J . Phys. Chem. 1990, 94, 4791-4795 hancement of these species in proteins. The present studies were made using laser excitation powers which minimize Raman saturation and allow us to measure for the first time the total differential Raman cross sections of Trp. Acknowledgment. We thank Paul A. Harmon for helpful discussions and assistance in the early stages of this work. We
4791
gratefully acknowledge support of this work from N I H Grant lROlGM30741-OS. S.A.A. is an Established Investigator of the American Heart Association. This work was done during the tenure of an Established Investigatorship of the American Heart Association, Pennsylvania, affiliate. Registry No. Trp, 73-22-3.
Theoretical Investigation of Chlorofluorocarbon Degradation Processes: Structures and Energetics of XC(O)O, Intermediates (X = F, CI) Joseph S. Francisco,* Avery N. Goldstein, Zhuangjie Li, Yao Zhao, Department of Chemistry, Wayne State University, Detroit, Michigan 48202
and Ian H. Williams School of Chemistry, University of Bath, Bath, BA2 7AY, U.K. (Received: August 25, 1989; In Final Form: November 14, 1989)
Photodissociation of stratospheric chlorofluoromethanes leads to carbonyl dihalide CX20, whose further degradation is also initiated by a photodissociation step. The energetics of possible oxidation pathways involving XCO, XC(O)O, and XC(0)02 radicals have been investigated by calculations at the UMP2/6-3 lG* and PMP2/6-3 lG* levels of ab initio molecular orbital theory. In the absence of experimental thermochemical data, calculated heats of reaction are used in discussions of the possible involvement of these intermediates in reactions of atmospheric significance.
Introduction It is generally accepted that a stratospheric chlorofluoromethane (CFM) undergoes UV photodissociation to yield a chlorine atom that participates in the ClO, catalytic cycle;'-3 less well understood is the subsequent fate of the trihalomethyl radical generated in the same process. We have embarked upon a theoretical survey of the mechanisms of oxidation of trihalomethyl radicals, to complement experimental studies of key reactions currently under way, and have suggested that the net result of the oxidation pathways for CX3 depicted in Scheme I is the formation of stable halocarbonyl compounds C X 2 0 along with the release of halogen atoms.4" Recent in situ measurements have indeed confirmed the presence of C F 2 0 and CCI2O in the stratosphere,'^^ thus providing indirect support for this view. Carbonyl dihalides CXzO may themselves undergo UV photodissociation, yielding haloformyl radicals XCO and halogen atoms."" To aid the assessment of the possible oxidation pathways for XCO depicted in Scheme 11, it is necessary to have knowledge of the heats of reaction for these various processes. Since experimental data are lacking, particularly for species XC(O)O,, we report here the results of ab initio quantum-chemical calculations for the energetics. of chlorofluoromethane degradation reactions in order to comment upon
SCHEME I: Oxidation Pathways for CX3 Radicals
their possible implications for stratospheric chemistry. ( 1 ) Cicerone, J . R. Science 1987, 237, 35. (2) Wayne, R. P. Chemistry of Atmospheres; Clarendon Press: Oxford, 1985. (3) Sridharan, U. C.; Klein, F. S.; Kaufman, F. J . Chem. Phys. 1985, 82, 592. (4) Francisco, J. S.; Li, Z.; Williams, I. H. Chem. Phys. Lett. 1987, 140, 531. (5) Francisco, J . S.; Williams, I. H. Int. J . Chem. Kinet. 1988, 20, 455. (6) Francisco, J. S.; Williams, 1. H. Proc. 16th Annu. Mtg. Natl. Org. Black Chem. Chem. Eng., Chicago, IL, 1989. (7) Rinsland, C. P.; Zander, R.; Brown, L. R.; Farme, C. B.; Park, J. H.; Norton, R. H.; Russel, J. M.; Raper, 0. F. Geophys. Res. Lett. 1986, 13, 769. (8) Wilson, S . R.; Crutzen, P. J.; Schuster, G.; Griffith, D. W. T.; Helas, G. Nature 1988, 334, 689. (9) Rowland, F. S.; Molina, M. J . Res. Geophys. Space Phys. 1975, 13, I. (IO) Sze, N . D. Geophys. Res. Lett. 1978, 5, 781. ( I I ) Simonaitis, R. In Proceedings of the NATO ASI on Atmospheric Ozone; Rep. FAA-EE-80-20, Aikin, A. C., Ed.; Washington, DC, 1980; pp 501-5 15.
