J. Phys. Chem. 1993,97, 12783-12788
12783
Ab Initio Investigation of the Heats of Formation of Several Trifluoromethyl Compounds 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 Received: August 10, 1993'
Ab initio calculations have been used to determine the heats of formation of CF3, CF3H, CFJO, CF30H, CF3O2, and CF302H. These species are potential intermediates in the atmospheric degradation of hydrofluorocarbons such as HFC-134a. The computational results differ by as much as 15 kcal/mol from previous estimates and permit a reassessment of the energetics of several reactions implicated in the decomposition of HFCs. The heats of formation are also used to evaluate the effects of the CF3 group on adjacent bond energies.
Introduction Hydrofluorocarbons (HFCs) have been proposed as environmentally friendly replacements for chlorofluorocarbons (CFCs) in many applications, primarily because the former have little potential for contributing to stratosphericozone depletion. HFCs contain no chlorine, which can catalyze stratospheric ozone depletion.' They do contain alkyl hydrogens, which are readily abstracted by atmospheric hydroxyl radicals, thus initiating tropospheric HFC decomposition:2
+OH
C,F,H,
-
+ H2O
C,F,H,,-
(1) Theultimateatmospheric fateof HFCs is stillnot fully understood, however, and the study of their decomposition mechanisms and products has been a topic of much current interest. The trifluoromethyl (CF3) group is a fundamental HFC building block, and recent work has demonstrated that it plays an important part in the degradation of these materials. CF3 radicals have been shown to be produced during the atmospheric oxidation of HFC-23,' HFC-125,4-6 and HFC-1 34a.73 The subsequent oxidation chemistry of CF3 is thus basic to understanding HFC atmospheric chemistry. In the oxygen-rich atmosphere, CF3 reacts rapidly to form a (trifluoromethy1)peroxy radical (CF302):9.10 CF,
-
+ 0, + M
CF30,
+M
(2) The peroxy radical thus formed likely reacts with NO to generate a trifluoromethoxy radical (CF3O):lI CF,O,
+ NO
CF,O
+ NO,
(3) The subsequent fate of CF30 is not certain, although its unimolecular decomposition,I2 reaction with N0,13J4and reaction with various have been investigated. Recent FT-IR evidence suggests that CF30 also reacts with water very slowly to form trifluoromethanol (CF30H):I8 CF30
+ H,O
-
CF30H
+ OH
(4) Reaction 4 may be an important decompositionpathway for CF30 in the troposphere, where water is abundant. This reaction is surprising in that it implies that the 0-H bond in CF3OH is at least as strong as that in water. In contrast, most alcohols have an 0-H bond strength approximately 15 kcal/mol less than that of water. Ab initio calculations indicate that reaction 4 is slightly exothermic and that the CF30-H bond strength is 120 3 kcal/ mo1.18 Basic to understanding the reactions of these and other HFC oxidation products is reliable estimates of their heats of formation.
*
Author to whom correspondence should be addressed. *Abstract published in Aduance ACS Abstracts, November 15, 1993.
0022-3654/93/2097-12783%04.00/0
The experimental evidence for the reaction of CF30 with water suggests that the thermodynamics of these products can differ significantly from that of nonfluorinated analogues. Such a result is not surprising; the CF3 group is well-known to have a strong perturbing effect on the energetics of adjacent bonds, as for instancein the 0-0 bond of bis(trifluoromethy1)peroxide (CF302CF3).I9 Thesourceof this behavior has been the subject of several investigations.2@22 Estimates of the heats of formation of CF30,CFJOH,CF3O2, and CF302H, along with several other species, are available and have been used extensively in discussions of HFC decomposition chemistry. Batt and Walsh used the group additivity approach to estimate the heats of formation of these and other compounds, along with several key bond energiesa23Francisco and Williams later used MNDO calculations along with group additivity and availableexperimentaldata to refine theearlier e~timates.2~ While these results do reflect the unusual energetics of the CF3substituted compounds, their quantitative accuracy is questionable. For instance, Batt and Walsh predict a CF30-H bond strength of 109 kcal/mol, and Francisco (using an ab initio method) predicts one of only 101 kcal/mol,2s both substantially less than the more accurate calculations.18 Similarly, estimates of the heat of formation of CF302 span at least 15 kcal/mol. Thus, a reassessment of the heats of formation of molecules involved in the atmospheric decomposition of HFCs is timely. In this paper we report an ab initio estimation of the heats of formation of CF3, CFJH, CF30, CF30H, CF3O2, and CF302H. The heats of formation of the methyl analogues have also been calculated for comparison with experiment and to estimate the accuracy of our approach. These heats of formation are used to evaluate internal bond energies and to explore the perturbing influence of the trifluoromethyl group. They also are used to obtain revised estimates of the heats of several reactions of atmospheric importance.
