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J. Phys. Chem. 1994, 98, 2217-2218
+ CF30 e COF, + F CF30 + F e C F 3 0 F
Reply to “Comment on the Thermochemistry of the CF3O Radical and CF30H”
CF300CF3 CF30 CF30
William F. Schneider,’ Timothy J. Wallington, and Michael D. Hurley Ford Research Laboratory, Ford Motor Company, P.O. Box 2053, MD 3083/SRL, Dearborn, Michigan 481 21 -2053
Jens Sehested and Ole John Nielsen Section for Chemical Reactivity, Environmental Science and Technology Department, Rise National Laboratory, DK-4000 Roskilde, Denmark Received: December 17, 1993
In a previous publication, experimental and theoretical evidence was presented suggesting that C F 3 0 radicals react with gasphase H20 in the atmosphere to give CF3OH and OH.’ The experimental evidence came from experiments performed by the UV irradiation of C12/CF3CFH2/02 mixtures with and without added HzO. The mechanism of the C1 atom-initiated oxidation of CF3CFHz in air has been studied extensively in our laborat ~ r i e s . ~C.F~3 0radicals are formed during the oxidation of CF3CFH2. CF3000CF3 and COF2 are both observed products of reactions involving C F 3 0 radicals. CF3OOOCF3 is formed by combination of C F 3 0 and CFlO2 radicals. COF2 is formed by the reaction of C F 3 0 radicals with H-containing species to give CF30H, which then decomposes to give COF2. In the presence of increasing amounts of H20 the yield of CF3000CF3 decreases while that of COF2 increases. The presence of H20 perturbs the chemistry associated with C F 3 0 radicals. As discussed in detail previously,’ we believe that the most likely explanation for this behavior is that C F 3 0radicals react with H20, albeit slowly. The experimental evidence is not proof that abstraction of an H atom from H2O by C F 3 0 radicals is thermodynamically feasible. However,the experimental evidenceis suggestiveof this possibility. Like Benson, we were surprised that C F 3 0radicals could react spontaneously with H2O in the gas phase, as this reaction implies an unusually strong CF30-H bond. To explore this result, we used ab initio methods t,o calculate DH0298(CF30-H) from the energetics of hydrogen metathesis reactions with molecules of known DHo298(R-H). Benson indicates that a discrepancy exists between our results and those in the literature and suggests that a better estimate of DH0298(CF30-H) is available from experimentally derived values for Aff?98(cF30) and AH898(CF30H). We briefly consider that experimental evidence here. As indicated by Benson, Batt and Walsh4 estimated (CF30) from the enthalpy of the reaction C F 3 0 0 C F 3& C F 3 0 F
+ COF,
(1)
in combination with DH02gs(CF30-OCF3) and AH?98(CF30F) and AH898(COF2). In fact, the heat of formation of CF3OF is known at best to only f 3 kcal/mol,s but the available experimental information does afford another route to AH?98(cF30). The equilibrium constant for reaction 1 has been studied as a function of temperature and A H 1 2 9 8 determined to be 24.5 f 0.7 kcal/ mo1.6 The kinetics of reaction 1 have been interpreted in terms of the following mechanism:7J 0022-3654/94/2098-2217$04.50/0
(2) (3) (4)
Arrhenius parameters for reaction 2 have been obtained by a number of gro~ps.~+9JO Equating the activation energy with the CF30-OCF3 bond energy yields AH2298 = 46.8 f 0.5 kcal/moL4 Similarly, Arrhenius parameters for the reverse of reaction q7J1 have been used to determine = 44.0 f 0.5 kcal/m01.