J. Phys. Chem. 1996, 100, 6097-6103
6097
Energetics and Mechanism of Decomposition of CF3OH William F. Schneider* and Timothy J. Wallington Ford Research Laboratory, Ford Motor Company, P.O. Box 2053, Mail Drop 3083/SRL, Dearborn, Michigan 48121-2053
Robert E. Huie Chemical Kinetics and Thermodynamics DiVision, National Institutes of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: September 14, 1995X
Ab initio calculations are used to examine the energetics of unimolecular and water-mediated decomposition of CF3OH into COF2 and HF. The calculations indicate that the barrier to unimolecular decomposition is large (42 ( 3 kcal mol-1) and that the rate of this reaction is negligible at room temperature. This reaction is of no importance under ambient atmospheric conditions. The calculations also reveal a substantially lower energy pathway for decomposition that is accessible via a reaction between CF3OH and water. This pathway involves formation of a six-membered-ring transition state, with water acting as a bridge between the fluorine and hydrogen of the alcohol. The existence of this lower energy pathway is consistent with experimental evidence for the intermediacy of H2O in the decomposition of CF3OH. From the computational results the second-order rate constant for homogeneous decomposition can be estimated to lie in the range 10-27 to 10-22 cm3 molecule-1 s-1 at 298 K and is likely too small to be atmospherically significant. The rate for heterogeneous decomposition cannot be estimated from the computational results, but the results are consistent with a prominent role for heterogeneous decomposition in the atmospheric chemistry of CF3OH. CF3O + NO f COF2 + FNO
Introduction Hydrofluorocarbons (HFCs) are “environmentally friendly” replacements for chlorofluorocarbons (CFCs) in industrial and consumer applications. Because of the potential widespread use and eventual environmental release of HFCs, considerable effort has been expended in understanding the atmospheric fate of these compounds.1 It is accepted that, unlike the CFCs they replace, HFCs have no significant adverse effect on stratospheric ozone.2 Further, the main features of the atmospheric degradation mechanisms have been delineated for the HFCs likely to find widest use, such as HFC-134a (CF3CFH2) used in automobile air conditioning systems. However, questions still remain about the details of the degradation processes, in particular of secondary compounds formed during the atmospheric oxidation of HFCs. Trifluoromethanol (CF3OH) is one such secondary compound that is of some importance. CF3OH is formed in the atmosphere from CF3 radicals, which in turn are produced during the degradation of many HFCs, including for instance HFC-23,3 HFC-125,4-6 HFC-134a,7,8 and HFC-143a.9 The mechanism of formation of CF3OH from CF3 radicals is well understood. In the oxygen-rich atmosphere, CF3 radicals react rapidly with O2 to form CF3O2 radicals,10,11 which then react with NO to generate CF3O radicals:12
CF3 + O2 + M f CF3O2 + M
(1)
CF3O2 + NO f CF3O + NO2
(2)
Two primary atmospheric degradation reactions of CF3O radicals have been identified. CF3O radicals react with NO to generate COF2 and FNO:13-16 * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-6097$12.00/0
(3)
Alternatively, CF3O radicals react with hydrocarbons15,17-20 or perhaps even H2O21,22 to form CF3OH:
CF3O + RH f CF3OH + R
(4)
CF3O + H2O f CF3OH + OH
(5)
Calculations indicate that the CF3O-H bond is unusually robust, with a bond strength of 119.4 ( 2 kcal mol-1 (1 kcal ) 4.184 kJ), which is slightly greater than that of water.23 From theoretical and experimental considerations it can be concluded that CF3OH formed via either reaction 4 or 5 will not react significantly with trace atmospheric constituents (such as OH radicals, O atoms, and Cl atoms)24 and will not be appreciably photolyzed25 and thus that other loss processes for CF3OH will be important. The fate of CF3OH is uncertain, but unimolecular decomposition to COF2 and HF is one possible loss mechanism:
CF3OH f COF2 + HF
(6)
The first ab initio study of the decomposition pathways available to CF3OH found reaction 6 to be endothermic by 15 kcal mol-1 at room temperature.26 While entropy favors reaction 6 by approximately 10 kcal mol-1 at 298 K, this contribution is not sufficient to drive the reaction thermodynamically. While these early computational results would predict CF3OH to be thermodynamically stable at room temperature, CF3OH formed in smog chamber experiments is observed to decompose readily to yield COF2. The rate of CF3OH decomposition varies widely depending upon the history of the reaction chamber, strongly suggesting a large heterogeneous component to the process.21 Chlorinated methanols have also been found to behave in a similar fashion.27 Recent ab © 1996 American Chemical Society
6098 J. Phys. Chem., Vol. 100, No. 15, 1996
Schneider et al.