0022-3654/90/2094-479 1 $02.50/0
Methods and Results The G A U S S I A N ~ and ~ ] ~ CADPAC'~program were used to perform ab initio molecular orbital calculations with the 6-31G* basesI4 for first-row atoms and the 6-31G* basisI5 for chlorine. The geometry of each species (Table I) was optimized by using the (12) Frisch, M. J.; Binkely, J. S.; DeFrees, D. J.; Raghavachair, K.; Schlegel, H. B.; Whiteside, R. A.; Fox, D. J.; Martin, R. L.; Fluder, E. M.; Melius, C. F.; Kahn, L. R.; Stewart, J. J. P.; Bobrowicz, F. W.; Pople, J. A. G A u s S l ~ N 8 6 ,Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1984. (13) Amos, R. D.; Rice, J. E. CADPACP.~,University of Cambridge, Cambridge, 1987. (14) Binkley, J. S . ; Pople, J. A,; Hehre, W. J. J . Am. Chem. Soc. 1980, 102, 939. Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (15) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S . ; Gordon, M . S.; DeFrees, D. J.; Pople, J. A. J . Chem. Phys. 1982, 77, 3654.
0 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 12. 1990
TABLE I: Optimized Geometries of Species in the CXIO System" UHF/ UMP2/ species sym param 6-31G* 6-31G* exptlb
F2
c12
CI F FO CIO
co
NO 0 2 (3.2
CC12 CFCl FCO CICO
FO2
NFF) R(CICI) R(CIF) NFO) R(CI0)
WCO) RNO) WOO) R(CF) 8(FCF) R(CCI) 8(ClCCl) R(CF) R(CCI) 8(FCCI)
WCO) WCF) B(FC0) NCOi R(CCI) e( CICO) NFO)
e( FOO) c102
co2
R(CI0)
WOO) e(Cl00) R(CO)
NO2
R(N0) 8(ONO)
C O F2
NCO) R(CF) B(FC0) R(C0) R(CC1) e(clc0) R(C0) R(CF) R(CC1) B(FC0) 8(ClCO) R(C=O) R(C-0 R(CFe S(OC0) 8( FC=O) R(C=O) R(C-0) R(CCI) e(OC0) 8(ClC=O) R(C=O) R(C-0) R(C0-0) R(CF) 8( FCO) 8(FC=O) 8(C00) R(C=O) R(C-0) R(0-0) R(CCI) e(clc0) 8(CCl=O) e(C00)
COCI, COFCI
FC(0)O
CIC ( 0 ) O
FC(O)O2
CIC(0)O2
1.345 1.990 1.613 2.877 1.621 1.114 1.127 1.168 1.283 104.5 1.711 110.3 1.276 1.743 106.2 1.152 1.304 128.3 1.136 1.838 128.9 1.380' 1.437 107.6 1.706 1.278 1 12.0 1.143 1.165 136.1 1.157 1.290 125.9 1.159 1.735 123.4 1.158 1.300 1.720 124.1 125.7 1.163 1.318 1.299 125.9 126.0 1.165 1.317 1.736 125.6 126.0 1.158 1.350 1.318 1.293 105.6 125.9 112.3 1.159 1.359 1.319 1.725 107.5 125.7 112.3
1.421 2.010 1.661 1.344 1.607 I.IS1 1.143 1.246 1.315 104.2 1.716 110.0 1.309 1.742 106.1 1.182 1.342 128.0 1.175 I .792 128.0 1.383 1.250 109.6 1.726 1.284 112.8 1.180 1.216 133.7 1.187 1.328 126.2 1.194 1.743 123.9 1.189 1.338 1.720 124.2 126.4 1.194 1.329 1,339 125.6 126.5 1.197 1.326 1.755 127.2 126.9 1.185 1.400 1.339 1.328 103.8 127.2 110.3 1.188 1.418 1.337 1.726 105.8 127.8 110.5
SCHEME 11: Degradation Pathways Consequent upon Photodissociation of C X 2 0
cx20
1.412 1.988 1.628 I .30 1.546 1.128 1.151 1.207 1.300 104.9 1.76 100
1.18 1.34 135.0 1.17 1.75 I20 1 .649d 1.200 112.2
1.16 1.197 134.2 1.174 1.312 126.0 1.166 1.746 111.3 1.173 1.344 1.725 123.7 127.5
" Distances in A, angles in deg. JANAF Thermochemical Tables, 3rd ed.; Chase, M. W., Jr.; Davies, C. A,; Downey, J. R., Jr.; Frurip, D. J.: McDonald, R . A,; Syverud, A. N . J . Phys. Chem. Ref Data Suppl. 1985, 14. ?Theoretical geometries of F 0 2 were calculated at the RHF/6-31G* level. dYamada. C.; Hirota, E. J . Chem. Phys. 1984, 80, 4694. unrestricted Hartree-Fock (UHF) and second-order MerllerPlesset (UMP2, all orbitals active) methods, to better than 0.001 A for bond lengths and 0.1' for angles. The UMP2 results show