Computational Details All calculationswere performed using the Gaussian 88 program suite.26Thecomputationalapproach was identicaltothat reported previously in our preliminary study of CH30, CHJOH, CF30, and CF30H.18 We report here the details of the calculations on CF3, CF3H, CH3O2, CH302H, CF3O2, and CF302H. All molecules were first gradient-optimized27928at the HartreeFock level using Pople's 6-31G(d,p) basis set29 and the harmonic vibrationalmodes evaluatedanalytically. From this starting point, the structures were then reoptimized using the same basis set but at the second-order Maller-Plesset (MP2) level,'O again with analytical gradients. The final structures agree well with experimental data. Unlike CH3, CF3 is pyramidal (C3" symmetry) with r(C-F) = 1.3272 A and L(F-C-F) = 111.20°. CF3H also has C3" 0 1993 American Chemical Society
Schneider and Wallington
12784 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
TABLE I: MP2/631C(d,p) Optimized Geometries' 40-0) r(C-0)
W-Xd r(C-X2) r(C-X3) r(0-H) dC-O-O) L(O-c-Xd L(O-c-X2) L(W-X3)
4H-O-O)
4x1-C-m
L(Xl*-x3) L(X1-C-O-O) L(H-O-O-C) 0
1.4568 1.3777 1.3323 1.3425 1.3326 0.9712
1.3331 1.4142 1.3261 1.3275 1.3275
1.4684 1.4187 1.0872 1.0895 1.0883 0.9691
1.3115 1.4481 1.0841 1.0846 1.0846
105.51 105.08 111.79 112.81 99.23 109.38 109.72 179.70 104.27
108.85 105.02 110.84 110.84
104.54 104.34 111.41 111.02 98.42 110.19 110.00 177.42 121.32
110.10 105.17 109.09 109.09
109.87 109.87 180.00
111.22 111.22 180.00
Distances in angstroms and angles in degrees.
symmetry, with r(C-F) = 1.3425 A, r(C-H) = 1.0854 A, L(FC-F) = 108.48O, and L(H-C-F) = 110.44O. The results for CF3H are in good agreement (f0.015 A, f 0.1 ") with commonly accepted experimental values.31 As expected, both structures differ from earlier Hartree-Fock results in predicting slightly longer bond l e n g t h ~ . ~The ~ s .geometric ~~ results for the peroxides are presented in Table I; the CH3O2 and CF3O2 structures have C, symmetry, while the CH302H and CF302H structures are asymmetric. Thecomputational results for C F J O ~ H are in equally good agreement with the gas-phase electron diffraction study of Marsden et al.33 The only significant discrepanciesare the H-O0-C dihedral angle, which was not included in the electron diffraction structural refinement, and the 0-0-C internal angle, which is underestimated by 2O by the calculations. To our knowledge the structures of the remaining peroxides have not been determined experimentally, although the MP2/6-3 lG(d,p) geometryof CHpO2 has been reported previously and, surprisingly, does differ slightly from our results, perhaps due to differences in geometry optimization strategy.34 For all molecules save CF3O2 and CF302H, the MP2/6-31G(d,p) geometries were used as input to single-point energy calculations at the full fourth-order Msller-Plesset (MP4) level using Pople's 6-31 l+G(d,p) b a s i ~ . This ~ ~ .basis, ~ ~ which is triple-l; in the valence space and includes polarization functions on all atoms plus diffuse functions on the heavy atoms, has been used successfully in our earlier thermodynamic work.18 It represents the best compromise we have found between flexibility and size in providing accurate thermodynamic information. The 1s core orbitals were frozen in the single-point MP4 calculations. For CF3O2 and CF~OZH, available resources restricted the singlepoint energy evaluations to the third-order Msller-Plesset level. The energetic results for all molecules of interest are summarized in Table 11, including those reported earlier.18 Harmonic vibrational frequencies for all molecules except CF302 and CF302H were obtained by numerical differentiation of the analytical MP2/6-31G(d,p) gradients. The MP2 vibrational frequencies were used unchanged to calculate zero-point vibrational energies and internal energy corrections. For CF3O2 and CF302H, computational resources again restricted us to determining second derivatives at the Hartree-Fock level. Vibrational frequencies are in general uniformly overestimated at this to compensate for this systematic error, the CF302 and CF302H frequencies were scaled by 0.92 before being used for the zero-pointand internal energy corrections. The final results are not sensitive to the value of this scaling constant. Internal translational, rotational, and vibrational energy corrections to 298.1 5 K were calculated using standard statistical mechanical m e t h o d ~ . ~The ~ J ~low-frequency CF3 internal rotational modes, as well as the hydroperoxy torsional modes, were treated as free rotations and thus contributed RT/2 (0.30 kcal/mol) to the internal energy. While zero-point vibrational corrections can
have a large (up to 5 kcal/mol in this study) effect on calculated reaction enthalpies, internal energy corrections are uniformly small for the reactions under consideration (< 0.5 kcal/mol).
