF radicals
3
produced in reaction (15) will add O and be converted into
2
CF
3
O radicals.
CF
OO + RO
3
+ O
2
- CF
3
OO
O + RO + O
2
(16)
(17)
CF
3
2
- CF
3
9
CF
3
O radicals react readily with organic compounds.
Reaction with CF
3
O radicals provides a plausible explanation
Fig. 3 Yield of COF
the chlorine initiated oxidation of C
/O diluent with [O ] = 10 (open symbols), 147 (grey symbols), or
00 Torr (filled symbols).
2
(circles) and CH
3
OCHO (triangles) following
for the observed curvature of the CH
Features attributable to CF
3
OCHO yield plot.
ꢂ1
2
F
5
CH OCH in 700 Torr of
2
3
OH at 1280 and 1283 cm were
3
N
2
2
2
observed in the product spectra.
7
Reaction of chlorine atoms at the –CH
3
group in
C F CH OCH will lead to the formation of C F CH OCH O
2
5
2
3
2
5
2
2
8
8.2 mTorr Cl
2
in 700 Torr of air. The consumption of
alkoxy radicals which will either react with O2 to give the
C F CH OCH was 43% (2.53 mTorr). Panel C is a reference
2
formate C F CH OCHO, or will decompose via O–C bond
5
5
2
3
2
2
2
760 Phys. Chem. Chem. Phys., 2011, 13, 2758–2764
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3
at a loss to explain why Oyaro et al. did not observe the
formation of CH OCHO and COF in their study. Using
3
2
3
GC-MS techniques Oyaro et al. observed a major and a
minor product ion both with an m/z ratio of 179. The major
+
product ion was identified as [C F C(O)OCH ]H while the
2
5
3
+
minor product ion was identified as [C F CH OCHO]H .
2
5
2
We suggest that the major product with m/z = 179 in the
3
study by Oyaro et al. was actually the proton adduct with
C
2
F
5
CH
major product of the chlorine atom initiated oxidation of
CH OCH is consistent with the findings by Nolan
et al. and Oyaro et al. that CF
2 2 5 2
OCHO. Our conclusion that C F CH OCHO is the
C
2
F
5
2
3
1
1
12
3 2
CH OCHO is the major
product (approximately 90% yield) of the chlorine atom
initiated oxidation of CF CH OCH .
3
2
3
3
.3 Products of Cl + C F CH OCH in 700 Torr of
2 5 2 3
Fig. 4 Yield of C
oxidation of CF OCF
open symbols), 147 (grey symbols), or 700 Torr (filled symbols).
2
F
5
CH
2
OCHO following the chlorine atom initiated
air with NO
3
2
CF
2
H in 700 Torr of N /O diluent with [O ] = 10
2
2
2
The products of the chlorine atom initiated oxidation of
C F CH OCH in the presence of NO were investigated
(
2
5
2
3
using mixtures of 9.11–10.9 mTorr C F CH OCH ,
2
2
5
3
scission to give HCHO and the C
2
F
5
CH
2
O alkoxy radical.
to give
3
9.7–57.3 mTorr Cl , and 38.2–83.7 mTorr NO in 700 Torr
2
Any C CH O radicals formed will react with O
5
2
F
2
2
of air diluent. Three products were observed: C
COF , and CH OCHO (we did not observe any formation of
alkyl nitrates such as CF ONO ). In the presence of NO the
fate of CF O radicals is reaction with NO to give COF and
2 5 2
F CH OCHO,
C
2
F
5
CHO. The observed carbonyl product feature evident in
ꢂ1
2
3
panel D of Fig. 4 (at 1766 cm ) does not match that
ꢂ1 10
3
2
of C F CHO (at 1776 cm
2
). We conclude that the
5
3
2
9
residual spectrum in panel D is attributable to the formate
C F CH OCHO produced in the following reactions:
FNO. It would be expected that when compared to experiments
conducted in the absence of NO, the COF2 yield would
increase by a factor of two reflecting this additional source.
2
5
2
Cl+C
CH
OCH
2
F
5
CH
OCH
OO + RO
2
OCH
3
- C
- C
- C
2
F
5
CH
CH
CH
2
OCH
OCH
OCH
2
+ HCl (5b)
CF O + NO - COF + FNO
(21)
3
2
C
2
F
5
2
2
+ O
2
2
F
5
2
2
OO (18)
Fig. 5 shows a plot of the formation of C F CH OCHO,
2
C
2
F
5
CH
2
2
2
2
F
5
2
2
O + RO + O
19)
2
2
5
COF , and CH OCHO versus the loss of C F CH OCH . The
3
(
2
3
2
5
2
lines through the data are linear least squares fits and give
C F CH OCH O + O - C F CH OCHO + HO (20)
2
5
2
2
2
2
5
2
2
molar yields of 83 ꢀ 6%, 36 ꢀ 3%, and 18 ꢀ 2% for
C
2
F
5
CH
2
OCHO, COF
2
,
3
and CH OCHO, respectively.