initio evaluations of the heats of formation of CF3OH23 and of COF228,29 are in accord with the experimental evidence, predicting reaction 6 to be endothermic by only 6.9 kcal mol-1 and thus thermodynamically favored in the gas phase to temperatures as low as 200 K.29 The activation barrier to reaction 6 has also been determined at various levels of theory, and a value for E0 of 45.1 kcal mol-1 has been obtained.26,30 This large barrier implies a negligible reaction rate at normal temperatures, and on the basis of these results, homogeneous decomposition of CF3OH in the atmosphere via reaction 6 has heretofore been considered unlikely. Alternative loss processes, such as incorporation into water droplets or decomposition on aerosols, are thought to dominate the fate of CF3OH. Given the earlier uncertainties in the energetics of reaction 6, we thought it appropriate to revisit the activation barrier to decomposition. In this work, the barrier to direct reaction is predicted to be slightly less than previously reported. However, the resultant reaction rate is predicted to be much too small for homogeneous unimolecular decomposition of CF3OH to be important in the atmosphere. The large activation barrier calculated for reaction 6 lends support to the argument that heterogeneous processes account for the loss of CF3OH observed in smog chamber experiments. Heterogeneous loss processes are well-known to be important in atmospheric chemistry, for instance on the surface of water/ acid aerosol particles.31 Further, the decomposition of CF3OH is accelerated in smog chamber experiments by the addition of H2O vapor, presumably through reaction of CF3OH on the H2Ocoated chamber surface and/or through chemistry on gas-phase H2O droplets. The apparent intermediacy of adsorbed H2O and/ or H2O vapor observed in the smog chamber experiments suggests a simple molecular model for the heterogeneous decomposition process. We consider here the interaction of a single H2O molecule with CF3OH and examine the effect of this interaction on the barrier to decomposition of CF3OH:
CF3OH + H2O f COF2 + HF + H2O
TABLE 1: RHF/SVP Optimized Geometriesa r(C-O) r(C-F1)b r(C-F2)b r(O-H) r(H-F) ∠(O-C-F1) ∠(O-C-F2) ∠(H-O-C) ∠(F2-C-O-F′2) ∠(H-O-C-F1)
CF3OH
[CF3OH]q
COF2
1.3314 1.3037 1.3203 0.9468 2.4192 108.95 111.97 110.55 120.10 180.0
1.2428 1.7663 1.2709 1.1737 1.1959 87.45 122.23 83.68 153.34 0.0
1.1585
HF
1.2893 0.9030 125.87 180.0
a Distances in angstroms and angles in degrees. b F is the one 1 symmetry-unique fluorine; F2 are the two symmetry-equivalent fluorines.
reactant and product states.37 The saddle points were also tested to ensure stability with respect to relaxation of the spin-restricted wave functions to spin-unrestricted forms. The energies were evaluated at the critical points at the fourth-order Møller-Plesset (MP4) level38 using a triple-ζ plus two polarization function (TZ2P, [5s3p2d/3s2p]) basis set.33 Spherical harmonic d functions were employed in the perturbation calculations, and the lowest energy core and corresponding highest energy virtual orbitals were kept frozen. The energetic results for all species studied are summarized in Table 4. The Hartree-Fock geometries and vibrational frequencies were used to calculate zero-point vibrational energies, internal energy corrections, and molecular partition functions, and these are listed in Table 3. Vibrational frequencies are in general uniformly overestimated at this level of theory;39 to compensate for this systematic error, the frequencies were scaled by 0.9 before being used in further calculations. Internal translational, rotational, and vibrational energy corrections to 298.15 K were calculated using standard statistical mechanical formulas.40,41 Rate constants were estimated using transition state theory based on standard formulations.42
(7) Results and Discussion
