J. Phys. Chem. 1994, 98, 11694-11696
11694
Determination of the Heats of Formation of FC(0)O and FC(0)OH Theodore S. Dibble and Joseph S. Francisco* Department of Chemistry, Wayne State University, Detroit, Michigan 48202 Received: May 26, 1994; In Final F o m : August 25, 1994@
We present a b initio calculations of the heats of formation of (fluoroformyl)oxyl, FC(O)O, which is an intermediate in the stratospheric photo-oxidation of halocarbons, and fluoroformic acid, FC(0)OH. The G 1 and G2 levels of theory are employed for both species. At 0 K G2 (Gl) values are -88.0 (-87.2) kcal mol-' for FC(0)O and -148.0 (-146.6) for FC(0)OH. An isodesmic reaction yields a heat of formation of FC(0)OH of -146.9 f 3, the uncertainty arising largely from possible errors in the experimental data for other species in the reaction. The 0 - H bond energy of FC(0)OH is 111.6 (1 11.O) kcal mol-', about the same as that of the C-H bond in methane (-110 kcal mol-').
n
This laboratory has been studying the degradation pathways of halogenated methyl and ethyl radicals formed in the atmospheric degradation of chlorofluorocarbons (CFCs)' and proposed substitutes for the CFCs, such as hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons ( H C F C S ) . ~A ~ ~recent concern has been the fate of FC(0)O radicals formed from the photo-oxidation of F2C0, FClCO, and HFCO? reservoir species in the degradation of some CFCs and HCFCS.~The dominant pathway appears to
+ hv -FCO + X (X = H, F, Cl) (1) FCO + 0, -FC(O)O, (2) FC(O)O, + NO - FC(0)O + NO, (3) FC(0)O + NO - [FC(O)ON=O]* - FNO + C 0 2 (4) FXCO
M
The heats of formation of FCO and FC(0)02 are reasonably well-known from both experiment and theory; that of FC(0)O is not. Our understanding of FC(0)O chemistry is improved by knowledge of its heat of f ~ r m a t i o n . ~ In the course of our work in ref 7, we were led to consider the possibility that FC(0)O could react with ethane or other hydrogen-containing species to form FC(0)OH (Figure 1). This idea seemed particularly relevant in light of recent studies indicating that CF30, another CFC/HCFC degradation product, would be more likely to react with hydrocarbons8 or water9 than NO in the troposphere. However, we determined that reactions of the type FC(0)O
+ RH - FC(0)OH + R
(5)
will not dominate FC(0)O chemistry. In our current study of FC(0)02 chemistry,'O we are considering FC(0)OH formation in the stratosphere via
+ HO, - FC(0)OH + 0, (6) (The analogous reaction of CH3C(0)02 + HOz has a 25% yield of CH3C(O)OH + O3.l') While reaction 6 is likely a minor FC(O)O,
fate of FC(0)02, and almost certainly does not affect stratospheric ozone, it illustrates another feasible path for FC(0)OH formation in the atmosphere. ~
~~
~~~
* Abstract published in Advance ACS Abstracts, October 1, 1994. 0022-365419412098-11694$04.5010
A
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Figure 1. Sketch of the molecular stmcture of FC(0)OH.
