J. Phys. Chem. 1992,96,4894-4899
41394
room temperature rate constants for peroxy radical self-reactions vary over several orders of magnitudes2This difference in behavior between the two sets of reactions presumably arises because the alkyl group behaves rather as a spectator in the R 0 2 H 0 2 reactions, but participates in the six-membered cyclic Russell3o intermediate in the R 0 2 self- reaction^.'^ Measurements of k6 and k8 to date conducted at the Ford Motor C O . ' ~ ~have ' * used relative rate techniques, with n-butane as a reference compound, using k(Cl+n-butane) = 2.25 X lO-'O cm3 molecule-' s-' (the only direct measurement of this rate con~tant).~' As discussed in detail elsewhere,15there is a growing body of data which suggests that this value for k(Cl+n-butane) is 25-3075 too high. Accordingly, here we have used values for k6 and k8 based upon k(Cl+n-butane) = 1.7 X 10-lo an3molecule-' s-'. Our values of k6 and k8 are in good agreement with previous room temperature determinations of 2.4 X cm3 molecule-' s-' (k5)18and 2.7 X 1O-Io (k6).17918 As the present values of k6 and k8 are obtained in a rather indirect manner, it is expected that the previous measurements will be more accurate. More importantly, we have shown the k6 and k8 are independent of temperature over the range 249-364 K and the good agreement between the present values and the literature values gives us confidence in our radical generation system.
+
Acknowledgment. The work at the University of Bordeaux was supported in part by an Environment Programme grant from the Commission of the European Communities. D.M.R. thanks the Royal Society for the award of an Overseas Fellowship. Registry No. c - C 6 H I 1 O 22143-59-1; , c - C 5 H 9 0 2 20682-76-2; , H02, 3170-83-0; C1, 22537-15-1; c - C S H ~ ~287-92-3; , c - C ~ H , ]110-82-7; , C H , O H , 67-56-1.
References and Notes (1) Cox, R. A. In Modern Gas Kinetics; Pilling, M. J., Smith, I. W. M., Eds.; Blackwell Scientific: Oxford, U.K., 1987. (2) Westley, F.; Herron, J. T.; Cvetanovic, R. J.; Hampson, R. F.; Mallard, W. G. NIST Chemical Kinetics Database, Version 3.0; NIST Standard Reference Database 17; U S . Department of Commerce: Gaithersburg, MD, 1991. (3) Finlayson-Pitts, B. J.; Pitts Jr., J. N. Atmospheric Chemistry: Fundamentals and Experimental Techniques; John Wiley and Sons: New York, 1986.
(4) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Molina, M. J.; Hamp son, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. "Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling. Evaluation Number 9"; JPL Publication 90-1; Jet Propulsion Laboratory: Pasadena, CA, 1990. (5) Lightfoot, P. D.; Roussel, P.; Lesclaux, R. J . Chem. SOC.,Faraday Trans. 1991, 87, 3213. (6) Moortgat, G. K.; Veyret, B.; Lesclaux, R. Chem. Phys. Lett. 1989, 160, 443. (7) Veyret, B.; Lesclaux, R.; Rayez, M. T.; Rayez, J. C.; Cox, R. A,; Moortgat, G. K. J . Phys. Chem. 1989, 93, 2368. (8) Burrows, J. P.; Moortgat, G.K.; Tyndall, G.S.; Cox, R. A,; Jenkin, M. E.; Hayman, G. D.; Veyret, B. J. Phys. Chem. 1989, 93, 2375. (9) Jenkin, M. E.; Cox, R. A. J . Phys. Chem. 1991, 95, 3229. (10) Murrells, T. P.; Jenkin, M. E.; Shalliker, S. J.; Hayman, G.D. J. Chem. SOC.,Faraday Trans. 