Clearly this model is not an accurate representation of the interaction of CF3OH with a H2O surface. However, it does provide a starting point from which to understand the mechanism of CF3OH decomposition on H2O surfaces. The reported calculations indicate that a single H2O molecule can serve as a template for the decomposition of CF3OH. Its intermediacy opens up an alternative, lower energy pathway to COF2 and HF, reducing the reaction barrier by 25 kcal mol-1 relative to the unimolecular pathway. The computational results suggest that the decomposition of CF3OH can be accelerated by H2O heterogeneously and provide a model for how such an interaction could occur. Computational Details Calculations were performed using the GAMESS32 and ACESII33 ab initio molecular orbital programs. The geometries of all species were determined by gradient optimization34,35 at the restricted Hartree-Fock level using Dunning and Hay’s split valence plus polarization (SVP) basis set.36 The structural parameters for the species studied are included in Tables 1 and 2. The structure of H2O was also determined and at this level of theory has an O-H bond length of 0.9439 Å and an H-O-H bond angle of 106.70°. Harmonic vibrational frequencies were evaluated analytically at all critical points and are listed in Table 3. The transition states were identified by a unique imaginary vibrational frequency, and intrinsic reaction coordinate following was used to ensure that the structures connected the desired
Unimolecular Decomposition. We consider first the unimolecular decomposition of CF3OH via reaction 6. The RHF/ SVP geometric parameters for CF3OH, COF2, HF, and the transition state connecting reactants to products, [CF3OH]q, are presented in Table 1. The Hartree-Fock structures obtained here do not differ appreciably from the MP2 structures reported previously,26,30 and we do not expect the use of the HartreeFock structures to introduce significant error into our results. CF3OH adopts a Cs structure with the hydroxyl hydrogen oriented gauche to the adjacent CF3 group. The saddle point [CF3OH]q also has Cs symmetry, but with the hydroxyl hydrogen oriented eclipsed to the leaving fluorine (Figure 1). A saddle point for HF elimination was searched for on the C1 surface between the Cs minimum and Cs saddle point, but none was found. The Cs transition state is highly distorted, with the C-F bond of the leaving fluorine lengthened by 0.35 Å and the H-O-C angle reduced to only 84°. The structure is in fact quite similar to the four-centered transition state found for the elimination of H2O from H2SO4.43 Examination of the intrinsic reaction coordinate connecting CF3OH to [CF3OH]q indicates that HF is lost in an asymmetric fashion. The C-F bond to be broken lengthens steadily as the transition state is approached, but the O-H bond length maintains a constant value until very near the transition state, when it begins to increase rapidly. The resultant transition state structure is reminiscent of the transition state for the symmetry forbidden addition of H2 to ethylene. While the addition of HF to COF2 (or the elimination of HF
Decomposition of CF3OH
J. Phys. Chem., Vol. 100, No. 15, 1996 6099
TABLE 2: RHF/SVP Optimized Geometriesa [CF3OH‚H2O]q CF3OH‚H2O r(C-Oa) r(C-F1) r(C-F2) r(C-F3) r(Oa-Ha) r(Ow-Hw) r(Ow-Hw′) r(Ow-Ha) r(F1-Hw) a
1.2194 1.6583 1.3031 1.3018 1.5587 1.0483 0.9490 1.0030 1.3248
1.3196 1.3257 1.3257 1.3082 0.9577 0.9451 0.9451 1.8426 3.6154
[CF3OH‚H2O]q CF3OH‚H2O ∠(Oa-C-F1) ∠(Oa-C-F2) ∠(Oa-C-F3) ∠(C-Oa-Ha) ∠(Hw-Ow-Hw′) ∠(Oa-Ha-Ow) ∠(Ha-Ow-Hw) ∠(F1-Hw-Ow) ∠(C-F1-Hw)
107.15 120.13 120.34 114.24 112.97 144.95 94.46 154.05 105.06
[CF3OH‚H2O]q CF3OH‚H2O ∠(F2-C-Oa-F1) ∠(F3-C-Oa-F1) ∠(F1-C-Oa-Ha) ∠(Oa-Ha-Ow-Hw) ∠(C-F1-Hw-Ow) ∠(Hw′-Ow-Hw-F1)
112.45 112.45 109.68 110.60 107.53 173.28 121.81
110.18 110.36 2.99 1.52 0.09 116.17
120.12 120.12 59.88 71.65
Distances in angstroms and angles in degrees. Subscripts a and w refer to atoms originating from the alcohol and from water, respectively.