Results The most straightforward approach to determining the heat of formation of a molecule from ab initio energetic results is to calculate the enthalpy change of a reaction connecting the molecule of interest with other species for which the heats of formation are available from experiment or other calculation. For a given set of target molecules, many different choices of reactions are possible. In practice, judicious selection of the reactions is essential to minimize computational error stemming from incomplete basis sets and limited treatment of correlation effects.4 In particular, the use of isogyric reactions (reactions in which the number of unpaired electronsis conserved)is essential to minimize errors due to incomplete treatment of electron ~ o r r e l a t i o n .Further, ~~ care must be taken to choose secondary reactant and product molecules for which experimental heats of formation are known with good accuracy. This selection is particularly important for reactions involving highly reactive species, for which experimental heats of formation may have large uncertainties. In this study, two types of isogyric hydrogenation reactions have been used to connect the molecules of interest with molecules with known heats of formation. The first reaction connects a radical with its saturated parent ( x = 0-2):
CF,O,
+ H,
-
CF,O,H
+H
(5) Combining the calculated AHsZg8 with the known heats of formation of H (52.1 kcal/mol) and HZ (0.0kcal/mol) yields
AZfACF,O,H) - AHf(CF30,) = AHSz9* - AHAH) (6) The advantage of this selection over other possible ones is that only one experimentally determined heat of formation-that of atomic hydrogen-is necessary for all values of x. The disadvantage is that in ( S ) , the radical center moves from an oxygen (or carbon, if x = 0) to a hydrogen, so that some possible cancellation of correlation effects between reactant and products is lost. Other selections for reaction ( S ) , such as:
-
+
+
CF,O, CH30xH CF,O,H CH,O, (7) would maximize this cancellation and thus permit a lower-level treatment of correlation effects, but would necessitate a larger database of experimental heats of formation. We chose to reduce the number of experimental parameters and to compensate by employing higher levels of correlation treatment. The second reaction connects the neutral, oxygenated compounds with the oxygen-free parent ( x = 1-2):
+
-
+
CF30,H H, CF3H H,O, (8) Combiningthecalculated AHgz98withtheknown heat offormation of H20 (-57.8 kcal/mol) or H202 (-32.6 kcal/mol) yields AHf(CF3H)- AHf(CF30,H) = AH8298 - AHAH,O,)
(9)
Two factors motivate the use of reaction 8. First, the oxygen environment is maintained from reactant to product, which will minimize errors in treating this structure. Second, only two additional experimental heats of formation are required. Again, alternate selections are possible that would permit less extensive basis sets and treatment of correlation but would necessitate the use of a larger number of experimental heats of formation. Using eq 6 and 9, the differencesbetween the heats of formation of all the molecules of interest can be determined. To determine the absolute heats of formation, the heat of formation of one of the members of the series must be obtained from either computation or experiment. For both series of interest, at least
Heats of Formation of Trifluoromethyl Compounds TABLE U
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12785
Calculated Electronic and Thermal Energies, in au
ZPEc ~ p 9 a . ci ~ total8 CF3OzH 'A 487.682 03 0.033 9gd 0.005 2Sd 487.64 804 2Alf -487.028 49 0.021 356 CF302 cs 0.004 7gd -487.00 236 'A -190.437 14 -190.469 41 0.056 18 CHjOzH C' 0.004 18 -190.40 905 ZA" cs -189.793 92 -189.825 51 0.044 64 0.003 62 -189.77 724 CH30z CF3OH C* 'A' -412.716 87 -412.726 47 0.029 69 0.00438 412.73 109 cs 2A' 412.026 10 CFJO -412.071 23 0.016 64 0.004 18 4 1 2.05 042 CH3OH C, 'A' -1 15.459 95 -115.477 29 0.053 04 0.003 39 -1 15.42 086 CH3O C* 2A' -1 14.789 42 -1 14.803 09 0.038 58 0.002 91 -114.76 155 CFjH c 3 v 'AI -337.625 22 -337.663 90 0.026 23 0.003 44 -337.63 423 CF3 C3" 'AI -336.948 75 -336.987 75 0.012 47 0.003 44 -336.97 184 CH4 Td 'A -40.398 45 40.405 14 0.046 61 0.002 86 -40.35 567 CH3 D3h 2A2" -39.726 67 -39.731 91 0.030 76 0.003 31 -39.69 784 HzO CZV 'A1 -76.277 32 -76.287 03 0.021 90 0.002 83 -76.26 230 c 2 'A -151.243 63 -151.267 32 HzOz 0.026 68 0.003 43 -151.23 720 HO c-v 2n -75.590 06 -75.595 39 0.008 76 0.002 36 -75.58 428 Hz D-h 'Z+* -1.166 21 -1.167 69 0.010 50 0.002 36 -1.15 483 H Kh 2s -0.499 8 1 -0.499 81 0.0 0.001 42 -0.49 839 a MP3(FC)/6-31 l+G(d,p) energy. MP4(FC)/6-31 l+G(d,p) energy. Zero point energy and 298.15 K internal energy corrections obtained from the MP2/6-3 lG(d,p) geometries and vibrational frequencies. Calculated from scaled HF/6-3 lG(d,p) vibrational frequencies. Based on MP4 energies except for first two molecules. sYm Cl