1991,87, 2351. (11) Rowley, D. M.; Lightfoot, P. D.; Lesclaux, R.; Wallington, T. J. J . Chem. SOC.,Faraday Trans. 1991, 87, 3221. (12) Rowley, D. M.; Lightfoot, P. D.; Lesclaux, R.; Wallington, T. J. J . Chem. Soc., Faraday Trans., in press. (13) Wallington, T. J.; Gierczak, G. A,; Ball, J. C.; Japar, S. M. Int. J . Chem. Kinet. 1989, 21, 1077. (14) Lightfoot, P. D.; Lesclaux, R.; Veyret, B. J . Phys. Chem. 1990, 94, 700. (15) Wallington, T. J.; Skewes, L. M.; Siegl, W. 0. J . Phys. Chem. 1989, 93, 3649. (16) Wu, D.; Bayes, K. D. Int. J. Chem. Kinet. 1986, 18, 547. (17) Atkinson, R.; Aschmann, S. M. Int. J. Chem. Kinet. 1985, 17, 33. (18) Wallington, T. J.; Skewes, L. M.; Wu, C. H.; Japar, S. M. Int. J . Chem. Kinet. 1988, 20, 867. (19) Braun, W.; Herron, J. T.; Kahaner, D. K. Int. J . Chem. Kinet. 1988, 20, 51. (20) Lightfoot, P. D.; Jemi-Alade, A. A. J . Phorochem. Photobioi. A: Chem. 1991, 59, 1 . (21) Crowley, J. N.; Simon, F. G.; Burrows, J. P.; Moortgat, G. K.; Jenkin, M. E.; Cox, R. A. J. Photochem. Photobiol. A : Chem. 1991, 60, 1 . (22) Lightfoot, P. D.; Lesclaux, R.; Veyret, B. J . Phys. Chem. 1990, 94, 708. (23) Vaghjiani, G. L.; Ravishankara, A. R. J . Geophys. Res. 1989, 94, 3487. (24) Jenkin, M. E.; Cox, R. A.; Hayman, G. D.; Whyte, L. J. J . Chem. SOC.,Faraday Trans. 2 1988,84, 913. (25) Jenkin, M. E., private communication. (26) Wallington, T. J.; Japar, S. M. Chem. Phys. Lett. 1990, 167, 513. (27) Wallingtog, T. J.; Japar, S. M. Chem. Phys. Lert. 1990, 166, 495. (28) Madronich, S.; Calvert, J. G. J . Geophys. Res. 1990, 95, 5697. (29) Atkinson, R. Atmos. Enuiron. 1990, 24A, 1. (30) Russell, J. A. J. Am. Chem. SOC.1957, 79, 3871. (31) Lewis, R. S.; Sander, S. P.; Wagner, S.; Watson, R. T. J. Phys. Chem. 1980,84, 2004.
Gas-Phase Hydrolysis of Trifluoromethyl Carbonyl Halides to Trifiuoroacetk Acid J. S. Francisco Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received: December 2, 1991; In Final Form: March 16, 1992)
The structures, energetics and vibrational features of the transition structures for gas-phase hydrolysis of CF3C(0)X (where X = H, F, or C1) compounds are described. The activated complexes are four-membered ring structures which involve the addition of the OH bond from water across the CO bond of CF3C(0)X. The calculated activation energies are 54.6, 35.5, and 50.1 kcal mol-' for CF,C(O)H, CF3C(0)F, and CF3C(0)CI, respectively. Decomposition of the hydrolysis product, CF3CX(OH),, into trifluoroacetic acid, CF3C(0)OH, has been examined, and activation energy barriers are 75.9, 38.5, and 7.6 kcal mol-', for X = H, F, and CI,respectively. The present calculations suggest that gas-phase hydrolysis of C F 3 C ( 0 ) H is more likely to form stable CF3CH(OH)2,while CF,C(O)F and CF3C(0)CIwill produce CF,C(O)OH and hydrogen halide.