TABLE 3: RHF/SVP Vibrational and Thermodynamic Parameters vibrational frequencies (cm-1) HF H2O COF2 CF3OH [CF3OH]q CF3OH‚H2O
[CF3OH‚H2O]q
a
4511 1754 638 253 998 277 1056 37 473 996 3950 56 576 1411 3174
4167 683 481 1246 355 1103 53 490 1291 4158 319 641 1467 4158
4291 880 493 1364 598 1564 74 670 1339 4275 368 672 1606 726i
1098 658 1474 618 1799 192 682 1502
1455 684 1563 739 2245 228 696 1594
2184 696 4181 929 2050i 264 790 1760
387 676 1754
465 848 1835
552 1057 2398
zero-point energya
thermal energya
ln Qb
9.25 20.94 14.22 28.89
2.36 2.84 3.29 4.47
61.98 63.27 67.00 73.79
23.13
4.39
73.87
52.31
8.24
81.04
50.07
6.23
76.98
Energies in milliHartrees based on scaled vibrational frequencies. b Log of total partition function.
TABLE 4: Electronic Energies (au) HF H2O COF2 CF3OH [CF3OH]q CF3OH‚H2O [CF3OH‚H2O]q
RHF/SVP
MP2/SVP
RHF/TZ2P
MP2/TZ2P
MP4/TZ2P
-100.047 83 -76.046 83 -311.697 81 -411.767 43 -411.667 23 -487.829 07 -487.761 18
-100.229 58 -76.241 47 -312.318 75 -412.564 43 -412.488 66 -488.824 13 -488.782 43
-100.063 64 -76.060 82 -311.744 72 -411.824 92 -411.727 43
-100.310 16 -76.300 24 -312.538 17 -412.858 91 -412.785 41
-100.318 85 -76.313 42 -312.570 28 -412.900 47 -412.827 71
-487.830 39
-489.132 59
-489.186 77
TABLE 5: Relative Energies (Including ZPE) (kcal mol-1) MP2/SVP MP2/TZ2P MP4/TZ2P
CF3OH (+H2O)
[CF3OH]q
CF3OH‚H2O
[CF3OH‚H2O]q
COF2+HF (+H2O)
0 0 0
43.94 42.51 42.05
-9.88
14.88 16.82 17.17
6.71 3.24 3.72
from CF3OH) is not symmetry forbidden, the large barrier to this process may arise in part from its similarity to the symmetry forbidden H2 plus ethylene reaction. The energetics of the unimolecular reaction were studied at the MP2 level using the SVP basis set and up to the MP4 level using the TZ2P basis set, and the results are presented in Tables 4 and 5. The larger basis set and higher level correlation treatment have a relatively minor effect on the calculated energetics and reaction barrier. The gas-phase enthalpy of reaction 6, corrected to 298 K, is calculated to be 8.0 and 5.0 kcal mol-1 at the MP2/SVP and MP4/TZ2P levels, respectively. These results are in good agreement with the enthalpy obtained by indirect calculation from the heats of formation (6.9 kcal mol-1)29 and when entropy contributions are incorporated are consistent with the observed spontaneous decomposition of CF3OH in smog chambers. Interestingly, the reaction enthalpy predicted for reaction 6 by direct calculations at the MP4/631G(d) level (14.9 kcal mol-1 corrected to 298 K)26 or even at the QCISD(T)/6-311G(2df,2p) level (9.1 kcal mol-1 corrected to 298 K)30 is greater by several kcal mol-1 than that obtained
here or calculated from the heats of formation. The degree of endothermicity of reaction 6 is of practical import; because the reaction is entropically driven in the gas phase, it is thermodynamically allowed only above temperatures determined by the endothermicity. The results here do not suggest any modification to the thermodynamics previously proposed for reaction 6.29 Because the earlier direct calculation overestimated the overall energetics of reaction 6, we thought it possible that the barrier to reaction also was overestimated and perhaps that the reaction may have a non-negligible room temperature rate. From Table 5, E0 for [CF3OH]q is calculated to be 43.94 and 42.05 kcal mol-1 at the MP2/SVP and MP4/TZ2P levels, respectively. As with the overall energetics, the larger basis set and level of correlation treatment have a relatively small effect on the calculated energy barrier. The MP4/TZ2P barrier is slightly less than that previously reported, although not enough so to greatly increase the rate of reaction.26,30 On the basis of comparisons between the MP2/SVP and MP4/TZ2P results, as well as with the earlier reported results for reaction 6, we believe
6100 J. Phys. Chem., Vol. 100, No. 15, 1996 that the MP4/TZ2P results are reasonably converged with respect to basis set size and level of correlation treatment. On the basis of these comparisons, we estimate that the relative energy of [CF3OH]q calculated here is converged to within (3 kcal mol-1. We choose to report the calculated barrier as 42 ( 3 kcal mol-1. The reduced barrier height calculated here results in larger estimates for the unimolecular rate constant. At 298 K, a 42 kcal mol-1 barrier leads to a first-order rate constant k6 of 1 × 10-18 s-1, based on a direct application of transition state theory.42 The imaginary harmonic vibrational frequency along the reaction coordinate is quite large (2050i cm-1 unscaled), indicating a narrow passage through the transition state, and tunneling will thus have a large effect on k6. A crude onedimensional