lifetime (obtained from the 2001 IPCC report18 or 2002 WMO
report19). It is also of interest to compare the rate information
obtained in this work with absolute rate data for the second-order
reaction of chloro radical with HFCs collated by the NASA Jet
Propulsion Laboratory.20 These too are listed in Table 1.
Figure 2 plots the data given in Table 1. The top plot (a) shows
all the data; the bottom plot (b) expands the data for compounds
with a lifetime of <20 y and applies a linear fit. We emphasize
that this is an empirical correlation; the linear fit is applied only
as a simple and acceptably accurate (R2 ) 0.87) function relating
the reaction kinetics and atmospheric lifetimes. It is not clear why
approximate linearity is observed in short-lived species but not
so in more stable ones. From an industrial perspective, any new
volatile compounds of potential interest for large-scale manufacture
would need to be short-lived and, hence, in the linear region of
the plot in Figure 2.
While the results above demonstrate the utility of the method,
they are limited to the particular experimental conditions chosen.
Moreover, in some cases, there was significant variation in the
results of replicate experiments, leading to the large confidence
intervals shown on certain data points in Figure 2. This is
attributed to subtle changes in the irradiation conditions (e.g.,
change in flux from the UV lamp, laboratory temperature), which
strongly affect the kinetics of the system. A more practical and
robust approach would acquire and use relative rate information.
With such an approach, small quantities of both an unknown
analyte and a known reference compound would be loaded in an
ampule in excess Cl2, and the relative rates of decomposition under
common irradiation conditions would be determined. In order to
validate this method, a set of experiments were carried out in
which two each of CF2HCH3, CF2H2, and CFH2CH3 were irradiated
in excess Cl2. CF2HCH3 was chosen as the reference compound
and was irradiated with each of the other analytes along a series
of timed intervals in duplicate experiments. The plots of concen-
tration versus time for each pair were fit to exponential decays,
and the relative pseudo-first-order rate constants were obtained.
Table 2 gives the results of this set of experiments. Using
CF2HCH3 as a referent, and assuming an inverse linear correlation
between reaction rate and atmospheric lifetime, one obtains a
predicted lifetime of 0.45 y for CFH2CH3 (literature lifetime 0.21)
and 6.1 y for CF2H2 (literature lifetime 4.9 y).
are given in Table 3. According to WMO, the stratospheric
degradation pathways of these ethers have not been adequately
measured, and the published atmospheric lifetimes are simply
upper bounds.19
A more challenging extension of this method is to olefinic
species. Reaction of the chloro radical with olefins does not
typically proceed by hydrogen atom abstraction. Instead, the key
propagation step, addition of chloro radical to the olefin with loss
of energy by collision with another molecule, is termolecular and
hence pressure-dependent in a manner that the hydrogen atom
abstraction pathway is not. For this reason, a valid relative rate
comparison should be possible only from olefin to olefin (or other
unsaturated species reacting by Cl• addition). As a brief test of
the simplest olefin-to-olefin comparison, ethene and propene were
chosen as analytes. An experiment proceeded as with HFCs and
HFEs, but using much shorter irradiation times (fractions of a
second) and using 1H NMR detection. Under these conditions
(23 ± 1 °C, 1.0 ± 0.1 bar internal pressure), propene was found to
react 1.25 times faster than ethene. The IUPAC-preferred values
for the absolute rates of reaction for these compounds with Cl•
at 298 K and 1 bar are 1.1 × 10-10 cm3 molecule-1 s-1 for ethene
and 2.3 × 10-10 cm3 molecule-1 s-1 for propene (2.1 times faster),
and ∆ log k ) ± 0.3 in both cases.21 The agreement of the relative
reaction rate obtained with this method and that of the literature
is encouraging, and supports our belief that this method should
be applicable to unsaturated species as well.
CONCLUSIONS
A method of estimating atmospheric lifetimes by chlorine
photolysis as measured by gas-phase NMR has been described.
The estimates so generated are only approximate measures of
the true lifetimes. Nevertheless, the method is sufficiently accu-
rate to serve as a “quick screen”, which is readily applicable early
in the process of consideration of a new industrial product. As
the method requires only a vacuum line, a UV lamp, a sealable
NMR tube, and a NMR probe capable of 19F (or 1H) detection, it
is applicable in most ordinary synthetic chemical laboratories.
Even the practice of flame sealing, facile in our laboratories, can
be obviated by use of commercially available valved NMR tubes.
Moreover, a duplicate set of experiments can be completed in a
day’s time with only milligrams of analyte, rendering the method
fast and inexpensive.
The use of NMR detection in the gas phase may be applicable
to more sophisticated indirect rate measurements. NMR intrinsi-
cally provides structural information beyond that furnished by GC,
IR, or even mass spectrometric techniques. Moreover, particularly
for fluorinated compounds, the efficient relaxation processes, and
attendant fast pulse repetition rate in the gas phase, yield greater
sensitivity than is typically appreciated for quantitative NMR
studies.
It is of interest to demonstrate the applicability of this method
to classes of compounds other than HFCs. An obvious extension
is to hydrofluoroethers, which should undergo free-radical de-
composition by the same hydrogen atom abstraction mechanism.
CF3CF2OCH3 was tested in two experiments with CF2HCH3 as
the referent, and CF2HCF2OCH3 was tested in one experiment
against CF2HCH3 and in another against CFH2CH3. The results
(18) IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of
Working Group I to the Third Assessment Report of the Intergovernmental
Panel on Climate Change; Houghton, J. T., Ding, Y., Griggs, D. J., Noguer,
M., van der Linden, P. J.; Dai, X., Maskell, K., Johnson, C. A., Eds.;
Cambridge University Press: New York, 2001.
(19) WMO, 2003: Scientific Assessment of Ozone Depletion: 2002. Global Ozone
Research and Monitoring Project-Report No. 47; World Meteorological
Organization: Geneva.
(20) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.;
Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical
Kinetics and Photochemical Data for Use in Stratospheric Modeling; National
Aeronautics and Space Administration, Jet Propulsion Laboratories: Pasa-
dena, CA,1997; Evaluation 12.
ACKNOWLEDGMENT
The authors thank Dr. Mack McFarland, DuPont Chief
Atmospheric Scientist, for a series of helpful communications, and
gratefully acknowledge the contributions of Dr. Barbara Minor
kinetic.ch.cam.ac.uk, 2006.
Analytical Chemistry, Vol. 80, No. 16, August 15, 2008 6321