Although the chlorine analog of fluoroformic acid, ClC(O)OH, has long been known,', FC(0)OH had not been detected until very recently, when Wiedmann and Wesdemiotis observed it using neutralization-reionization mass spectrometry.13 Havlas, Korar, and Zahradnik14 discussed some reasons for the failure to observe FC(O)OH, and Francisco and Ghoul15 conducted studies of its gas phase dissociation pathways. Questioning whether the proposed reactions 5 and 6 were thermodynamically feasible, we realized that no good value of the heats of formation of FC(0)O or FC(0)OH existed. Heats of formation can be determined accurately from an isodesmic reaction (one in which the number and types of bonds are conserved), if heats of formation of the other species in the reaction are well-known.16 For FC(0)OH we consider FC(0)OH
+ CH, - HC(0)OH + CH,F
(7)
For both FC(0)OH and HC(O)OH, we considered the isomers with the carboxylic hydrogen syn to the carbonyl oxygen, which are the lowest energy isomers of both species.15 No isodesmic scheme appears helpful for FC(0)O. Therefore, we apply G1 and G2 theory, which have yielded very accurate atomization energies,17J8 to determine the heats of formation of FC(0)O and FC(0)OH (Table 1). To aid experimenters searching for FC(O)OH, we carry out a frequency analysis using second-order Mdler-Plesset perturbation theory with a moderately large basis set. All calculations were carried out using the GAUSSIAN 92 series of ~r0grams.l~ Geometries were optimized using the gradient method of Schlege120 to better than 0.001 for distances and 0.1' for angles. The specific calculations necessary for G1 and G2 are listed in refs 17 and 18, but we describe the principle here, in brief. In G1 and G2 theory, MP2, MP4, and QCISD(T) single-point energies are determined with various
0 1994 American Chemical Society
Heats of Formation of FC(0)O and FC(0)OH
J. Phys. Chem., Vol. 98, No. 45, 1994 11695
TABLE 1: G1 and G2 Energies and Relative Energies of FC(O)O, FC(O)OH, and Their Component Atoms
TABLE 2: Energy of Isodesmic Reaction 7 and Heat of Formation of FC(0)OH (kcal mol-') level of theory
G1
G2
-288.008 81 -288.009 70 energy (H) sum of atom -287.381 47 -287.381 20 energies (H)y atomization 0.627 41 0.628 50 energy (H) AHf (kcal mol-'), -87.2 -88.0 OK AHf (kcal mol-'), -85.2 -86.0 298 Kb
G1
G2
-288.685 74 -288.687 74 -287.881 47 -287.881 20 0.804 27
0.806 54
-146.6
-148.0
-144.4
-145.9
From ref 18. Thermal correction computed at the UHF/6-31G(d) level of theory.
basis sets at the MP2/6-31G(d) geometry of the target molecule. By appropriately combining these results, and assuming additivity of basis set effects on energy, one obtains an energy close to that of QCISD(T)/6-311+G(2df,p) or 6-31 l+G(3df,2p) at significant savings in computer time. Addition of a scaled HF/ 6-3 1G(d) zero-point energy and an empirical correction gives the G1 or G2 energy of the target molecule. Comparison to G1 and G2 energies of the constituent atoms yields the atomization energy of the target, and the heat of formation is calculated from those of the atoms. A spreadsheet used to calculate G1 and G2 energies was checked against the published datal8 for 0 2 to ensure its accuracy. All FC(0)O calculations were for the 2B2 ground state.21 Optimized geometries of all species have been reported elsewhere and compared to experiment.'5,21*23MP2/6-31G(d) geometries of FC(0)O are very good.22 Heats of formation of the atoms were taken from JANAF thermochemical tables.24 For the isodesmic reaction 7, geometries were optimized at the MP2 level of theory with all orbitals active. Basis sets employed included 6-31G(d), 6-3 1lG(d,p), 6-3 1lG(2d,2p), and 6-31 lG(2df,2p). Zero-point energies were obtained using the 6-31G(d) basis sets. Analysis of the vibrational frequencies of FC(0)OH was carried out at the MP2/ 6-3 1lG(d,p) and