Consistent with expectations outlined above, for experiments
Given the lack of any discernable C F CHO we proceed on
2
5
in the presence of NO the COF
CH OCHO. From the COF and CH
= 0.18 ꢀ 0.02 and, hence, k5b/k = 0.82 ꢀ 0.02.
2
yield is twice that of
the assumption that the carbonyl feature in panel D of Fig. 4 is
attributable solely to C CH OCHO and that this species
accounts for the balance of the loss of C CH OCH not
accounted for by formation of COF and CH OCHO. Using
this approach we estimate that the concentration of
3
2
3
OCHO yields we derive
2
F
5
2
k
5a/k
5
5
2
F
5
2
3
2
3
C
2
F
5
CH
2
OCHO responsible for the carbonyl absorption
ꢂ1
evident at 1766 cm
is 2.11 mTorr. The integrated
ꢂ1
absorption of C
ꢂ17
2
F
5
CH
2
OCHO at 1740–1790 cm is then
ꢂ1
3
.8 ꢁ 10
cm molecule which is similar in magnitude to
ꢂ1
that for the carbonyl band at 1720–1790 cm of CH OCHO
3
ꢂ17
ꢂ1
of 4.6 ꢁ 10 cm molecule . Having established a reference
spectrum, the yields for C F CH OCHO shown in Fig. 4 were
2
5
2
estimated. As seen from Fig. 4, there was no discernable effect
of varying [O ] over the range 15–700 Torr on the yield of
OCHO. We conclude that the atmospheric fate of
OCH O radicals is reaction with O to give the
CH OCHO.
2
C
C
2
2
F
F
5
CH
CH
2
2
5
2
2
formate C
2
F
5
2
The results from the product study are inconsistent with
3
those from the study by Oyaro et al. in which it was
concluded that H-atom abstraction from the –CH3 group
was the minor reaction path (o5%) and that abstraction from
the –CH – group led to formation of C F C(O)OCH . We are
Fig. 5 Formation of C F CH OCHO (diamonds), COF (circles),
2
2
5
2
and CH OCHO (triangles) from the reaction of Cl atoms with
3
2 5 2 3
C F CH OCH in the presence of NO in 700 Torr of air.
2
2
5
3
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Fig. 7 Loss of C
circles) following exposure to OH radicals in 700 Torr air. The loss
of C CH OCH was calculated from the formation of COF
2 5 2 3 2 4 2 2
F CH OCH versus C H (squares) and C H
Fig.
6
Formation of COF
2
versus loss of CF
3
CF
2
CH
/CH
2
OCH
ONO/
3
(
observed following UV irradiation of CF
NO/air mixtures.
3
CF
2
CH OCH
2
3
3
2
F
5
2
3
2
.
somewhat unusual behavior may reflect the substantial
inductive effect of the C – group which will strengthen the
The linearity of the C F CH OCHO yield plots in Fig. 4
2
2
5
2 5
F
and 5 indicates that the formate is much less reactive than the
C–H bonds in the –CH
ꢂ1
2
– group. The H–Cl bond strength is
ꢂ1
parent ether. This observation is consistent with the measurement
3
by Oyaro et al. of k(Cl + CF
4
32 kJ mol while the H–OH bond strength is 497 kJ mol
3
CH
2
OCHO) = (6.3 ꢀ 1.9) ꢁ
and the rate of hydrogen atom abstraction reactions slows
ꢂ14
3
cm molecule
ꢂ1 ꢂ1
3
s . As discussed by Oyaro et al., the
1
0
down more rapidly for chlorine atom than OH radical reactants
ꢂ1
atmospheric fate of the formate is most likely uptake into rain,
cloud, and ocean water, followed by hydrolysis.
as the strength of the C–H bond approaches 432 kJ mol
.
3
.5 Kinetics of OH + C F CH OCH in 700 Torr of air
2 5 2 3
3
.4 Products of OH + C F CH OCH in 700 Torr of
2 5 2 3
air with NO
The rate of reaction (22) was measured relative to reactions
23) and (24):
(
The products of the OH radical initiated oxidation of
C F CH OCH in the presence of NO were investigated using
2
5
2
3
OH + C F CH OCH - Products
2
5
2
3
(22)
(23)
(24)
mixtures of 10.3–29.4 mTorr C F CH OCH , 150 mTorr
3
2
5
2
OH + C H - Products
2
4
CH ONO, and 7.1–17.0 mTorr NO in 700 Torr of air diluent.
