Environ. Sci. Techno/. 1995, 29, 247-250
Atmospheric Chemistry of CF30H: Is Photolysis Important? W. F. SCHNEIDER* A N D T. J. W A L L I N G T O N Ford Research Laboratory, Ford Motor Company, P.O.Box 2053, Mail Drop 3083ISRL, Dearborn, Michigan 48121 -2053
K. MINSCHWANER' National Center for Atmospheric Research, Boulder, Colorado 80307-3000
E . A . STAHLBERG Cray Research, Inc., 655E Lone Oak Drive, Eugen, Minnesota 55121
The likely impact of photolysis of CF30H to produce CF30 radicals in the stratosphere is considered. Combination of the known absorption spectrum of CH30H with the calculated solar flux as a function of altitude provides an estimate of its lifetime with respect to photolysis. CISD calculations on the ground and first excited states of CF30H indicate that the first valence absorption maximum has an energy of 9.05 eV, blue-shifted 2.24 eVfrom that of CH30H. Estimates of the minimum likely lifetime of CF30H with respect to photolysis are obtained by shifting the absorption spectrum of CH30H to higher energy and recombining with the calculated solar flux. For altitudes below 40 km, the lifetime of CF30H with respect to photolysis is on the order of millions of years. Photolysis of CF30H is of no atmospheric importance.
Introduction Chlorofluorocarbons(CFCs) are being replaced with more environmentally acceptable alternatives such as hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs). For example, HFC-134a is areplacement for CFC12 in automotive air conditioning systems. With their large scale industrial use, the environmental impact of the atmospheric release of HFCs and HCFCs needs to be considered (I). CF3 radicals are produced during the atmospheric oxidation of a variety of HFCs, including HFC-23 (21, HFC125 (3-3, HFC-134a (6, 7), and HCFC-123 (4, 8-10). In the atmosphere, CF3 radicals react with 0 2 to give CF3O2 radicals which, in turn, react rapidlywith NO to form CF30 radicals (11, 12): CF, 0, M CF,O, M (1)
+ + + CF30, + NO - CF,O + NO,
(2) The atmospheric chemistry of CF30 radicals has been the subject of intensive research over the past 2 years. It is now recognizedthat the fate of CF30 radicals is the reaction with organic compounds (13-17) andpossiblyH20 (18) to produce CF30H and the reaction with NO to produce COF, and FNO (13, 19-23): CF,O RH CF,OH R (3)
-
+ + CF,O + NO - COF, + FNO
(4) Some workers have speculated that CF30, (CF30and CF302) radicals could participate in catalytic ozone destruction cycles in the stratosphere. Many potentially catalytic ozone destruction cycles involving CF30, radicals are possible (24). Examples include:
+ 0, - CF,O + 20, CF,O + 0, - CF,O, + 0,
I
CF,O,
and CF30,
I1
+ 0, - CF,O + 20,
+ C10 + M - CF,OOCl + M CF,OOCl + hu - CF,O, + C1 c1 + 0, - c10 + 0,
CF,O
(5) (6)
(5) (7)
(8)
(9) The ability of cycles such as I and I1 to destroy stratospheric ozone depends upon the balance between the efficiency of chain propagation reactions and chain termination reactions. Recent experimental studies of reaction 6 have detected no (25-28) or a very slow (29) reaction between CF30 radicals and 0,. Upper limits for k~ lie in the range (0.2-6) x 10-14 cm3molecule-'s-l at ambient temperature. In addition, upper limits of ks < (0.3-1) x cm3 molecule-' s-' have been derived for the reaction of C F ~ O Z radicals with ozone (25, 27, 29). Given the low reactivity of CF30, radicals toward 0 3 and the presence of efficient chain termination reactions, it is now generally accepted that CF30x-basedcatalytic cycles have a negligible impact on stratospheric ozone (25-29). * Author to whom correspondence should be addressed. Present address: Department of Physics, New Mexico Institute of Mining and Technology, Socorro, NM 87801. +
