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J. Phys. Chem. 1994,98,2216
Comment on the Thermochemistry of the CF30 Radical and CF30H Sidney W. Bensont Loker Hydrocarbon Research Institute, University of Southern California, University Park, Los Angeles, California 90089-I661 Received: November 18, I993 In a recent paper on the atmospheric chemistry of CF30 radicals,' the authors have reported an apparent H atom abstraction reaction with H20 at 296 K from which they concluded that the bond strength, DH0298(CF30-H), is comparable to that in H20, namely 119 kcal/mol. Then on the basis of some ab initio calculations of the heats of three metathesis reactions of CF30 with H20, CH4, and H2 they assign a probable value DH0298(CF@-H) = 120 f 3 kcal/mol. While persuasive, these arguments have ignored a substantial volume of experimental data which establishes DH0298(CF30H) = 109 kcal/mol with an estimated uncertainty no greater than 2.5 k ~ a l / m o l . ~ - ~ The thermochemistry of CF2O is known with good preci~ion.~ That of CF3OF has been obtained from an equilibrium study with F2 and CF204and was assigned a lesser accuracy because of possible interference from side reactions or the equilibrium of CF2O and CF30F to give CF300CF3. Independent study of this last equilibrium has been the source of thermochemical data for the p e r ~ x i d e .Studies ~ of the kinetics of the pyrolysis of the peroxide have been used by Batt and W a l ~ h to ~ . establish ~ thermochemical values for the C F 3 0 radical. They then used group additivity values to estimate thermochemistry for CF30H, CF30CF3,and some related compounds. Of interest here are thevaluesfor AfH0298(CF30)= -157 f 1.5 kcal/mol, which can be used to establish the bond strength DH0298(CF30-H) = 109 f 2.5 kcal/mol. The uncertainty assigned is my estimate. This is very far from the 120 f 3 kcal/mol estimated in ref 1 . The experimental basis for postulating a reaction between CF30' and H2O was the observation that when H20 was added to a system in which CF30' radicals were produced, there was an increase in the production of one of the minor products, CF20. Since the authors ascribed the production of CF2O to the t DistinguishedProfessorof Chemistryand Scientific CeDirector, Emeritus.
0022-3654/94/2098-2216$04.50/0
heterogeneous decomposition of CF,OH, itself produced in the reaction of CF30*with H atom donors in the system, the increase in CF2O with added H2O was interpreted as an increase in the production of CF30H, which heterogeneously decomposes to give the observed CF2O plus H2O. The CF30H presumably could only come from the reaction CF30+ HzO. However, the reaction system is very complex involving 17 discrete steps and 19 species and the yields of CF20 are about 20%of the molar disappearance of initial reactant CF3CH2F. In the presence of the maximum amount of HzO ( 1 Torr) the yields increase to 32%. Many plausible reactions have been omitted, and a t least two reactions are very surface sensitive. This seems a very tenuous basis for deriving a result in conflict with established thermochemistry. It might be argued that group additivity is subject to larger than the usual uncertainties of f l kcal/mol when applied to highly fluorinated compounds. It is thus of interest to use an alternative empirical approach involving fluorinated species. It has been observed that the replacement of an atom by another atom or group of comparable electronegativity in a saturated compound is accompanied by a constant change in heat of formation.5 In the case of fluorocarbons we have available AfH0298(CH3F) = -56 f 1 kcal/mo16 and AfH0298(CH30H) = -48.0 kcal/mol, leading to a F/OH replacement value of 8 f 1 kcal/mol. In similar fashion, AfH029~(CH2F2)= -108.2 kcal/ mol while A1H02~8(CH2[OH]2)= -93.5 f 1 kcal/mol.' The average of the double replacement is +7.4 f 0.5 kcal/mol, in good agreement with the first value. On this basis we would estimate AfH02ss(CF30H)= -215 f 1 kcal/mol using AfHo298(CF4)= -223 f 1 kcal/mol.4 This is in excellent agreement with the group additivity value of -213.5 f 1.5 derived by Batt and Walsh.3 N
References and Notes (1) Wallington, T. J.; Hurley, M. D.; Schneider, W. F.; Sehested, J.; Nielsen, 0. J. J . Phys. Chem. 1993, 97, 7606. (2) Batt, L.; Walsh, R. Int. J . Chem. Kinet. 1983, 15, 605. (3) Batt, L.; Walsh, R. Int. J . Chem. Kinet. 1982, 14, 933. (4) JANAF Thermochemical Tables, 2nd ed.;Stull, D. R., Prophet, H., Eds.; U S . Government Printing Office: Washington, DC, 1971. (5) Benson, S. W. Chem. Rev. 1978, 78, 23. Such atom groups have been labeled 'homothermal". Phenyl and vinyl area homothermal pair as are Br and SH. (6) Luo, Y.-R.; Benson, S.W. J . Phys. Chem. 1988,92, 5255. (7) Benson, S. W. ThermochemicalKinetics,2nded.; Wiley: New York, 1976.
0 1994 American Chemical Society