Introduction Presently, there is a major search for alternative halocarbons that are suitable replacements for chlorofluorocarbons. Alternative halocarbon that are being considered are of the type of CF3CX2H, where X = H, F, or C1. However, little is known about the environmental acceptability of these compounds. Some studies suggest that a major byproduct from the atmospheric degradation of these compounds is trifluoroacetic acid.' This compound is of major concern since it is believed that soil bacteria metabolizes trifluoroacetic acid to produce monofluoroacetic acid-a potent 0022-3654/92/2096-4894%03.00/0
mammalian toxin. To understand how to minimize the production of trifluoroacetic acid from alternative halocarbons, it is necessary to establish the route by which it is formed. Major byproducts from degradation of alternative halocarbons such as CF3CX2H are CF3C(0)X compound~.~J The removal of these compounds by reaction with OH radials will be slow. Moreover, these compounds only have significant UV absorption thresholds below 330 nm, which suggests that photooxidation in the troposphere is not a major removal process of CF3C(0)X compounds. One route for the removal of CF3C(0)Xcompounds in the troposphere could 0 1992 American Chemical Society
Hydrolysis of Trifluoromethyl Carbonyl Halides
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4895
TABLE I: Optimized Geometries (A and deg) for Reactants and Products for CF3C(0)X Hydrolysis to Trifluoroecetic Acid HF/3-21G HF/6-31G* species coordinate H F c1 H F HX HX 0.735 0.937 1.293 0.730 0.91 1 HO 0.967 0.947 H20 HOH 107.6 105.5 1.192 1.178 CF$(O)OH co 1.510 1.531 cc 1.343 1.316 CF 1.338 CF’ 1.307 1.336 CO’ 1.316 0.969 O’H 0.953 124.9 occ 123.4 O’CC 108.8 110.4 113.9 HO’C 109.3 FCC 110.5 110.0 110.6 F’CC 110.0 180.0 F’CCO 180.0 180.0 180.0 HO’CC 1.175 1.199 1.178 CF$( 0 ) X co 1.175 1.161 1.524 1.508 1.507 cc 1.516 1.527 1.321 1.341 1.349 CF 1.339 1.314 1.306 1.335 1.336 CF’ 1.337 1.304 cx 1.077 1.089 1.339 1.830 1.307 127.1 occ 122.8 122.1 125.8 126.1 123.5 xco 112.6 114.0 123.9 124.2 109.3 110.0 110.1 109.4 FCC 109.5 112.3 F’CC 112.4 109.7 109.5 110.5 108.2 107.2 FCF 107.8 108.3 108.7 109.4 109.1 F‘CF 109.0 109.4 109.3 0.0 0.0 F’CCO 0.0 0.0 0.0 1.406 CF,CX(OH), co 1.372 1.372 1.377 1.353 1.405 CO’ 1.380 1.384 1.376 1.360 0.966 OH 0.968 0.968 0.949 0.951 0.967 0.972 0.969 O’H 0.951 0.950 1.074 cx 1.361 1.855 1.082 1.336 1.507 1.503 cc 1.510 1.521 1.530 1.343 CF 1.335 1.316 1.338 1.311 1.352 1.352 CF’ 1.354 1.324 1.323 113.0 113.6 114.7 O’CO 112.7 113.3 104.9 occ 106.4 106.3 106.0 107.5 111.6 HOC 109.2 112.6 112.7 109.1 110.5 111.9 HOT 109.2 111.0 109.6 xco 113.5 111.3 112.4 111.0 110.9 111.0 FCC 110.5 111.5 110.8 111.1 110.0 108.1 F’CC 110.7 107.3 109.2 FCF 108.7 108.7 108.4 109.0 108.7 108.0 F’CF 108.8 108.9 108.0 108.6 0.0 F’CCO 0.0 0.0 0.0 0.0
be homogeneous gas-phase hydrolysis. If hydrolysis does play a major role in the removal of CF,C(O)X compounds, the question raised is, What new species are produced in the troposphere that could be of concern? An expected byproduct of the hydrolysis of CF,C(O)X compounds is trifluoroacetic acid, CF3C(0)OH. It is important to know the mechanism by which trifluoroacetic acid is formed from the homogeneous hydrolysis of CF3C(0)X compounds. Knowledge of the mechanistic steps are important to finding ways to suppress the production of trifluoroacetic acid. Kinetic data for the homogeneous gas-phase hydrolysis of CF3C(0)X compounds are seriously lacking in the open literature. Consequently, it is impossible to make timely assessments in the absence of useful data. To assess the importance of homogeneous hydrolysis as an effective removal process for CF,C(O)X compounds, ab initio molecular orbital calculations are carried out to locate activation energy barriers for the hydrolysis of CF,C(O)X compounds. The energetics for the decomposition of the hydrolysis byproducts of CF3C(0)X, namely CF3CX(OH)2,to form trifluoroacttic acid is also examined. Computational Approach Ab initio molecular orbital calculations were performed with the GAUSSIAN 88 system4 using split valence (3-21G)5 and polarization (6-31G*)6basis sets. All equilibrium geometries and transition-state structures were fully optimized at the HartreeFock level using analytical gradient methods? Electron correlation
c1 1.266
1.164 1.537 1.313 1.305 1.740 122.8 123.7 110.0 109.7 108.8 109.1
0.0 1.358 1.365 0.951 0.952 1.78 1 1.536 1.310 1.324 112.9 106.0 109.6 108.5 111.5 112.0 108.1 108.9 107.8 0.0
energy was estimated by the Morller-Plesset perturbation theory8 up to fourth order, including all single, double, and quadruple excitations ( M P a D Q , frozen core). Vibrational frequencies and zero-point energies were obtained from analytical second deriva t i v e ~calculated ~ at the HF/3-21G level using the HF/3-21G optimized geometry.