0013-936W95/0929-0247$09.00/0
0 1994 American Chemical Society
VOL. 29, NO. 1.1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 1247
TABLE 1
Calculated Results for Ground and First Singlet Excited States of CHJOH and CF3OH CHiOH
RHF M P2 MP3 CISD dipoleC
ChOH
'A (eu)
' A (au)
Aa (eV)
A' (nm)
'A (au)
' A (au)
An (eV)
An(nm)
Mb (eV)
-115.06137 -115.42147 -115.43938 -115.41286 1.7238
-114.83711 -115.16495 -115.18274 -115.16281 5.9863
6.10 6.98 6.98 6.80
203 178 178 182
-41 1.70435 -412.64440 -412.64430 -412.51738 2.0198
-41 1.39478 -412.30397 -412.30439 -412.18494 4.9921
8.42 9.26 9.25 9.05
147 134 134 137
2.32 2.28 2.27 2.24
Mulliken Analyses CHjOH chargesd
C 0 H X
x popse
OPX OP, OPZ
CFjOH
'A (au)
' A (au)
Aa (eV)
'A (au)
' A (au)
Aa (eV)
0.8835 -0.4999 0.1024 -0.2055 -0.1403 1.3689 1.3369 1.9152
0.9123 -0.0353 -0.5989 -0.2082 -0.0350 1.5420 1.4538 1.0916
0.0288 0.4646 -0.7013 -0.0027 0.0487 0.1731 0.1169 -0.8236
1.9028 -0.5222 0.1409 -0.4843 -0.5186 1.1953 1.4926 1.8979
2.0460 -0.1726 -0.5959 -0.4414 -0.4181 1.3439 1.6893 1.0234
0.1432 0.3496 -0.7368 0.0429 0.1006 0.1485 0.1967 -0.8745
a Energy difference between 'A' and ' A states. Difference between CFJOH and CHJOH excitation energies. Debye. X = out-of-plane H or F centers; X = in-plane H or F centers. Z-axis normal to the symmetry plane.
However, uncertainties remain in our understanding of the atmospheric chemistry associated with CF30, radicals. For example, as noted by KO et al. (241, if CF30 radicals are regenerated from CF30H, then the efficiency of reaction 3 as a sink for CF30 radicals will be diminished and ozone destruction cycles may be enhanced. We have previously demonstrated that chemical processes that convert CF3OH into CF30 radicals are of negligible significance in the atmosphere (30). Another possibility is the regeneration of CF30 radicals from CF30H by a photolytic process: CF30H hv CF30 H (10) Both CH30H and CH3CH20H dissociate photolytically to generate alkoxy radicals (34,and CF30H may also dissociate in this manner. CH30H and other aliphatic alcohols absorb light only in the vacuum ultraviolet region of the spectrum (32, 33). The first band maximum for CH30H and the only one of any potential atmospheric import occurs at 183 nm with an intensity of about 170 L mol-' cm-' (6.5 x cm2 molecule-') (33). This band is essentially unchanged in other small aliphatic alcohols, but is blue-shifted dramatically when these higher alcohols are fluorinated (33). The instability of CF3OH with respect to HF elimination makes experimental examination of its vacuum ultraviolet spectrum difficult. In this work, we perform two separate sets of calculations to examine the possibility of atmospheric photolysis of CF3OH. First, ab initio molecular orbital calculations are used to examine the relative energetics of the first accessible excited state of CH30H and CF30H. The first band maximum for CF30H is predicted to be blueshifted 2.3 eV from that of CH30H to deep in the vacuum ultraviolet region of the spectrum. Second, the solar radiation flux is calculated as a function of altitude and is combined with the known absorption cross section of CH3OH (321,both unaltered and blue-shifted by several energy increments, to obtain an estimate for the photolysislifetimes of CH30H and CF30H in the stratosphere. On the basis of these calculations, we conclude that the photolysis of CF3OH proceeds extremely slowly and is of no atmospheric importance.
