22
T.J. Wallington, M.D. Hurley / Chemical Physics Letters 507 (2011) 19–23
experiments is small, but discernable. Least squares analysis of the
While there have been no previous studies of k19, we can
compare our result with k(OH + CH2@CHCH@CH2) = 6.78 ꢀ 10ꢁ11
[12] and k(OH + CF3CF@CF2) = (2.4 0.3) ꢀ 10ꢁ12 cm3 moleculeꢁ1
sꢁ1 [12]. Comparing k19 with k(OH + CH2@CHCH@CH2) we see that
the presence of electron withdrawing fluorine substituents results
in decreased reactivity with OH radicals. This behavior is expected
considering the electrophilic nature of OH radicals. Unlike the sit-
uation for the chlorine atom reactions discussed in Section 3.3,
there is no simple relationship between the number of >C@C< dou-
ble bonds and the reactivity of OH radicals towards linear perflu-
oroalkenes with k19 being approximately a factor of four times
larger than k(OH + CF3CF@CF2).
data in Figure 4 gives k18/k17 = 0.13 0.02. Using k17 = 7.8 ꢀ 10ꢁ13
[8] gives k18 = (1.01 0.16) ꢀ 10ꢁ13 cm3 moleculeꢁ1 sꢁ1
.
There
have been no previous studies of k18 to compare with our result.
As might be expected from the similarity of the molecular struc-
ture, the reactivity of OH radicals towards octafluorocyclopentene
is similar to that towards hexafluorocyclobutene (see Section 3.4).
3.6. Kinetics of the OH + hexafluoro-1,3-butadiene reaction
The rate of reaction (19) was measured relative to reactions (17)
and (20):
OH þ hexafluoro-1; 3-butadiene ! products
OH þ C2H2 ! products
ð19Þ
ð17Þ
ð20Þ
3.7. Atmospheric lifetimes and global warming potentials
As with other perfluoroalkenes [7], the atmospheric lifetimes of
the compounds investigated in the present work will be dictated
by the rates of their reactions with OH radicals. Rate constants
were measured in the present work at 295 K. However, the appro-
priate temperature to use for atmospheric lifetime calculations is
272 K [13]. Taking the behavior of the reaction of OH radicals with
perfluoropropene as a guide [14], we expect the rate constants for
reactions of OH radicals with the title perfluoroalkenes to increase
by approximately 10% between 295 and 272 K. Proceeding on this
assumption and using a global average OH radical concentration of
106 cmꢁ3 [15] gives atmospheric lifetimes of 135, 104 and 1.1 days
for hexafluorocyclobutene, octafluorocyclopentene, and hexa-
fluoro-1,3-butadiene, respectively. The approximate nature of
these lifetime estimates should be stressed; the average daily con-
centration of OH radicals in the atmosphere varies significantly
with both location and season [16]. The values above are estimates
of the global average lifetime; local lifetimes could be significantly
different.
OH þ C2H4 ! products
Initial reaction mixtures consisted of 6.2–7.3 mTorr of hexa-
fluoro-1,3-butadiene, 100 mTorr CH3ONO, and either 1.5–1.8 mTorr
C2H2 or 2.6–4.4 mTorr C2H4 in 700 Torr total pressure of air diluent.
Figure 5 shows the loss of hexafluorocyclobutene plotted versus the
loss of the reference compounds. Linear least squares analysis of the
data in Figure 5 gives k19/k17 = 14.0 1.0 and k19/k20 = 1.06 0.08.
Using k17 = 7.8 ꢀ 10ꢁ13 [8] and k20 = 7.9 ꢀ 10ꢁ12 cm3 mole-
culeꢁ1 sꢁ1 [8] gives k19 = (1.09 0.08) ꢀ 10ꢁ11 and (8.37 0.63) ꢀ
10ꢁ12 cm3 moleculeꢁ1 sꢁ1. We choose to cite a final value which is
the average of the individual determinations together with error
limits which encompass the extremes of the determinations, hence
k19 = (9.64 1.76) ꢀ 10ꢁ12 cm3 moleculeꢁ1 sꢁ1
.
Acerboni et al. [11] conducted a relative rate study of reaction
(19) and reported k19 = (1.1 0.3) ꢀ 10ꢁ11 cm3 moleculeꢁ1 sꢁ1. This
value of k19 was obtained from experiments conducted using three
different reference compounds (ethene, propene, and cyclohexane)
and is in good agreement with that measured here. However, the
rate constant ratio k19/k20 = 1.3394 0.0253 measured by Acerboni
The halocarbon global warming potentials relative to CFC-11
(HGWP) for the title perfluoroalkenes (PFA) can be estimated using
the expression [17]:
et al. [11] is approximately 25% higher than the ratio k19/k20
=
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
REPFA
RECFC-11
s
s
PFAMCFC-11
CFC-11MPFA 1 ꢁ expðꢁt=sCFC-11
1 ꢁ expðꢁt=sPFAÞ
1.06 0.08 measured here and hence for reasons which are unclear
there is small, but discernable, difference between our results and
those of Acerboni et al. [11].
HGWPPFA
¼
Þ
where REPFA, RECFC-11, MPFA, MCFC-11, sPFA, and sCFC-11 are the radia-
tive efficiencies, molecular weights, and atmospheric lifetimes of
the perfluoroalkenes and CFC-11, and t is the time horizon over
which the forcing is integrated. Given its very short (1.1 day) atmo-
spheric lifetime, hexafluoro-1,3-butadiene will not make any signif-
icant contribution to radiative forcing of climate change and
estimation of a global warming potential for this molecule is not
1.8
1.6
C2H2
1.4
meaningful. Using the lifetimes estimated above;
s(CFC-
11) = 45 years [18]; RE = 0.28 and 0.32 W mꢁ2 ppbꢁ1 for hexafluoro-
cyclobutene and octafluorocyclopentene, respectively [19]; and
RECFC-11 = 0.25 W mꢁ2 ppbꢁ1 [18] gives 100 year time horizon
HGWPs of 8.7 ꢀ 10ꢁ3 and 5.9 ꢀ 10ꢁ3 for hexafluorocyclobutene
and octafluorocyclopentene. Scaling these values using the GWP va-
lue of CFC-11 of 4750 [18] gives GWPs of 42 and 28 for hexafluoro-
cyclobutene and octafluorocyclopentene.
1.2
1.0
0.8
C2H4
0.6
0.4
0.2
0.0
4. Discussion
Kinetic data are reported for reactions of chlorine atoms and
OH radicals with hexafluorocyclobutene, octafluorocyclopentene,
and hexafluoro-1,3-butadiene. With the exception of the reaction
of OH radicals with hexafluoro-1,3-butadiene, the present work is
the first study of these reactions. For reactions with both chlorine
atoms and OH radicals the order of reactivity is hexafluorocyclob-
utene ꢃ octafluorocyclopentene < CF3CF@CF2 < CF2@CF–CF@CF2.
There is no evidence for a dependence of reaction rate on ring
0.0
0.2
0.4
0.6
0.8
1.0
Ln ([reference]to/[reference]t)
Figure 5. Loss of hexafluoro-1,3-butadiene versus C2H2 (triangles) and C2H4
(circles) following exposure to OH radicals.