state
MP3'
MP46
TABLE IIk Heats of Reactions 5 and 8, in kcal/mol
TABLE IV: Comparison of Calculated and Accepted Experimental Heats of Formation.* All Energies in kcal/mol
m 9 8
+
-
CH3 H2 C& + H CH3O + H2 CH3OH + H CH3Oz + H2 CH302H + H CH3OH + H2 CH4 + H2O CH302H Hz-CH4 H202 CF3 H2 CF3H + H CF3O + HZ CF3OH + H CFlOz + H2 CF302H + H CF3OH + Hz CF3H + HzO CHiOzH + H2 CF3H + H202 +
+
+
+
+
+
+
+
+
+
+
(MP3) -0.9 -0.4 15.0 -28.0 -20.3 4.9 -0.4 9.1 -7.5 -8.1
(MP4) -0.9 -1.8 15.5
-26.5 -18.2 -3.7 -1.8 -6.6
exptl' -0.5 -0.3 18.16
-27.4 -19.1b -2.1 4.76 17.2b -10.9' -2.gb
MP
AH+98
calcd CH4 CH3 CH3OH CH3O CH302H CH30z H Hz0 HzOz
35.2 49.1 4.8 -32.2 4.5
exptl -17.8 34.8 48.2 4.2 -31.3d 2.7' 52.1 -57.8 -32.6
calcd CF3H CF3 CF3OH CF3O CFsOzH CF302 HO H02
-1 10.8 -217.7 -150.4 -190.8 -147.8
exptl -166.6' -1 12.4' -213.5' -156.7' -196.3d -161.4d 9.4 3.5
'Experimental values based on data from Table IV. bBased on estimated heats of formation for some species.
a Unlessotherwise noted, experimentalvalues from ref 43. Reference 3 1. Estimated using group additivity. Reference 23. Estimated using group additivity. Reference 24. e Reference 41.
one heat of formationis experimentally known with high accuracy, and these will be used to calibrate the other heats of formation. This approach maximizes the accuracy and internal consistency of thecalculated heats of formationwhile reducing computational effort. It is used below first for the unfluarinated methyl compounds, for which a complete list of experimental results are available for comparison, and then for the trifluoromethyl compounds. Methyl Compounds. The calculated heats of reactions 5 and 8, with CF3 replaced by CH3, are shown in the top half of Table 111. The agreement between the computational results and experimental data, obtained from the experimental heats of formation, is excellent. All of the deviations save one are less than 1.5 kcal/mol. The exception involves hydrogenation of CH3O2 to form CH302H. The experimental heat of formation of the former has been reported to f0.8 kcal/mol but is difficult toobtain e~perimentally,4~~~~ and that of the latter is only available from estimates based upon group additivity.24 Thus the rather large (2.6 kcal/mol) deviation in this case likely results as much from uncertainties in the experimental as the computational results. The calculations appear to do very well in modeling the energetics of these reactions. The data from Table I11 can be substituted into eqs 6 and 9 to obtain the relative heats of formation of the six species. The heat of formation of methane (-17.8 kcal/mo1)43 is known experimentally to within fO.l kcal/mol, and it serves as a convenient reference molecule. Combining the experimental AHr (CHI) with the calculated relative heats of formation yields the absolute heats of formation listed in Table IV. The available experimental data is also listed for comparison. The agreement with experiment surpassesthat in Table 111. With the exception of CH302, the computed and experimentalheats of formation all
agree within 1 kcal/mol. For CH3O2, the difference increases to 1.8 kcal/mol, which is unsurprising given the uncertainties in the experimental data just mentioned. The improvement from the reactions in Table I11 to the heats of formation arises in some part from fortuitous cancellation of error in the combination of eq 6 and 9 but also benefits from the calibration of the results to the experimental heats of formation of the reference compounds. Given these comparisons, a conservative estimate of the accuracy of the combination of computational method and selection of reactions is f 2 kcal/mol for the heats of formation. This level of accuracy is adequate for applicationto the problems of interest here. TrifluoromethylCompounds. The calculated heats of reactions 5 and 8 are shown in the bottom half of Table 111. Actual experimental data is available only for the first reaction, and in that case agreement is equal to that noted above for the methyl compounds. Because of limited computational resources, two of the reactions under considerationwere studied only at the MP3 level. Comparison between other MP3 and MP4 results in Table I11 shows that the deviations are in general less than 2 kcal/mol. In fact, for the methyl compounds, the MP3 results are slightly closer to the experimentalvalues than are the MP4. In contrast, results at the MP2 level, or with smaller basis sets, do not agree nearly as well with the higher level computations. Deviations at these lower levels are as large as 10 kcal/mol. Similarly, the heats of formation derived at the MP3 and MP4 levels agree within 2 kcal/mol, while those obtained at lower levels deviate to a much larger extent. Thus, in the reactions under consideration, thecomputationalresults appear tobeclose toconvergence with respect to larger basis sets and levels of correlation treatment. Use of MP3 energies introduces at most 2 kcal/mol of increased uncertainty in our final results.