estimate of the effect of tunneling is provided by the Wigner correction,44 which in this case leads to almost a 2 orders of magnitude increase in k6, to 8 × 10-17 s-1. While this rate constant is 5 orders of magnitude greater than that first suggested by Francisco,26 it is still negligibly small (corresponding to a lifetime of 400 million years!). Of course the estimated rate constant is highly sensitive to the magnitude of the barrier. For example, reducing the barrier to 35 kcal mol-1, or by more than twice the estimated error, increases the calculated rate constant to 1 × 10-11 s-1, corresponding to a lifetime of 3000 years. While the actual barrier may be less than that calculated here and the effects of tunneling greater, it is highly unlikely that either of these factors will be large enough to increase k6 by the amount necessary to make it atmospherically significant. In accord with Francisco’s comments, we conclude that unimolecular decomposition of CF3OH is of no atmospheric importance.30 Huey and co-workers45 have very recently reported experimental results for the unimolecular decomposition of CF3OH. From experiments at elevated temperatures, they obtain Arrhenius parameters consistent with the results reported here and further confirm that unimolecular decomposition of CF3OH is slow and unimportant under conditions typical in the atmosphere. Water-Mediated Decomposition. While the computational results (and recent experiments) make it clear that unimolecular decomposition of CF3OH cannot be important under typical laboratory or atmospheric conditions, CF3OH is known to decompose readily to form COF2 in the laboratory. The decomposition of CF3OH is likely accelerated by the interaction of the molecule with the surfaces of reaction chambers. While the chemical nature of the reaction chamber surfaces is complex and varies greatly with the chamber history, introduction of H2O vapor into reaction chambers is known to accelerate the presumably heterogeneous decomposition process.46,47 Further, Lovejoy and co-workers have recently found a large increase in the rate of decomposition of CF3OH on bulk water and sulfuric acid solutions.48 These experimental results indicate that H2O, either supported on the chamber walls or in aerosols, plays an important part in promoting CF3OH decomposition. To better understand the role of H2O in the decomposition of CF3OH and to demonstrate a possible mechanism for such an interaction, we have examined computationally the interaction of CF3OH with a single H2O molecule. A simple mechanism for the interaction of H2O with CF3OH leading to decomposition can readily be drawn. In this mechanism, H2O serves as a “hydrogen shuttle” between oxygen and fluorine in CF3OH. As shown in Scheme 1, by reducing the distortion of CF3OH necessary to transfer a hydrogen from oxygen to fluorine, H2O (in vapor or on surfaces) may promote decomposition to COF2 and HF. Similar mechanisms have been proposed in earlier computational investigations of the hydrolysis of SO343
Schneider et al.
Figure 1. RHF/SVP structures of [CF3OH]q and [CF3OH‚H2O]q.
SCHEME 1
and of HC(O)NH2,49 in which one additional H2O beyond the H2O of hydrolysis permits formation of a relatively low-energy six-membered-ring transition state structure and promotes the hydrolysis reaction. As we will show, such a mechanism is also reasonable for CF3OH. Table 2 contains the RHF/SVP geometric parameters for the transition state corresponding to the process described in Scheme 1, identified here as [CF3OH‚H2O]q. The structure is also given in Figure 1. In both the table and figure, the atoms are labeled with a subscript a to denote those originating from the alcohol and subscript w to denote those originating from water. That this structure is a transition state for the desired reaction is confirmed by the harmonic vibrational analysis, which reveals a unique imaginary mode (Table 3) pointing along the desired reaction coordinate, and the results of intrinsic reaction coordinate following, which clearly indicates that the transition state is connected smoothly with both reactants and products. Further, to examine the effects of correlation on the transition state geometry, MP2/SVP single-point calculations were performed at several points along the RHF/SVP reaction coordinate. The MP2/SVP energy was found to be a maximum at the RHF/ SVP transition state geometry, suggesting that this geometry is a reasonable approximation to the MP2/SVP transition state geometry. The [CF3OH‚H2O]q transition state itself contains a nearly planar six-membered ring, although the vibrational analysis reveals a very low energy out-of-plane ring deformation mode.