3
The formation of substantial amounts of HCHO as a product
OH + C H - Products
2
2
of CH
region of the spectra and precluded the measurement of
methyl formate or C CH OCHO. However, it was possible
to identify and quantify the formation of COF in the system
3
ONO photolysis in reaction (3) obscured the carbonyl
Experiments were performed using mixtures of 8.37–19.1 mTorr
CH OCH , 23.5–35.0 mTorr NO, 146 mTorr CH ONO,
and 3.08–7.79 mTorr of either C or C in 700 Torr air
diluent at 296 ꢀ 2 K. The loss of C CH OCH was modest
(1–10%) and difficult to measure directly. To improve the
2
F
5
2
C
F
2
5
2
3
3
H
4
2 2
H
2
ꢂ1
2
using its intense absorption feature at 774 cm (see panel C
of Fig. 2). Fig. 6 shows the formation of COF versus
loss following the UV irradiation of
/CH ONO/NO mixtures. The line is a least
2
F
5
2
3
2
C
C
2
2
F
F
5
CH
CH
2
2
OCH
OCH
3
precision of the measurements the loss of C
2
F
5
CH
2
OCH
3
2
was
and
monitored indirectly by observing the formation of COF
5
3
3
squares fit which gives a yield of 87 ꢀ 10%. As discussed in the
using the product yield described in Section 3.4 to calculate the
loss of C CH OCH
previous sections the formation of COF is a marker for
2
2
F
5
2
3
.
oxidation initiated at the –CH – site with 2 molecules of
2
The lines through the data in Fig. 7 are linear least squares
fits which give k22/k23 = 0.08 ꢀ 0.006 and k22/k24 = 0.67 ꢀ
0.04. Quoted uncertainties are two standard deviations from
the linear regressions together with our estimate of the
uncertainty associated with the product determination. The
relative rate data can be placed upon an absolute basis using
COF
the COF
hence, k22b/k22 = 0.56 ꢀ 0.05.
2
2 5 3
formed for each C F CHOCH radical produced. From
2
yield we conclude that k22a/k22 = 0.44 ꢀ 0.05 and,
OH + C F CH OCH - C F CHOCH + H O (22a)
2
5
2
3
2
5
3
2
ꢂ12 7
ꢂ13 7
literature values of k23 = 7.9 ꢁ 10
to give k22 = (6.32 ꢀ 0.47) ꢁ 10
and k24 = 7.8 ꢁ 10
2 5 2 3 2 5 2 2 2
OH + C F CH OCH - C F CH OCH + H O (22b)
ꢂ13
and (5.23 ꢀ 0.31) ꢁ
ꢂ1 ꢂ1
cm molecule s . Taking an average of the individual
ꢂ13
3
It is interesting to compare the branching ratios for reactions
of OH radicals and chlorine atoms with C CH OCH ; we
10
2
F
5
2
3
determinations with error limits which encompass the
extremes of the individual determinations and propagating
an additional 11% uncertainty associated with conversion of
find that chlorine atoms are more discriminating. Interestingly,
this finding runs counter to the typical behavior of chlorine
atoms which typically tend to be less discriminating than OH
radicals in their reactions with organic compounds. This
COF2 formation into C F CH OCH loss gives k
3
=
2
5
2
22
(5.78ꢀ1.02) ꢁ 10ꢂ13 cm molecule
3
ꢂ1 ꢂ1
s
. Our result is
2
762 Phys. Chem. Chem. Phys., 2011, 13, 2758–2764
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ꢂ2
ꢂ1
consistent with, but less precise than, the previous determination
ꢂ1 ꢂ1
0.33 W m ppb . The HGWP for C
18
estimated using the expression:
2
F
5
CH
2
3
OCH can be
of (6.42ꢀ0.33) ꢁ 10ꢂ13 cm molecule
3
s
by Oyaro et al.