Schneider and Wallington
12786 The Journal of Physical Chemistry, Vol. 97, NO. 49, I993
The heats of formation of CF3H and CF3have been investigated experimentally, and the former is known with somewhat higher accuracy, so it is chosen as our reference compound.31 Based on this choice, the heats of formation of the other trifluoromethyl compounds can be established as above and are listed in Table IV. For CF3, CF30, and CF3OH, we adopt the error limits suggested by the methyl calculationsof f 2 kcal/mol. Systematic differences between the methyl and trifluoromethyl calculations, for instance in the selection of fluorine basis set or in the experimental AHr(CFsH), may increase the error. However, the agreement obtained between the computational results and the limited experimentaldata makes us believe these systematicerrors to be small. For CF302 and CF302H, the error estimate is increased to f 4 kcal/mol to account for the use of MP3 rather than MP4 energies. The heat of formation of CF3 is known with nearly as high an accuracy as that of CF3H.31 In this case, the calculated heat of formation (-110.8 kcal/mol) is 1.6 kcal/mol higher than the experimental one. This difference is only slightly larger than the reported experimental uncertainty of f 1 kcal/mol31 and well within the estimated error bounds for the calculations. Thus, this limited data supports the accuracy estimates of the computational approach. For the remaining compounds of interest, the differences between the calculated heats of formation and the earlier estimates uniformly exceed our error limits. The heat of formation of CF30 has been obtained from the heat of formationof CF302CF3 (-360.2 kcal/mo1)44and the CF30-OCF3 bond dissociation energy (46.7 kcal/mol).45 The resultant AHr(CF30) is -156.7 kcal/moL23 The calculated AHr(CF30) is 6.6 kcal/mol greater, or -150.1 kcal/mol. The discrepancy likely rests in the heat of formation of CF302CF3. The 0-0 bond dissociation energy of CFsOzCF3 was obtained directly from kinetic data and is consistent with the bond energies of related compounds.45 In contrast, the CF3O2CF3 heat of formation was obtained indirectly by measurement of the temperature dependence of the equilibrium constant of the reaction: CF30,CF3 F! CF,OF
+ COF,
(10) in combination with the heats of formation of COF2 and CF3OF.4" The resultant AHr(CF302CF3) contains accumulated error both from the equilibrium measurement and the other heats of formation. A revised estimate of AHr(CF302CF3) can be obtained by combining the experimental bond dissociation energy with the calculated AHr(CF3O). Therevisedvalue thus obtained is-346.9 kcal/mol, which we believe to be a more accurate estimate. The most widely quoted value for the heat of formation of C S O H , which was estimated based upon group bond contributions, is -213.5 k c a l / m ~ l . The ~ ~ value reported here is 3.9 kcal/mol lower (-217.4 kcal/mol), in excellent agreement with the value (-217.15 kcal/mol) computed by Sana et a1.4 This difference is not great, but combined with the revised value for AHr(CF3O) above, it has important implications for reactivity. The group additivity work assumed a standard value for the group contributionof the hydroxyl group in CF30H. In fact the hydroxyl group has a considerably different bond energy in CF3OH than in other alcohols. The earlier estimates of M r ( C F 3 0 ) and AHdCFaOH) yield a value for the CF3O-H bond strength of 109 kcal/mol.23 Because the calculations shift AHr(CF30) and AHdCF3OH) in opposite directions relative to the earlier work, the resultant CF30-H bond strength is increased 10.5 kcal/ mol-making it at least as strong as the 0-H bond in water. Recent experimental evidence suggests that CF30 does abstract hydrogen from water in the gas phase, albeit slowly.l* The present work is in agreement with this observation, while the earlier heats of formation are not. The calculated heats of formation of CF302 and CF302H differ by an even greater extent from those suggested previously. AHr
60
CH3 40
E 3Y
2o
CH,O -
CF30 cF302
CH302
C
'U
9
E1 d
0
CH'I
CF3H
CH~O~H
CF,02H
-20
CH30H -40
CF,OH -
Figure 1. Comparison of the relative heats of formation of the CH3 and CF3 series.