Decomposition of CF3OH
J. Phys. Chem., Vol. 100, No. 15, 1996 6101
Figure 2. Intrinsic reaction coordinates for [CF3OH]q and [CF3OH‚H2O]q, calculated at the RHF/SVP level. The results reveal the qualitative shapes but not the quantitative energetics of the two reaction pathways.
In accord with the Hammond postulate for an endothermic reaction, the structure is more product-like than reactant-like. The reaction products can be clearly identified in the transition state structure (Figure 1). The C-O bond is reduced to within 0.06 Å of its length in COF2, or 65% of the way from CF3OH to COF2, and the alcohol Oa-Ha and C-F1 bonds are lengthened to 1.56 and 1.66 Å, respectively, so that COF2 is nearly completely formed. The forming water O-H bond (Ow-Ha) is close to its equilibrium length. In contrast, the forming HwF1 bond is still quite long at the transition state, and scission of the water Ow-Hw bond apparently occurs late along the reaction path. The bond angle distortions necessary to reach the transition state are not nearly as great as those in [CF3OH]q, which contributes to the diminished barrier for this pathway. The internal ring angles vary between 105° and 121° about the heavy atoms, while the hydrogen-centered angles are close to linear. Further, any symmetry-derived contribution to the HF elimination barrier in CF3OH is relieved in the water-assisted pathway. These geometric results may be modified by a treatment that includes electron correlation effects, but the qualitative characteristics will be unchanged. Similar results for the energy of the transition state are obtained with the two basis sets (Table 5). The MP2/SVP calculations yield a reaction barrier E0 of 15 kcal mol-1, and the MP4/TZ2P calculations, a barrier of 17 kcal mol-1. Both results are considerably less than the [CF3OH]q energy. A comparison of the direct and water-assisted pathways, based upon the intrinsic reaction coordinate (IRC)50 following results, is presented in Figure 2. The IRCs were computed at the restricted Hartree-Fock level with the SVP basis, so that the absolute energetics in Figure 2 differ from those obtained by the correlated calculations, but the relative barrier lowering is accurately represented. The unimolecular reaction exhibits a steep rise in energy as the transition state is approached, and the barrier is sharply peaked, as characterized by the highfrequency imaginary mode. In contrast, the water-mediated pathway rises gradually to a broad and flat reaction barrier characterized by a small imaginary vibrational mode. Thus energetically [CF3OH‚H2O]q is much more accessible than [CF3OH]q. Because of the broad barrier, direct tunneling will likely be less important for [CF3OH‚H2O]q than for [CF3OH]q.
At the RHF/SVP level, one or more weakly bound complexes can also be located on the CF3OH + H2O surface, as evidenced by the dip in the [CF3OH‚H2O]q intrinsic reaction coordinate below the separated molecule limit. One such complex has been characterized here. It is designated CF3OH‚H2O, and the corresponding geometric parameters are listed in Table 2. The complex has Cs symmetry, with bonding occurring primarily between the water oxyen and alcohol hydrogen. Secondary interactions between both water hydrogens and two of the alcohol fluorines are also present. At the MP2/SVP level this complex is bound by 10 kcal mol-1. Basis set superposition error contributes approximately 1.6 kcal mol-1 to the binding energy, based on full counterpoise calculations,51 so that the net binding is closer to 8 kcal mol-1. Because of the small basis set used and limited treatment of correlation, the calculated binding energy for this weak, hydrogen-bonding interaction should be viewed with caution, but the results clearly indicate that such a complex is bound. The harmonic vibrational frequency analysis reveals a number of very low energy vibrational modes (Table 3) characteristic of a highly fluxional molecule. Bound CF3OH‚H2O complexes would likely exist only as transient species in the gas phase at ambient temperatures; they are not expected to have appreciable lifetimes of their own as distinct, strongly bound entities. However, the relatively strong association between CF3OH and H2O suggests that CF3OH may be readily accommodated on water surfaces or in the bulk and that these interactions may play a role in further promoting the elimination of HF from CF3OH. Qualitatively, then, the computational results are consistent with a role for H2O in the decomposition of CF3OH. While the barrier to unimolecular decomposition of CF3OH is large, a single H2O molecule is found both to interact with CF3OH in a stabilizing fashion and to provide a lower energy pathway for elimination of HF. The results closely parallel those obtained previously for the hydrolysis of SO343 and HC(O)NH2;49 while the direct reaction between either substrate and H2O involves formation of a four-membered transition state and is quite unfavorable, an additional H2O molecule both binds to the substrates and provides a lower energy pathway for hydrolysis through the formation of a six-membered transition state. This six-membered-ring structure may well be a common