Results and Discussion Geometries. In the reaction of CF3C(0)X compounds with water, the expected major hydrolysis product is the formation of CF,CX(OH)2, viz. CF,C(O)X HzO CF3CX(OH)2 (1)
+
+
Decomposition of the hydrolysis product to form trifluoroacetic acid occurs by four-center elimination of HX via CF3CX(OH)2 CF,C(O)OH HX (2) +
+
The optimized equilibrium geometries for the reactant and product species involved in reactions 1 and 2 are given in Table I. The bond lengths for these species follow a well established trend for H a r t r e F o c k calculations,I0in that increasing the size of the basis set results in shorter bonds. Thus, the bond lengths with the 6-31G* basis set are systematically underestimated at the HartreeFock level. Similar basis-set dependencies of the bond angles in these molecules also occur. The 6-31G* basis set does predict bond lengths and angles for HF, HCl, H 2 0 , and CF3C-
4896 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
Francisco
TABLE II: Optimized Geometries (A and deg) for Transition States for CF3C(0)X Hydrolysis to Trifluororcetic Acid
-
reaction process CF,C(O)X H2O CFICX(OH)2
+
coordinate
H 1.322 1.623 1.402 0.970 1.073 1.500 1.342 1.356 127.8 79.1 91.9 117.5 122.0 111.2 109.8 2.5 1.324 1.311 1.300 0.971 1.608 1.042 1.509 1.341 1.337 113.0 91.9 71.6 109.1 111.8 -46.0
co
CO’ OH O’H’ ~~
~~
cx cc
CF C F’ H’O’C HO’C OCO‘
cco xco CFICX(OH),
-
CF$(O)OH
+ HX
FCC F’CC HOCO’
co
CO’ OH O‘H
cx cc
XH
CF C F’ HO‘C HOC
xco
FCC F’CC HOCX
HF/3-21G F 1.289 1.574 1.389 0.971 1.358 1.499 1.339 1.346 125.4 79.6 93.5 120.4 119.6 110.3 110.4 -3.3 1.294 1.304 1.212 0.97 1 1.792 1.206 1.499 1.340 1.334 114.0 86.3 85.0 109.1 112.0 -1.9
C1 1.289 1.540 1.399 0.972 1.903 1.506 1.339 1.342 123.9 78.9 94.8 121.2 119.5 109.8 110.8 -2.9 1.236 1.283 1.163 0.996 3.247 1.678 1.516 1.338 1.330 116.9 115.4 57.1 109.0 110.1 0.0
H 1.291 1.587 1.352 0.956 1.083 1.522 1.315 1.325 116.2 76.5 94.7 117.4 120.3 110.5 110.7 1.1 1.290 1.312 1.311 0.954 1.476 0.995 1.530 1.312 1.313 108.2 98.5 65.0 109.2 111.8 -2.6
HF/6-31GS F
C1
1.267 1.542 1.35 1 0.957 1.337 1.526 1.311 1.318 113.8 76.7 96.2 119.8 118.4 109.8 110.8 -4.1 1.267 1.282 1.138 0.955 1.853 1.241 1.527 1.312 1.304 110.1 86.3 84.5 108.9 111.5 -2.6
1.271 1.538 1.358 0.957 1.798 1.534 1.311 1.316 115.1 76.8 96.0 117.8 118.0 109.4 111.7 -3.1 1.228 1.262 1.095 0.985 3.21 1 1.708 1.533 1.309 1.301 112.2 111.6 58.0 108.7 110.5 0.0
3 995
1352
&311
1 0 1 39 85 ’”
31 8251 31 “’
‘Tr’
1.228”
-
-
Figure 1. Transition-state structures for the CF,C(O)X + H 2 0 CFICX(OH)2 reaction (where X = H, F, and C1). No asterisk refers to X = H parameters, a single asterisk shows X = F parameters, and a double asterisk refers to X = CI parameters.