(CF302H) was approximated by Francisco and Williams24 and Batt and WalshZ3as the average of AHr(H202) and AHr(CF3OzCF3),or -196.4 kcal/mol. The value obtained here is 5.9 kcal/ mol higher, or -190.5 kcal/mol. However, if we use the revised AHr(CF302CF3) obtained above, the discrepancy between the two results disappears. Theapproximation yields a AHr(CF302H) of -189.7 kcal/mol, remarkably close to our calculated value. This approach of averaging of heats of formation is equivalent to averaging the bond strengths of HO-OH and CF30-OCF3 to obtain the bond strength of CF30-OH. Thecalculations indicate that the CF3O-OH bond strength is very nearly halfway between that in H202 and in CF302CF3. The spread in these 0-0 bond strengths is less than 5 kcal/mol, so that replacement of CF3 with H perturbs the 0-0 bond strength only slightly in this case. The heat of formation of CF3O2 was originally estimated by Batt and Walsh to be -144 kcal/mol, based upon the assumption that the bond dissociationenergy of CF3-02 was the same as that of CH3-02.23a The same authors later revised their estimate downward to -161.3 kcal/mol based on a more elaborate treatment of the r bond energy of ~ x y g e n . ~The ~ b available experimental evidence, based upon kinetic studies of the dissociation reaction CF,O, F? CF3
+ 0,
(1 1) is consistent with the higher although the M N D O work of Francisco supports the lower one.u Combining the value reported here for AHr(CF3) with Antonik's value for the heat of reaction 1 14'yields AHr(CF302) = -148 kcal/mol. Thecalculated AHdCF3O2) of -147.5 kcal/mol is in excellent agreement with this lower value and is consistent with Batt and Walsh's original work. In fact, thecalculations indicate that DO(CF3-02) exceeds DO(CH3-02) by only 6.3 kcal/mol, rather than the 17.5 kcal/ mol later suggested by Batt and Walsh.23b
Discussion Energetic Influence of the Triflwromethyl Group. Figure 1 contains a comparison of the relative heats of formation of the methyl and trifluoromethyl compounds studied here, scaled such that AHr(CH4) = AHr(CF3H). In general, the trifluoromethyl compounds are relatively more stable than the methyl analogues. The one exception is CF3 itself, which is surprising considering
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12787
Heats of Formation of Trifluoromethyl Compounds
TABLE V kcal/mol
Calculated Bond Dissociation Energies, in
CX3-H CX3-0H
cx3-0
CX3-02H cx3-02
CX3O-H CX30-0H
cx30-0
CXsOrH
a
X=H
X=F
Aa
105.1 93.7 90.0 70.9 30.7 106.0 46.4 59.8 88.8
107.9 116.2 99.2 83.5 37.0 119.4 49.7 57.0 95.1
2.8 22.6 9.2 12.7 6.3 13.4 3.5 -2.8 6.4
CF3-X minus CH3-X.