marked improvements in bond lengths as compared with UHF results, and agree with experimental values to within fO.01 A for bond lengths and f 1 ' for angles. For some species, such as FO and F02, it is absolutely essential to use an electron-correlated method, such as UMP2, in order to obtain reasonable geometries. Harmonic vibrational frequencies and zero-point energies (Table 11) were obtained from analytical second derivatives computed at the UHF/6-31G* level. Consistent with expectation for results calculated at this level, the frequencies are generally overestimated by 10-15% with respect to observed fundamentals, where these are available. The tabulated zero-point energies for FO and F 0 2 are derived from experimental frequencies, since the UHF/6-3 1G* geometries and force constants are not reliable for these radicals. The effects of spin contamination of the UMP2 wave function were reduced by annihilation of the next highest spin component using the PMP2 method16 to reevaluate the energies" for UMP2/6-3 1G* optimized geometries. Table 111 contains experimental heats of formation (corrected to 0 K) for the various species involved in the processes whose theoretical and experimental heats of reaction are presented in Table IV. Theoretical heats of reaction (0 K) are obtained as the sum of potential energy changes calculated at a particular level and zero-point energy changes calculated by using UHF/6-3 lG* frequencies. Calculations for a small selection of simple reactions a t the UMP4SDTQ/6-3 IG*//UMP2/6-31G* level did not yield significant changes in the results as compared with results from the UMP2/6-3 lG* level. Consequently correlation-energy corrections for larger XC(O)O, species were determined at the second-order of Mdler-Plesset perturbation theory but not at higher orders. The rms error in the theoretical heats of reaction (for 11 reactions not involving XO, species) is actually slightly lower at the UMP2 level (5.3 kcal mol-') than at the PMP2 level (6.4 kcal mol-'), although the latter results are tabulated. The errors for reactions involving XO and X 0 2 are much larger, but we note that this results from the need of higher levels of electron correlation and large basis sets for these systems. Molecules such as FO, FOz, and F 2 0 2are' a class of species that are difficult to describe with reasonable accuracy. For example, the geometry of the FO radical has been shown to be poorly described at the Hartree-Fock leve1.l8.l9 Post-Hartree-Fock, CASSCF, and MRCI methods have been shown to give reasonable descriptions of the geometries of t h e species.20,21 More recent studies2' on FO have shown that to obtain reasonable relative energies extraordinary measures have to be employed to get results to within f 2 kcal mo1-l. These findings are consistent with theoretical studies of F202.22p23A (16) Schlegel, H. B. J . Chem. Phys. 1986, 84, 4530. (17) See Supplementary Table I. (18) Lathan, W. A,; Curtiss, L. A,; Hehre, W. J.; Lisle, J. B.; Pople, J. A. Prog, Phys, o,,g,
1974,
(19) OHare, P. A. G.; Wahl, A. C. J . Chem. Phys. 1970, 53, 2469. (20) Langhoff, S. R.; Bauschlicher, Jr., C. W.; Partridge, H. Chem. Phys. Lett. 1983, 102, 292. (21) Zhao, Y.; Francisco, J. S. Chem. Phys. Left. 1990, 167, 285. (22) Rohlfing, C. M.; Hay, P. J. J . Chem. Phys. 1987, 86, 4518. (23) Clabo, Jr., D. A,; Schaefer, 111, H. F. Inr. J . Quantum. Chem. 1987, 31. 429