Figure 2. Transition-state structures for the CF3CX(OH)2 CF,C( 0 ) O H + HX reaction (where X = H, F, and CI). No asterisk refers to X = H parameters, a single asterisk shows X = F parameters, and a double asterisk refers to X = C1 parameters.
(0)OH that are in very close agreement with experimental results.” No experimental structures are available for the CF3CX(OH)2compounds, but the present results obtained with the 6-31G* basis sets currently represent the best prediction for the geometry of these compounds to date. Geometries optimized using the 3-21G and 6-31G* basis sets for the transition state in reaction 1 are given in Table I1 and are illustrated in Figure 1. The most striking feature of the transition structure is the nearly coplanar ring defined by the heavy atoms and the transferring hydrogen from water. The largest degree of nonplanarity occurs for the fluorine case. The forming OH bond is, for the most part, similar at each level of theory. The effects of substituents have little influence on the OH bond in the transition state. However, substituent effects do play a role in the formation of the CO’ bond and the change in hybridization of the CO bonds. The substituents interact through the lone-pair system for the OCO’ system. Differences in the CO bond length
in going from hydrogen to chlorine substituents range from ca. 0.02 to 0.05 A. Optimizedgeometries for the transition state for species involved in reaction 2 are given in Table I1 and are illustrated in Figure 2. The transition state, which is characterized as a four-center 1,2-HX elimination process, is quite similar to its products, which indicates that these transition states are late. The breaking CX and HO bonds occur unsymmetrically,leading to a rather distorted four-center transition-state, which is a consequence of the orbital symmetry forbidden nature of these reactions. The dihedral angle defining the four-center ring HOCX in the case of X = H and F is 2.6O at the HF/6-31GS level of theory, which suggests that the transition structure is noncoplanar. However, for the chlorine case the transition structure has a coplanar ring. All harmonic frequencies of the transition state structures listed in Table I1 show one imaginary frequency for the dissociation as shown in Table 111.
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4897
Hydrolysis of Trifluoromethyl Carbonyl Halides
TABLE III: Calculated HF/3-21C Vibrational Frequencies (cm-I) and Zero-Point Energy (kcal mol-') for Reactions, Products, and Transition States for CF3C(0)X Hydrolysis to Trifluoroacetic Acid zero-point species frequency energy (ZPE) Reactants and Products H2 4654 6.6 HF 406 1 5.8 HCl 2850 4.1 H20 3947, 3814, 1800 13.7 CF,C(O)OH 3871, 2016, 1538, 1445, 1405, 1338, 1209, 846, 836, 700,622,610, 524,454, 26.2 410, 253, 241, 46 3282, 1962, 1562, 1439, 1393, 1362, 1122, 897, 742, 552, 545,460, 324, 259, 76 CFIC(0)H 22.8 2094, 1454, 1449, 1408, 1246, 850, 826, 724, 620, 533,451, 403, 248, 226,47 CF,C(O)F 18.0 2023, 1451, 1420, 1360, 961, 763, 757,607, 525, 510, 421, 344, 251, 203, 50 CFIC(0)Cl 16.6 CF,CH(OH), 3897, 3857, 3330, 1619, 1534, 1462, 1431, 1410, 1379, 1277, 1226, 1110, 882, 41.7 734, 622,616, 545,485, 441, 382, 322, 253, 320, 89 3887, 3880, 1599, 1479, 1450, 1415, 1354, 1329, 1217, 1098, 829, 704, 660, 642, CF,CF(OH)z 35.8 568, 538,454,407, 375, 371, 271, 239, 224, 75 CF,CCl(OH)2 3882, 3824, 1542, 1457, 1420, 1399, 1340, 1296, 1119, 916, 766, 664,629, 580, 34.4 491,464, 414, 379, 339, 337, 301, 238, 196, 76