~The enthalpies of reactions 1, 2, and 4, combined with the heats of formation of COF2 (-152.7 f 0.4 kcal/mol)s and of F (18.97 f 0.07 kcal/mol),12 yield a fully determined system for the heats of formation of CF30, CF3OOCF3, and CF3OF. Solving this linear system yields AH?98(CF30) = -155.4 f 1.1 kcal/mol, AH?98(CF300CF3) = -357.6 f 1.4 kcal/mol, and AH?98(CF3OF) = -180.4 f 1.2 kcal/mol, where the errors are estimated by standard propagation techniques.13 We believe these values to be the best available experimental estimates. They differ by up to 2.6 kcal/mol from the previous results of AH?98(cF30) = -156.7 f 1.5 kcal/mol,4 AH?98(CF3OOCF3) = -360.2 f 3 kcal/ mo1,6 and AH?98(CF30F) = -182.8 f 3 kcal/mol.5 Unfortunately, no experimental information beyond our inference of a slow reaction between C F 3 0 and H20 is available to estimate AH?98(CF30H). Batt and Walsh4used AH898(CF3OOCF3) to determine the group contribution for C-(F)3(0)(4 which from the revised AH898(CF300CF3) is -174.3 kcal/mol. Combining this value with the group contribution for 0-(C)(H) then yields AH?98(CF30H) = -212.2 kcal/mol and thus DH0298(CF30-H) = 108.9 kcal/mol. This result is clearly at variance with our calculated value of 120 f 3 kcal/mol and with the postulated spontaneous reaction between C F 3 0and water. Both the group additivity and ab initio methods have impressiverecords for the calculation of these types of molecular energetics. It is certainly surprising that the two methods lead to such widely divergent results in this case. However, we believe the paucity of experimental information makes it impossible to recommend one result over the other. Clearly, more work is necessary to resolve this conflict. We have recently published calculated values for the heats of formationof CF3Oand CF30H of-150.1 and-217.4 kcal/mol.ls These results differ from the values obtained above by 5.3 and -5.0 kcal/mol, respectively, which leads to the net discrepancy of greater than 10 kcal/mol in DH0298(CF30-H). Using the same computational approach, the heats of formation of C H 3 0 and CH30H were calculated to within f l kcal/mol of the accepted experimental values, which lends support to the validity of the method. In fact, the calculated value for AH?98(CF30H) is in slightly better agreement with the estimate obtained by Benson’s homothermal estimation than is the group additivity result. Thus, as with the bond strengths, the heats of formation of these compounds still require further investigation. In summary, we certainly agree that a large discrepancy exists between calculated and literature values for DH0298(CF30-H), but we do not believe that sufficient information is available to resolve thedifference. Further experimental and theoretical work is in progress.
References and Notes (1) Wallington, T. J.; Hurley, M.D.; Schneider, W. F.; Sehested, J.; Nielsen, 0. J. J . Phys. Chem. 1993, 97, 7606-7611. (2) Wallington, T. J.; Hurley, M. D.; Ball, J. C.; Kaiser, E.W. Emiron. Sci. Technol. 1992, 26, 1318-1324. (3) Sehested, J.; Wallington, T. J. Emiron. Sci. Technol. 1993,27, 146152.
0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 8, 1994
(4) Batt, L.; Walsh, R. Int. J. Chem. Kinet. 1982, 14, 933-944. (5) Chase,M. W.;Davies,C.A.;Downey, J.R.;Frurip,D. J.;McDonald, R. A,; Syverud, A. N. JANAF Thermochemical Tables, 3rd ed.;J. Phys. Chem. Ref.Data 1985, 14, (Suppl. 1). (6) Levy, J. B.; Kennedy, R. C. J. Am. Chem. SOC.1972.94.3302-3305. (7) Kennedy, R. C.; Levy, J. B. J. Phys. Chem. 1972, 76, 3480-3488. (8) Descamps, B.; Forst, W. J. Phys. Chem. 1976, 80, 933-939. (9) Czarnowski, J.; Schumacher, H. J. Z . Phys. Chem. (Munich) 1974, 92, 329-337. (10) Descamps, B.; Forst, W. Can. J . Chem. 1975, 53, 1442-1448
Comments (1 1) Czarnowski, J.; Castellano, E.; Schumacher, H. J. Chem. Commun. 1968, 1255. (12) Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA Key Values for Thermodynamics; Hemisphere: New York, 1989. (13) Skoog, D. A.; West, D. M.Fundamenrals of Analyrical Chemistry; 4th ed.;Saunders: New York, 1982. (14) Benson, S.W. ThermochemicalKinerics,2nded.; Wiley: New York, 1976. (15) Schneider, W. F.; Wallington, T. J. J. Phys. Chem. 1993,97,1278312788.