Chlorofluorocarbon Degradation Processes
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 4793
TABLE 11: Calculated and Experimental Frequencies (cm-I) of Species in the CX20 System‘
species Fz FO NO CF2
mode
UHF/6-31G* 1245
VI
VI
y3
2221 1390 729 1295 1210 708 2140 713 453 1191 633 2079 331 983 498 65 I 560 832 I998 193 367 847 580 207 2166
FCO’ bend FCO bend wag FC str CO‘ str CO str
544 655 853 1076 1459 2129
CO str CF str 0‘0“ str C O str OCO’/COO’, asym FCO/bend OCO’/CO’O’, sym wag, torsion
2169 1422 I273 1075 764 585 387 838 173
VI VI
u2 V3
FCO
VI
V2
UP
CIO,
VI
V2
v3
C0Cl2
VI
V2
V3 y4
u5 y6
c12 CIO
VI VI
0 2
VI
CCI,
81
V2
VI
ClCO
VI
V2
FC
/O
‘u
\oo
exptl 894 1056 I904 1222 665
species
mode
UHF/6-31G* 1518 746 2585 1094 2187 63 1 I463 683 873 914 2439 1346 488 817
co, COF2
1110
1018 626 1855 954 447 1 IO9 570 1827 285 849 440 585 560 866 1580 72 1 350 748 570 281 i a80
CIF
co
CFCl F02
1612 832 1880 854 2137 446 1294 554 748
NO2
COFCl
0
CIC/
\o
CICO’ bend ClCO bend wag CIC str CO‘ str CO str CO str CCI str 0’0” str CO’ str OCO’/CO’O”. asvm CICO’bend OCO’/CO’O”, sym wag,
t0r si 0n
exptl 1385 667 2349 965 1928 626 1294 584 774 785 2170
1495 376 585 1358 ’, 57 1666 776 1868 501 1095 41 5 667
405 519 739 788 1283 2072 2109 563 1262 1172 967 457 327 726 154
’JANAF Thermochemical Tables, 3rd ed.; Chase, M . W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J . Phys. Chem. ReJ Data, Suppl. 1985, 14. TABLE 111: Experimental Heats of Formation of Reactants and Products‘
~~
0 F CI
F2 CI, FCI FO
CIO CO NO 02
CF2
58.98 f 0.03 18.47 f 0.08 28.59 f 0.002 0 0 -12.00 f 0.01 26.0 i I O 24.21 f 0.5 -27.20 f 0.04 21.46 f 0.04 0 -43.61 f 1.5
CCI,
FCO ClCO FO2
c102
co2
NO2
COF, COCI, COFCI
56.1 f 5 -42.0 f 4‘ -4.0 f 3‘ 6.24 f 0.5,d 3.44 f 5, 12 f 3,‘ 5.90 f 0.41 21.3 f 1.28 -94.12 f 0.0 1 8.59 f 0.2 -151.96 f 0.4 -52.19 f 0.8 -101.41 f 7.9
‘ J A N A F Thermochemical Tables, 3rd ed.; Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A,; Syverud, A. N. J . Phys. Chem. Ref. Data Suppl. No. I 1985, 14. bBowers, M. T.;Chau, W. J . Phys. Chem. 1976,80, 1738. CWalkerL. C.; Prophet, H. Trans. Faraday SOC. 1967, 63, 879. dPagsberg, P.; Ratajczak, E.:Sillesen, A.; Jodkowski, J. T. Chem. Phys. Lett. 1987, 141, 88. eBenson, S. W. In Thermochemical Kinetics; Wiley: New York, 1976. /Lyman, J. J. Phys. Chem. Ref Daia 1989, 18, 199. SClyne, M. A . A.; McKenney, D. J.; Watson, R. T. J . Chem. Soc., Faraday Trans. I 1975, 71, 332.
geometry optimization of F 0 2 at the UMP4SDTQ/6-31 1++G(2d,2p) show no significant difference from results at the UMP2/6-31G* level, Le., FO = 1.389 A, 00 = 1.245 A, FOO = 1 10.6O. Moreover, the heat of formation at this level is ca. 14
kcal mol-’ underestimated, while at the UMP2/6-31G* level it is ca. 18 kcal mol-’ underestimated. Consequently, the errors for reactions involving XO and XOz species could have 10-18 kcal mol-’ uncertainties in them at the UMP2/6-3 1G level of theory. Nevertheless, in view of the encouraging agreement between theoretical and experimental heats of reactions for other species besides XO and X 0 2 reasonable estimates of the heats of enthalpies for reactions involving XC(O)O, species, for which at present there are no known experimental heats of reaction, can be made and used to assess consequences of possible chemistry implicating XC(O)O, species.