6102 J. Phys. Chem., Vol. 100, No. 15, 1996 feature of all H2O-promoted reactions involving the transfer of H atoms. The available experimental evidence is most consistent with a heterogeneous reaction between CF3OH and H2O. It is difficult to make a quantitative connection between the results reported here and the likely actual heterogeneous process. Many important factors such as the influence of multiple water molecules in stabilizing the transition state and the perturbations due to aggregated or surface-bound water participating in the reaction have not been considered. Solvation is likely to further stabilize the transition state described above, as well as the products formed, and may accelerate the reaction to a greater extent than that suggested only by the lowering in the activation barrier. While the calculations cannot provide a precise estimate of the rate of heterogeneous reaction, they do indicate unambiguously that an alternative pathway for CF3OH decomposition is available in the presence of water and provide an indication of the mechanism of heterogeneous decomposition of CF3OH. It is interesting to consider the implications of these results for a homogeneous second-order reaction between CF3OH and H2O vapor.26 The calculated energy barriers and partition functions can be used to estimate the second-order rate constant k7 for the CF3OH plus H2O reaction using standard transition state theory.42 Because the transition state for this homogeneous process arises from the tight coupling of two independent, closed-shell molecules, the prefactor is expected to be very small. At 298 K it is calculated to be only 5.0 × 10-14 cm3 molecule-1 s-1. The prefactor is dominated by the reduced translational degrees of freedom in the transition state, but it is also sensitive to the vibrational degrees of freedom and thus to the vibrational frequency scaling. It is likely accurate to (50%. The rate constant is very sensitive to the height of the reaction barrier, and as mentioned above, with the two basis sets used here the barrier is calculated to be 15 and 17 kcal mol-1. It is difficult to estimate the accuracy of these results, but based on the estimated accuracy of the energetics of the unimolecular reaction, the range 17 ( 3 kcal mol-1 would appear to be reasonable. Even this relatively narrow range leads to widely varying estimates for the rate constant. Combining the reaction barrier and prefactor estimates yields k7 ) 10-(25.8(2.5) cm3 molecule-1 s-1 at 298 K. Tunneling will enhance the reaction rate; again using the Wigner correction as a crude estimate of tunneling efficiency44 leads to a rate enhancement factor of 11. Thus, k7 is predicted to have a value of 2 × 10-25 cm3 molecule-1 s-1, with an error range extending from 6 × 10-23 to 6 × 10-28 cm3 molecule-1 s-1. The wide range obtained for k7 reflects the great difficulty associated with calculating absolute rates from first principles. The calculated range for k7 corresponds to a very broad range of lifetimes for CF3OH in the presence of H2O vapor. Thus, an H2O concentration of 1 × 1017 cm-3 (representative of that typically found in the atmosphere between 0 and 5 km altitude) yields a lifetime of CF3OH with respect to homogeneous reaction with H2O vapor of between 50 h and 600 years at 298 K, with a most likely value on the order of 2 years. The predicted lifetime will obviously be greater at the lower temperatures more typical of the atmosphere. Such a range is clearly much too broad to make a definitive statement concerning the potential atmospheric importance of homogeneous reaction between CF3OH and H2O, except to note that the lifetime is probably much greater than competing heterogeneous processes, particularly in the troposphere, where decomposition on cloud particles likely occurs on the order of several days.48 More accurate calculations would be necessary to provide a more accurate estimate of the rate of any homogeneous reaction.