MP2/6-3 11G(2d,2p) levels of theory. Singlepoint energies were determined using the quadratic configuration interaction with single, double, and perturbative correction for triple substitutions (QCISD(T)) with core orbitals frozen and employing the 6-3 1lG(2df,2p), 6-31 1++G(2df,2p), and 6-31 l++G(3df,3pd) basis sets. The heat of formation of CH4 (16.0 kcal mol-') was taken from JANAF, that of HC(0)OH (-88.0 kcal mol-') from ref 25, and that of CH3F (-57 kcal mol-') from ref 26. For FC(O)OH, the G1 and G2 atomization energies differ by 1.4 kcal mol-', which is not an unusually large difference. The G2 (Gl) heat of formation is -148.0 (-146.6) kcal mol-'. The isodesmic scheme arrives at a heat of formation for FC(0)OH of -146.9 kcal mol-' at the QCISD(T)/ 6-3 11++G(3df,3pd)//MP2/6-3 1lG(2df,2p) level of theory, a value intermediate between the G1 and G2 values (Table 2). While this agreement is reassuring, an uncertainty of k 2 kcal mol-' in AHf for CH3FZ6 contributes to a &3 kcal mol-' uncertainty for FC(0)OH. By comparison, the G2 energy of HC(0)OH calculated by Smith and RadomZ7yields a AHfo (0 K) of -90.8 kcal mol-', in reasonable agreement with the single tabulated value.25 Without knowing more about the possible errors in the cited results one cannot determine the significance of the 2.8 kcal mol-' discrepancy; we are satisfied that our results for FC(0)OH are accurate to within 2-3 kcal mol-'. Table 3 lists calculated vibrational frequencies and intensities for FC(0)OH at the MP2 level with two moderately large basis sets. We note that the zero-point energy of the
AW(FC(O)OH)
MP2/6-31G(d) MP2/6-31 lG(d,p) MP2/6-31 l G ( 2 d . 2 ~ ) MP2/6-311G(2df,2p) QCISD(T)/6-3 1lG(2df,2p)" QCISD(T)/6-3 11++G(2df,2p)" QCISD(T)/6-3 11++G(3df,3pd)a
+20.4 +20.0 +20.0 +21.1 f19.7 f16.9 $16.3
-151.0 - 150.6 -150.6 -151.7 -150.3 -147.5 -146.9
a At MP2/6-311G(2df,2p) geometry. Includes AZPE of 0.8 kcal mol-' for reaction 7.
TABLE 3: Vibrational Frequencies (cm-') and Intensities (km mol-') for FC(0)OH' MP2/6-31lG(d,p)
MP2/6-311G(2d,2p)
mode symmetry description frequency intensity" frequency intensity" 1 2 3 4
A'
5
6 7 8 9
A"
OH str c=o str COH bend c-0 str C-F str FC=O bend FC-0 bend umbrella torsion
3875 1946 1421 1232 973 626 570 798 593
108 469 141 370 66 78 10 41 117
3855 1916 1420 1211 963 625 567 780 593
113 434 115 360 56 15 9 37 107
"The hydrogen is at 0" dihedral to the carbonyl oxygen, 180' dihedral to the fluorine.
MP2/6-3 11G(2d,2p) level lies only 0.4 kcal higher than the scaled HF/6-31G(d) value. For the FC(0)O radical, G1 and G2 atomization energies differ little and the implied values of AH; (0 K) are -87.2 and -88.0 kcal mol-', respectively. These values are much more negative than the approximate value (-79 kcal mol-') derived previously from the MP4/6-3lG(d)//MP2/6-31G(d) C-F bond dissociation energy.28 Inadequacies of both basis set and perturbation theory in the earlier calculation might be invoked to explain the large discrepancy. The close agreement of the G1 and G2 results implies that the basis set additivity assumptions of these methods are quite accurate. On the basis of its history of success with other difficult molecules, we believe that these results are as accurate as those usually obtained.18 Therefore, we estimate AHf" (0 K) for 2B2 FC(0)O to be -88.0 k 1.8 kcal mol-'. Returning to the question which motivated this work, we determine the 0-H bond energy for FC(0)OH to be 111.6 (1 11.0) kcal mol-' from G2 (Gl) theory. Therefore, reaction 5 , hydrogen abstraction by FC(O)O, should be thermodynamically feasible from any hydrocarbon (De(C-H) 5 110 kcal mol-'), but not water (D,(O-H) = 120 kcal mol-' in the gas phase).28 Reaction 6 should be exothermic by about 38 kcal mol-' .28 References and Notes (1) Francisco, J. S.; Goldstein, A. N.; Li, 2.;Zhao, Y.; Williams, I. H.