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Electronic Structure Calculations Hartree-Fock and single-reference configuration interaction singles and doubles (CISD)calculationswere performed on the ground VA') and first singlet excited states ( ' A ) of both CH30H and CF30H using the Columbus programs (34). The geometric parameters were fixed at their MP2/ 6-31G(d,p) optimized values (18). The CI calculations employed Dunning's augmented valence double [ basis set (aug-cc-pVDZ) (39, with the 1 s orbitals remaining uncorrelated. The resulting CI expansions consisted of 67 017 and 152 688 configuration state functions (CSFs) for the 'A'and 'A" states of CH30H, respectively, and 683 175 and 1 633 365 CSFs for the 'A' and 'A" states of CF30H, respectively. Population analysisand dipole moments were calculated for each state using the CISD natural orbitals. The computational results are presented in Table 1. The first electronic transition for CH30H has its band maximum at about 6.78 eVand has previously been assigned as n 8 in character (33). From Table 1, the energy of the transition is poorly reproduced at the Hartree-Fock level, but inclusion of electron correlation at any level greatly improves the results. The calculated vertical transition energy is 6.80 eV or 182 nm at the CISD level, in excellent agreement with the experimental band maximum. Population analysis of the ground and excited states using the CISD natural orbitals is consistent with the n u* assignment. At all levels of theory considered here, the vertical transition energy to the l A state of CF30H is predicted to increase by approximately 2.3 eV relative to CH30H. At the CISD level, the predicted vertical transition energy is 9.05 eV, corresponding to a band maximum at 137 nm. The changes in populations of the CISD natural orbitals are similar to those observed for CH30H and again are consistent with an n u* description of the transition. The calculated excited states of CH30H and CF30H are clearly of the same origin and character. Further, consideration of the Hartree-Fock orbitals indicates that no other lower energy valence transitions are accessible to CF30H. Rydberg transitions of comparable or lower energy may be
-
-
-
Wavelength ( n m )
I
200
100
300
400
- j/ Q, -
3
NS L
0
X 0)
tl
I
A /v 170
180
190
t
lo'*
v Y
X
o ' " " " ' " " " ' ' ' ~ ~ ~ ' " 160
'
N v,
200
2
210
Wavelength ( n m ) FIGURE 1. UV spectrum of CHsOH taken from ref 33.
availablebut were not investigated. The abilityto reproduce the CH30H band maximum along with the consistency of the calculated blue shift from CH30H to CF3OH both provide strong support for the calculated results. Based on the ab initio calculations, the first valence band maximum for CF3OH is predicted to be considerably blue-shifted compared to CH3OH and to reside deep in the vacuum ultraviolet region of the spectrum. In considering the source of the dramatic hypsochromic shift from CH30H to CF30H, we note that the HartreeFock energies of the lowest unoccupied orbitals (LUMOs) of the two species are very similar, but that the highest occupied orbital (HOMO) drops in energy by several eV from CH30H to CF30H. The a" HOMO is predominantly oxygen lone pair in character and as previously discussed is stabilized by a hyperconjugativeinteraction with the CF3 group (36). This same effect accounts for the unusually strong C-0 and 0-H bonds in CF30H (36).
10"
LL L
-
6
loto lo9 1 o8
160
170
190
180
200
Wavelength ( n m ) FIGURE 2. Direct solar flux over the wavelength range 100-400 nm at the top of the atmosphere (labeled -1 and at 80,40,20, end 0 km altitude (indicated curves). The bottom panel is en expanded region of the spectrum between 160 and 200 nm corresponding to absorption in the Schumann-Runge bands of 02. See ref 43.