thepossibilityofresonancestabilizationoftheradical.Thegreater spread in the trifluoromethyl values reflects the strengthening effect of the CF3 group on adjacent bonds. This effect extends at least to bonds y to the CF3 group, although it does diminish with distance. The CF3 group has a particularly great stabilizing effect upon CF30Hand CF302H,which reflects marked increases in the C-O bond energies. Surprisingly, the stabilization (and bond strengthening) is not nearly as pronounced in the corresponding radical species. These bond energy trends are explored further below. The heats of formation obtained above can be combined with previously available heats of formation to calculate a number of bond dissociation energies, and these are collected in Table V, grouped by like type. For comparison, the corresponding calculated bond dissociation energies for the methyl compounds are also presented, as is the difference in the two. From the last column of the table, the CF3 group clearly has a stabilizing influence on adjacent bonds, as is ~ e l l - k n o w n .More ~ ~ intriguing are the trends in stabilization. The effect on a hydrogen a to CF3 is relatively small. In contrast, C-O bonds a to CF3 are markedly strengthened relative to the CH3 analogues. The effect is most pronounced for the neutral compounds CF3OH and CF302H but is diminished in the corresponding alkoxy and peroxy radicals. According to the calculations, the C-O bond in CF3OH is 22.6 kcal/mol stronger than that in CH3OH, and the C-O bond length is correspondingly reduced from 1.420 A in CH3OH to 1.349 A in CF30H.18 Similarly, the CF3-02H bond is 12.7 kcal/mol stronger than CH3-02H and is 0.041 A shorter in the former than in the latter. One explanation for these trends is the presence of a hyperconjugation effect in CF3OH and CF302H. Hyperconjugation arises from a interactions between vacant u antibonding orbitals on one group (CF3 in this case) and lone pairs on an adjacent group (OH or OZH here).20-2-22 Because of the strong electron-withdrawing ability of fluorine, the u antibonding orbitals on CF3 are at relatively low energy and are readily accessible to secondaryA interactions. The OH and O2H groups can (formally,at least) donate two sets of lone pair electronsinto the CF3 antibondingorbitals, contributing to the bond shortening and strengthening effects noted. The resultant stabilization of CF30H in particular, as shown in Figure 1, is quite large. Regardless of the explanation, the magnitude of the effect is striking. The bond shortening and strengtheningeffect is less pronounced in the radical species CF30and CF3O2. The C-O bond energies are increased relative to their methyl analogues9.2 and 6.3 kcal/ mol, respectively, and the bond length reductions are correspondingly less. Why the a bond stabilizing influenceof the CF, group is so much greater in the closed shell compared to the open shell molecules is unclear. In CF30,one might argue qualitatively that only three electrons are available for donation into the CF3 u antibondingorbitals, because one of the lone pairs is hybridized away from oxygen along the C-O axis. Thus, C - O hyperconjugation would be diminished. If the radical possessed C3" symmetry this hybridizationpicture would be rigorously correct, but under C,symmetry the lone pairs are not restricted in this
fashion. The argument is even more difficult to make for CF3O2. These bond strength trends warrant further investigation. The CF3 group has a lesser effect on bonds in the /3 position relative to the a. The 0-0 bond in CF302H is only slightly stronger (3.5 kcal/mol) and shorter (0.041 A) than that in CH302H. In the peroxy radicals, CF3O-O is actually calculated to besomewhat weaker (2.8 kcal/mol) althoughstill shorter (0.034 A) than CH3O-O. Both of these energetic differences are near the accuracy limit of the calculations, so that the direction and magnitude of the changes may be incorrect. However, that the /3 bond energies change only a small amount in these molecules is unquestionable. In fact, the /3 effects in the peroxides studied here are much less than those observed in CF3-substituted hydrocarbons or dialkyl peroxides.49 In contrast, the /3 stabilizing effect in CF3OH is considerable: CF3O-H is 13.4 kcal/mol stronger than CH@-H. The large increase from unfluorinated to fluorinated alcohol can be related to differences in the C-O bond energy in CF3OH and CFpO. The CF3-OH bond is 17.0 kcal/mol stronger than the CF3-O bond; or in other words, cleavage of the CF3O-H bond results in a loss of 17.0 kcal/mol of C-O bond energy. By comparison, cleavage of the CHpO-H bond results in a loss of only 3.7 kcal/mol of C-O bond energy. The much greater bond energy loss in the former case increases the apparent strength of the CF3O-H bond relative to CH3O-H. Of course all /3 (or y, etc.) bond energy effects can be cast in terms of a bond energy effects. Such a comparison is not particularly revealing for CF3O2 or CF~OZH.But because the a C-O bond energy changes so radically from CF30H to CF30, the comparison becomes important. It again points out the surprising energetic stability of CF30H with respect to direct bond cleavage. The stabilizing influence of the CF3 group on adjacent bonds extends at least to they position. The data from this work indicates that CF3OrH is 6.4 kcal/mol stronger than CH3OrH. Based on energetics alone CF3O2 should be able to abstract hydrogen from some hydrocarbons, such as isobutane. As with CF3OH. the strength of CF302-H can be related to the differencein C-O bond strengths of CF302H and CF3O2, but the effect is much less dramatic in this case. Implications for Atmospheric Chemistry. The calculated heats of formation of the HFC decomposition intermediates can be combined with other available heats of f ~ r m a t i o n ~tol .obtain ~~ revised estimates of the energetics of some fundamental HFC decomposition reactions. No attempt is made at completeness here; we only comment on the energetics of some of the more important reactions touched on in the introduction. The reaction of CF30with water has already been touched on a number of times. Reaction 4 is calculated to be exothermic by 0.5 kcal/mol and, given the uncertainties in the calculations, can best be characterized as close to thermoneutral. Clearly, because C-H bonds are generally at least 15 kcal/mol weaker than HOH, the reaction of CF30 with hydrocarbons will be more exothermic by at least 15 kcal/mol. As previously mentioned, C F 3 0 has been observed to react with a variety of hydrocarbons.1gl7 Another CF30 reaction of current interest is that with ozone
+ 0,
-
+NO
-
CF,O
CF,O,
+ 0,
COF,
+ FNO
(12) because of its possible importance in an ozone depletion cycle. Reaction 12 is exothermic (AH12 = -31.5 kcal/mol), although recent work indicates that the reaction is very slow, if it occurs at all.SG52 The energetics of this reaction are similar to that of H O ozone. C F 3 0 is known to react rapidly with NO:13-1533
+
CF,O
(13)
and this reaction is very exothermic (AH13 = -39 kcal/mol).