J. Phys. Chem. 1990, 94, 4791. (2) Francisco, J. S.; Ghoul, W. A,; Williams, I. H. J. Mol. Srruct. (THEOCHEM) 1993, 279, 35. (3) Francisco, J. S.; Li, Z.; Bradley, A.; Knight, A. E. W. Chem. Phys. Lett. 1993, 214, 77. (4) (a) Wallington, T. J.; Hurley, M. D.; Maricq, M. M. Chem. Phys. Lett. 1993, 205, 62. (b) Maricq, M. M.; Szente, J. J.; Khitrov, G. A,; Francisco, J. S. J. Chem. Phys. 1993, 98, 9522. (c) Wallington, T. J.; Elleman, T.; Nielsen, 0. J.; Sehested, J. J. Phys. Chem. 1994, 98, 2346. ( 5 ) Wallington, T. J.; Schneider, W. F.; Worsnop, D. R.; Nielsen, 0. J.; Sehested, J.; DeBruyn, W. J.; Shorter, J. A. Environ. Sci. Technol. 1994, 28, 320A.
(6) Dibble, T. S.; Francisco, J. S. J. Phys. Chem. 1994, 98, 5010.
11696 J. Phys. Chem., Vol. 98, No. 45, 1994 (7) Maricq, M. M.; Szente, J. J.; Dibble, T. S.; Francisco, J. S. J . Phys. Chem., submitted. (8) See for example: Barone, S. B.; Tumipseed, A. A.; Ravishankara, A. R. J . Phys. Chem. 1994, 98,4602. (9) Wallington, T. J.; Hurley, M. D.; Schneider, W. F.; Sehested, J.; Nielsen, 0. J. J . Phys. Chem. 1993, 97, 7606. (10) Maricq, M. M.; Szente, J. J.; Foltz, K. B.; Dibble, T. S.; Francisco, J. S. Unpublished research. (1 1) Lightfoot, P. D.; Cox, R. A.; Crowley, J. N.; Destriau, M.; Hayman, G. D.; Jenkin, M. E.; Moortgat, G. K.; Zabel, F. A m o s . Environ. 1992, 26A, 1805. (12) Jensen, R. J.; Pimentel, G. C. J. Phys. Chem. 1967, 71, 1803. (13) Wiedmann, F. A,; Wesdemiotis, C. J . Am. Chem. SOC. 1994, I 1 6 , 248 1. (14) Havlas, Z.; Kovar, T.; Zahradnik, R. J.Am. Chem. SOC.1985,107, 7243. (15) Francisco, J. S.; Ghoul, W. A. Chem. Phys. 1991, 157, 89. (16) Hehre, W. J.; Radon, L.; Schleyer, P. V. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (17) Pople, J. A,; Head-Gordon, M.; Fox, D. J.; Raghavachari, K.; Curtiss, L. A. J . Chem. Phys. 1989, 90, 5622. (18) Curtiss, L. A,; Raghavachari, K.; Trucks, G. W.; Pople, J. A. J . Chem. Phys. 1991, 94, 7221.
Dibble and Francisco (19) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Forest", J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92, Revision C;Gaussian, Inc.: Pittsburgh, PA, 1992. (20) Schlegel, H. B. J . Comput. Chem. 1982, 3, 214. (21) Francisco, J. S.; Ostdin, A. Mol. Phys. 1989, 68, 255. (22) Maricq, M. M.; Szente, J. J.; Li, Z.; Francisco, J. S. J . Chem. Phys. 1993, 98, 784. (23) Francisco, J. S.; Zhao, Y. J . Chem. Phys. 1990, 93, 9203. (24) Chase, M. W., Jr.; Davies, C. A,; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J . Phys. Chem. Re$ Data 1985,14, Suppl. 1. (25) Gurvich, L. V.; Veyts, I. V.; Alcock, C. B. Thermodynamic Properties of Individual Substances; Hemisphere: New York, 1991; Vol. 2. (26) (a) Lias, S. G . ;Karpas, Z.; Liebman, J. F. J . Am. Chem. SOC. 1985, 107, 6089. (b) Dixon, D. J . Phys. Chem. 1988, 92, 88. (27) Smith,B. J.; Radom, L. J . Am. Chem. SOC.1994, 115, 4885. (28) Li, Z.; Dibble, T. S.; Francisco, J. S. Adv. Phys. Chem., in press. Francisco, J. S.; Ostafh, A. J. Phys. Chem. 1990, 94, 6337.