Solar Flux and CFlOH Photolysis Calculations
The rate of photolysis of CF30H at a particular altitude can be obtained by combiningthe absorption cross section for the molecule with the solar radiation flux. Calculation of the entire absorption envelope of CF30H is not possible. We use instead the known spectrum of CH30H (33) as shown in Figure 1as a surrogate for CF30H and investigate the dependence of photolysis rate on the location of the band maximum as it is shifted to the blue. Figure 2 shows the solar flux expressed in photons cm-2 s-l nm-l as a function of wavelength at the top of the atmosphere and at altitudes of 40, 20, and 0 km. The calculation employs the SUSIM solar irradiances (37), the U.S. Standard Atmosphere (38), and a solar zenith angle of 30". The resolution is approximately 1.0 nm. Ozone cross sections were taken from DeMore et al. (39);oxygen cross sections in the Herzberg continuum were from Yoshino et al. (40); and the Schumann-Runge band absorption follows the high-resolution treatment of Minschwaner et d. ( 4 1 ) ,with transmission spectrally degraded to 1.0 nm. Schumann-Runge continuum cross sections are from Ogawa and Ogawa (42). The fluxes correspond to the direct solar beam and do not include scattered radiation, which to a good approximation can be neglected below 195 nm (43). For a given wavelengh, multiplication of the solar flux by the molecular absorption cross section gives the pho-
1 0 - * i o - ~i o o
io' io2 lo3
10'
io5 io6
10' l o B
Lifetime ( y e a r s ) FIGURE 3. Calculated lifetimes of CHjOH and of species with the seme absorption spectrum but blue-shifted by the indicated amounts with respect to photolysis as a function of altitude.
tolysis rate, assuming unit quantum efficiency. Integration over all wavelengths then gives the total photolysis rate, and the reciprocal of the photolysis rate is then the molecular lifetime with respect to photolysis (44). In Figure 3 the lifetimes with respect to photolysis of CH30H and of hypothetical species with band maxima shifted to the blue by 10, 15, and 20 nm are plotted as a function of altitude. The absorption envelope is shifted such that the shape is constant in the frequency domain, so that the band narrows slightly in the wavelength domain. In deriving the data in Figure 3, allowance was made for the diurnal variation of the solar flux by dividing the photolysis rate at the 30" solar zenith angle by a factor of 2. VOL. 29. NO. 1,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY a 249
As seen from Figure 3, photolysis of CH30H is slow at altitudes below 25 km, and the rate of photolysis decreases rapidly as the absorption maximum is shifted to the blue. AblueshiftoflOnm(0.39eVstartingfrom 183nm)increases the lifetime with respect to photolysis below 25 km to 1000 years and a blue shift of 20 nm (0.83 eV starting from 183 nm) to 100million years even at 40 km. The band maximum for CF30H was calculated above to be at 137 nm. If this result is correct, then CF30H has essentially an infinite lifetime with respect to photolysis in the stratosphere, and photolysis of CF30H will be completely unimportant until the very highest reaches of the atmosphere. Even if the blue shift of this band maximum is grossly overestimated by the calculations, and if the absorption cross section for CF30H is an order of magnitude greater than that for CH3OH, photolysis will occur to only a negligible extent in the stratosphere. Physical transport of CF30H from the stratosphere occurs on the time scale of 2-3 years, and once in the lower atmosphere, CF30H is expected to be removed rapidly by incorporation into rainwater-seawater-cloudwater. From the results presented above, it is clear that photolysis is of no importance in the atmospheric chemistry of CF30H.
Conclusions CF30 radicals in the stratosphere react with hydrogen donors (e.g., HOZradicals or CH4)to generate CSOH. We show here that photolysis of CF30H to regenerate CFsO radicals is of no importance and have previously shown that chemical processes that convert CF30H into CF30 radicals are also unimportant (30). There is no known mechanism by which CF30H can be readily converted into CF30radicals in the stratosphere. CF30H is removed from the stratosphere either by heterogeneous decomposition on aerosol particles to give COF2 and HF or by transport to the lower atmosphere. The rate of heterogeneous decomposition is unknown. Transport from the stratosphere takes place over a time scale of 2-3 years. Once in the lower atmosphere, CF30H will be rapidly removed by incorporation into rainwater-seawater-cloudwater where hydrolysis w ill give COz and HF. CF3OH is a permanent sink for CF30, radicals and should be treated as such in model evaluations of the ozone depletion potentials of HFCs containing CF3 groups, such as HFC-134a.
Acknowledgments We thank Steve Japar and Matti Maricq (Ford)and Malcolm KO (Atmospheric and Environmental Research, Inc.) for helpful comments and Ole John Nielsen and Jens Sehested (Rise, National Laboratory, Denmark) for ongoing discussions regarding CF30, chemistry.
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ES940339J
@
Abstract published in AdvanceACSAbstracts, November 1,1994.