Schneider et al. Conclusions The aim of the present work is to further our understanding of the atmospheric chemistry of CF3OH. The unimolecular decomposition of CF3OH into COF2 and HF is thermodynamically allowed. However, the activation energy for this process is calculated to be 42 ( 3 kcal mol-1. With such a high activation barrier unimolecular decomposition is negligibly slow and is of no importance in the atmospheric chemistry of CF3OH. Experimental evidence indicates that CF3OH decomposes heterogeneously on H2O surfaces. A simple mechanism has been proposed to account for the role of H2O in the decomposition reaction, in which H2O serves as a bridge between the two ends of the CF3OH molecule. Ab initio calculations indicate that such a mechanism does lead to a substantial lowering of the energy barrier for loss of HF from CF3OH. The calculations are suggestive of what may occur in heterogeneous reactions on water surfaces. They do indicate that homogeneous reaction between CF3OH and H2O is slow under atmospheric conditions and is unlikely to be competitive with heterogeneous reactions. Acknowledgment. The authors thank Bill Hase of Wayne State University for helpful discussions concerning the dynamics of CF3OH decomposition, Steve Japar of Ford Motor for a critical reading of the manuscript, and John Montgomery and co-workers for a preprint of their work on COF2. The authors also thank Mattias Hallquist, Sarka Langer, and Evert Ljungstro¨m for sharing their experimental results on the decomposition of CF3OH. References and Notes (1) Wallington, T. J.; Schneider, W F.; Worsnop, D. R.; Nielsen, O. J.; Sehested, J.; Debruyn, W. J.; Shorter, J. A. EnViron. Sci. Technol. 1994, 28, 320A. (2) Wallington, T. J.; Schneider, W. F.; Sehested, J.; Nielsen, O. J. Discuss. Faraday Soc. 1996, 100, 55. (3) Nielsen, O. J.; Ellermann, T.; Sehested, J.; Bartkiewicz, E.; Wallington, T. J.; Hurley, M. D. Int. J. Chem. Kinet. 1992, 24, 1009. (4) Edney, E. O.; Driscoll, D. J. Int. J. Chem. Kinet. 1992, 24, 1067. (5) Tuazon, E. C.; Atkinson, R. J. Atmos. Chem. 1993, 17, 179. (6) Sehested, J.; Ellerman, T.; Nielsen, O. J.; Wallington, T. J.; Hurley, M. D. Int. J. Chem. Kinet. 1993, 25, 701. (7) (a) Wallington, T. J.; Hurley, M. D.; Ball, J. C.; Kaiser, E. W. EnViron. Sci. Tech. 1992, 26, 1318. (b) Wallington, T. J.; Nielsen, O. J. Chem. Phys. Lett. 1991, 187, 33. (8) Tuazon, E. C.; Atkinson, R. J. Atmos. Chem. 1993, 16, 301. (9) Nielsen, O. J.; Gamborg, E.; Sehested, J.; Wallington, T. J.; Hurley, M. D. J. Phys. Chem. 1994, 98, 9518. (10) Ryan, K. R.; Plumb, I. C. J. Phys. Chem. 1982, 86, 4678. (11) Caralp, F.; Lesclaux, R.; Dognon, A. M. Chem. Phys. Lett. 1986, 129, 433. (12) Wallington, T. J.; Dagaut, P.; Kurylo, M. J. Chem. ReV. 1992, 92, 667. (13) Li, Z.; Francisco, J. S. Chem. Phys. Lett. 1991, 186, 336. (14) Chen, J.; Zhu, T.; Niki, H. R. J. Phys. Chem. 1992, 96, 6115. (15) (a) Bevilacqua, T. J.; Hanson, D. R.; Howard, C. J. J. Phys. Chem. 1993, 97, 3750. (b) Jensen, N. R.; Hanson, D. R.; Howard, C. J. J. Phys. Chem. 1994, 98, 8574. (16) Sehested, J.; Nielsen, O. J. Chem. Phys. Lett. 1993, 206, 369. (17) Chen, J.; Zhu, T.; Niki, H.; Mains, G. J. Geophys. Res. Lett. 1992, 19, 2215. (18) (a) Sehested, J.; Wallington, T. J. EnViron. Sci. Tech. 1993, 27, 146. (b) Wallington, T. J.; Ball, J. C. J. Phys. Chem. 1994, 99, 3201. (19) Saathoff, H., Zellner, R. Chem. Phys. Lett. 1993, 206, 349. (20) Kelly, C.; Treacy, J.; Sidebottom, H. W.; Nielsen, O. J. Chem. Phys. Lett. 1993, 207, 498. (21) Wallington, T. J.; Hurley, M. D.; Schneider, W. F.; Sehested, J.; Nielsen, O. J. J. Phys. Chem. 1993, 97, 7606. (22) Turnipseed, A. A.; Barone, S. B.; Jensen, N. R.; Hanson, D. R.; Howard, C. J.; Ravishankara, A. R. J. Phys. Chem. 1995, 99, 6000. (23) Schneider, W. F.; Wallington, T. J. J. Phys. Chem. 1993, 97, 12783. (24) Wallington, T. J.; Schneider, W. F. EnViron. Sci. Tech. 1994, 28, 1198. (25) Schneider, W. F.; Wallington, T. J.; Minschwaner, K.; Stahlberg, E. A. EnViron. Sci. Technol. 1995, 28, 247.
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