3
ꢀ
ꢁꢀ
ꢁ
REHFE
RECFC-11
ꢀ
tHFEMCFC-11
tCFC-11MHFE
ꢁ
3
.6 Atmospheric lifetime of C
2
F
5
CH
2
OCH
3
HGWPHFE
¼
Organic compounds are removed from the atmosphere via
1
ꢂ expðꢂt=tHFEÞ
photolysis, wet deposition, dry deposition, and reaction with
ꢁ
1
ꢂ expðꢂt=tCFC-11Þ
OH radicals, NO
3
3
radicals, Cl atoms, and O . For saturated
compounds such as CF
3
CF CH OCH reactions with NO
2
2
3
3
where REHFE, RECFC-11, MHFE, MCFC-11, tHFE, and tCFC-11
are the radiative efficiencies, molecular weights, and atmospheric
lifetimes of the HFE and CFC-11, and t is the time horizon
over which the forcing is integrated. Using t(C F CH OCH ) =
radicals and O are too slow to be of importance. The average
3
concentration of chlorine atoms in the troposphere is several
1
3
orders of magnitude less than that of OH radicals,
and reaction with chlorine atoms will not be a significant
atmospheric loss mechanism for CF CF CH OCH . Ethers
do not absorb at UV wavelengths >200 nm and photolysis
of CF CF CH OCH will not be important in the
troposphere. Highly fluorinated molecules such as
CF CF CH OCH are hydrophobic and wet deposition is
unlikely to be of importance. The volatility of
CF CF CH OCH will render dry deposition an unlikely
2
5
2
3
1
9
2
0
days, t(CFC-11)
=
45 years,
gives an HGWP of 0.00136 for a
00 year time horizon. Scaling this value using the GWP value
and RECFC-11 =
3
2
2
3
ꢂ2 ꢂ1 19
ppb
1
4
0.25 W m
1
3
2
2
3
1
9
2 5 2 3
of CFC-11 of 4750 gives a GWP for C F CH OCH of 6.
3
2
2
3
4. Implications for atmospheric chemistry
3
2
2
3
We present here a large body of self-consistent kinetic and
mechanistic data concerning the atmospheric chemistry of
removal mechanism. The only significant removal process
for C F CH OCH in the atmosphere is reaction with OH
2
5
2
3
3 2 2 3
CF CF CH OCH which extends and refines the information
radicals. The value of k(OH + C F CH OCH ) derived in the
2
5
2
3
3
supplied in the study by Oyaro et al. Taking the combined
data from the two studies, the atmospheric chemistry of
CF CF CH OCH is well established.
present work can be used to provide an estimate of the
atmospheric lifetime of C CH OCH . Using a global
weighted-average OH concentration of 1.0 ꢁ 10 molecules cm
leads to an estimated lifetime for C CH OCH with
2
F
5
2
3
6
ꢂ3 15
3
2
2
3
With regard to the environmental impact of
CF CF CH OCH we can make the following statements.
F
2 5
2
3
3
2
2
3
respect to reaction with OH radicals of 20 days. The approxi-
mate nature of this lifetime estimate should be stressed; the
average daily concentration of OH radicals in the atmosphere
First, CF
3 2 2 3
CF CH OCH does not contain any chlorine and
will not contribute to stratospheric ozone depletion via the
well established chlorine based chemistry. As with all hydro-
fluorocarbons (HFCs) and hydrofluoroethers (HFEs), the
1
6
varies significantly with both location and season. The value
above is an estimate of the global average lifetime; local
lifetimes could be significantly different.
ozone depletion potential of CF
3
CF
Second, the atmospheric lifetime
is approximately 20 days, and
2 2 3
CH OCH is for all
2
0,21
practical purposes zero.
of CF CF CH OCH
3
2
2
3
3
.7 IR spectra and global warming potential of
consequently this compound has a negligible GWP (see
Section 3.7). Third, the atmospheric oxidation of
C F CH OCH
2
5
2
3
2 5 2 3
The IR spectrum of C F CH OCH recorded in 700 Torr of
CF CF CH OCH
3
gives C F CH OCHO, COF2 and
2 5 2
2
2
3
air diluent at 296 K is shown in Fig. 8. The integrated
ꢂ1
CH OCHO. The atmospheric fate of C F CH OCHO and
3
2
5
2
absorption cross section (650–1500 cm ) value is (2.07 ꢀ
COF
2
is uptake into rain, cloud, and ocean water, followed by
CH OH, HC(O)OH, CO , and
HF. The OH radical initiated oxidation of CH OCHO gives
HC(O)OC(O)H, HCOOH, HCHO, CO and CO . At the levels
anticipated in the environment, the atmospheric oxidation
products of CF CF CH OCH are not of concern.
ꢂ16 ꢂ1
0
.10) ꢁ 10
cm molecule . This result is in agreement,
hydrolysis to produce C
2
F
5
2
2
within the combined experimental uncertainties, with the value
ꢂ1
3
ꢂ16
ꢂ1
of (1.946 ꢀ 0.024) ꢁ 10
cm molecule (490–1525 cm )
2
3
17
reported by Oyaro et al. Using the method of Pinnock et al.
and the IR spectrum of C CH OCH shown in Fig. 8
we calculate a radiative efficiency for C CH OCH of
2
F
5
2
3
3
2
2
3
2
F
5
2
3
Acknowledgements
We thank Mads Sulbaek Andersen for help with calculations
of radiative forcing. OJN, DLT and VFA acknowledge
financial support from the Villum Kann Rasmussen
Foundation and EUROCHAMP2.
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