12788 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
Reaction 13is important in that it provides a pathway out of CF3 atmospheric chemistry. The reaction of CF3O2 with NO is an important step in the formation of atmospheric CF30 radicals,lS and reaction 3 is calculated to be exothermicby 16.3 kcal/mol. Like reaction 13, reaction 3 is reasonably fast.I5 The reaction of CF302 with ozone is of current interest: CF,O,
-
+ 0,
CF30+ 2 0 ,
As with reaction 12, reaction 14 is both quite exothermic (AH14 = -36.7 kcal/mol) and evidently quite S ~ O W . ~ ”Further ~~ work is necessary to understand the kinetics of reactions 12 and 14. As present the principal atmospheric fate of CF3O radicals is believed to be reaction with hydrocarbons and/or H20 to give CF3OH.54 The fate of CF3OH is uncertain, but unimolecular decomposition is one possible loss mechanism. A number of unimolecular decomposition pathways for CF3OH have been studied computationally by Francisco.25 All pathways were found to be endothermic, but 1,2-elimination of H F was found to be least so CF,OH
-
COF,
+ HF
(15) with an energy of 13.6kcal/mol. This reaction has been observed tooccur in reaction chambers containing CF3OH, although with at least some heterogeneouscomponent.16 Chlorinatedmethanols are also known to decompose by a similar pathway.55 Using the AHdCF3OH) calculated here along with experimental values for the heats of formation of H F and COFZyields A H 1 5 = 0.4 kcal/mol, much less endothermic than previously suggested by Francisco.25 Entropic effects will also favor reaction 15 by about 10 kcal/mol at room temperature, so that the reaction is thermochemically allowed. It seems likely that the activation barrier for reaction 15 calculated by Francisco (47.9 kcal/mol) may also be significantly overestimated. Hence, homogeneous decomposition of CF3OH may be substantially more important than previously thought. Indeed, as the competing loss mechanisms for CF3OH in the atmosphere (photolysis, reaction with OH radicals, and incorporation into rain-sea-cloud water) are probably slow (lifetimes > 1 month) then it is possible that homogeneous decomposition is a major atmospheric fate of CFSOH. Clearly, further computational and experimentalstudies of reaction 15arerequired toinvestigatethispossibility. Such studies are underway in our laboratory.
Conclusions Revised values for the heats of formation of several CF3containing species have been obtained from the calculated energetics of their hydrogenation reactions. The new values differ by chemically significant amounts from earlier estimates and permit the reevaluation of the energetics of a number of important atmospheric reaqions. As previously noted, the CF3 group has a perturbing effect on heats of formation and bond energies, although the magnitude of the effect varies widely from system to system. This effect needs further study. While these heats of formationprovide a good starting point for understandingHFC decomposition reactions, further work is necessary to characterize the kinetics of the significant reactions.
Acknowledgment. The authors wish to thank Steve Japar of Ford Motor Co. for a critical reading of this manuscript and Joe Francisco of Wayne State University for providing a preprint of a review of atmospheric CX3 chemistry. References and Notes (1) Rowland, F. S. Annu. Rev. Phys. Chem. 1991, 42, 731-768. (2) Atkinson, R. In Scientifle Assessment of Stratospheric Ozone: 198% Vol 11, World Meteorological Organization, Global Ozone Research and Monitoring Project, Report No. 20.
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