Discussion Thermochemical arguments have been presented elsewhere6 to suggest that catalytic cycles for ozone depletion (reactions 1-3) R 03 --* RO + 0 2 (1)
+
RO + 0 - R
-
+0 2
(2)
net 03 + 0 202 (3) are in principle feasible for R = CX3 or C X 3 0 but that their viability would depend upon the lifetime of the CX30 radical. Ab initio MO calculations at the UMP4SDQ/6-31G* level for the energetics of dissociation 4 of CF30, together with RRKM CF30 CF20+ F (4)
-
calculations for its unimolecular dissociation rate constant, have led to the conclusion4 that this species exists only transiently at stratospheric pressures: the excess energy with which it is formed by reaction I (R = CF,) is more than sufficient to overcome the
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The Journal of Physical Chemistry. Vol. 94, No. 12, 1990
TABLE IV: Energy Changes (kcal mol-') for Reactions Pertinent to CX20 for Degradation LE
--
reaction
C C l 2 0 CCI, + 0 C C l 2 0 C12 + co CCI,O CI + ClCO ClCO CI co ClCO + o2 c102 + co ClCO + 0 2 - C l O + co, ClCO + o2 CIC(0)O2 cIc(o)o, CIO + CO2 CIC(O)O2 + N O NO2 + C l C ( 0 ) O ClC(0)O CI + c02 C l C ( 0 ) O CIO + co CF2O CF2 + 0 CFZO F2 + C O C F 2 0 -- F FCO FCO F + CO FCO + 02 FO2 + C O FCO 0 2 FO + C 0 2 FCO + 02 FC(0)Oz FC(O)O2 FO C 0 2 FC(0)Oz NO NO2 + F C ( 0 ) O FC(0)O F + C02 FC(0)O FO C O CFCIO CFCl 0 CFCIO F + ClCO CFCIO CI FCO CFCIO FCI + CO
-- -+ ---+
-+
-. +
-+-
+
+
--- + -- + -- +
+
UHF/ 6-31G 115.1 18.1 44.3 -15.5 23.8 -54.8 -15.4 39.4 -7.6 31.4 38.4 107.1 125.3 86.8 5.8 55.4 -9.1 -25.9 16.2 -6.6 0.6 99.9 113.9 75.3 53.9 55.6
IJMP2/ 6-31G* 177.9 35.6 75.6 21 23.0 -58.5 -23.0 35.4 -27.3 -38.0 45.9 164.8 124.3 126.5 36.0 48.7 -22.6 -30. I 7.4 -26.8 2.5 88.3 174.0 117.4 83.5 64. I
PMP2/ 6-31G' 177.1 35.6 12.6 3.7 23.0 -54.3 - 1 8.4 -36.0 -27.1 -37.4 45.4 164.0 124.3 123.9 37.4 51.0 -18.9 -25.7 6.8 --26.5 3.0 87.5 172.8 114.5 80.8 64. I
AZPE UHF/6-31G* -4.5 -3. I -3.2 -0.7 -0.2 2. I 4.0 -1.9 0.2 -0.3 -3.6 -5.0 -4.6 -4. I -2.3 -1.7 0.9 3.1 -2.9 0.2 -1.6 -4.6 -4.8 -4.4 -2.8 -3.8
AH0
theor" 172.6 32.5 69.4 3.0 22.8 -52.2 -14.4 -37.9 -26.9 -37.7 41.8 159.0 I 19.7 1 19.5 35.1 49.3 -18.0 -22.0 3.9 -26.3 1.4 82.9 168.0 110.1 78.0 60.3
exptib 167.9 f 5 25.0 f 0.8 76.8 f 3.1 5.4 f 3 -1.9 f 3 -65.9 f 3
167.3 f 2 124.8 f 0.4 128.4 f 4 33.3 f 4 (18.2-26.8) f 5 -26.1 f 1 1
115.9 f 8.5 88.0 f 8.8 62.2 f 8
'Theoretical value is the sum of the PMP2/6-31G* value and AZPE. bCalculated by using the enthalpy data in Table 111. endothermocity of reaction 4. S e m i e m p i r i ~ a and l ~ ~ab ~ ~initio25 MO calculations for CCI,F3-,0 radicals ( n = 1-3) suggest that CI atom dissociation occurs more readily in exothermic processes with very low activation barriers. Thus C X 3 0 radicals are likely to have very short lifetimes, and the net result of the reactions shown in Scheme I is the formation of C X 2 0 and X. Recent experimentsz6 on oxidation of CF3 by NOz also yield the product C F 2 0 in accord with this general conclusion. Under stratospheric conditions the initial step in the degradation of C X 2 0 is photodissociation, which could occur in three ways (reactions 5 - 7 ) Although reaction 6 is the least endothermic
cx*o2 xco + x cx*o22 co + x* cx20 -JLcx2 + 0
(5)
of these for X = CI, as the data in Table I V show, it has an energetic barrier considerably greater than that for the dissociation 5 , which is the preferred pathway; calculations at the UMP2/631G* level indicate a barrier height of 150 kcal mol-' for reaction 6, whereas there is no activation barrier to reaction 5.27 However, photodissociation by radiation of any wavelength