CF3C(O)H + H20 CF$(O)F
+ H20
-
CFJC(O)Cl+ H20
+
CF$H(OH)2
CF$F(OH)z
+
CF$Cl(OH)2
+ H2
CF$H(OH)2-CF$(O)OH CF$F(OH)Z
+
CF,C(O)OH + HF
CF,CCl(OH)2+ CF3C(O)OH + HCl
Transition States 3838,3334,2427, 1602, 1469, 1420, 1380, 1135, 1328, 1268, 1030,907,840,751, 682, 593, 540, 441, 393, 373, 250, 207, 56, 16811 3845,2439, 1611, 1486, 1439, 1411, 1333, 1252,968,953,919,790,652,614, 576, 468, 440, 408, 379, 323, 219, 203, 74, 17921 3831, 2431, 1542, 1436, 1426, 1379, 1329, 1136,959,923,799,696, 610, 579, 479, 459, 411, 374, 324, 288, 215, 194, 76, 18261 3841, 2032, 1725, 1620, 1493, 1450, 1422, 1346, 1309, 1166,885,801,752,671, 621, 600, 537, 463, 439, 381, 266, 225, 85, 30201 3847,2296, 1667, 1591, 1463, 1414, 1346, 1205, 1104,970,839, 791,669, 639, 616, 555, 459, 399, 366, 283, 232, 196, 80, 17661 3408, 1854, 1701, 1520, 1471, 1427, 1356, 1282, 1121, 897, 868, 832, 770, 682, 585, 538, 451, 379, 269, 242, 162, 76, 18, 6261
Energetics. Total energies for HF/3-21G and HF/6-31G* optimized geometries for each reactant, product, and transition state, along with electron correlated energies, using HF/6-3 lG* geometries, are included in Table IV. Heats of reactions and activation energies are listed in Table V. CFjC(0)H+ H 2 0 System. The first step in the hydrolysis of CF3C(0)H by water is the formation of CF3CH(OH)2,viz. CF,C(O)H H20 CF3CH(OH)2 (3)
+
+
This reaction is exothermic by -13.8 kcal mol-'; however, a rather large activation energy of 54.6 kcal mol-' exists for the addition of water to CF3C(0)H, as calculated at the MP4SDQ/6-31G* level of theory. The predicted activation energy compares well with the estimated activation energy for the addition of water to formaldehydeI2of 48 kcal mol-', calculated at the HF/6-3 1G* level of theory. Trifluoroacetic acid is produced from decomposition of the hydrolysis product, CF3CH(OH)2,viz. CF3CH(OH)2
+
CF3C(O)OH
+ H2
(4)
by reaction through a four-center transition state. Decomposition into CF,C(O)OH H2 has an activation energy barrier of 75.9 kcal mol-' and is exothermic by 3.8 kcal mol-'. The estimated activation energy is not unreasonable when compared with the energy barrier of 86 kcal mol-' reported by Harding et al.13 for the addition of hydrogen to formaldehyde to form methanol via a four-center transition state calculated at the MP4/6-31G**/ /UMP2/6-31G* level of theory. It is interesting to note that to form trifluoroacetic acid from the reaction of CF3C(0)H and H20, an effective activation barrier of 62.1 kcal mol-' must be surmounted, which is an additional 7.5 kcal mol-' above the necessary 54.6 kcal mol-' activation barrier for the first addition step. CFjC(0)F+ H 2 0 System. The reaction of CF,C(O)F with water to form CF3CF(OH)2is calculated to be exothermic by 10.8 kcal mol-' and with a barrier of 35.5 kcal mol-'. The barrier is 19.1 kcal mol-' lower than that for CF,C(O)H with water. Consequently, one would expect the hydrolysis rate to be significantly increased. The activation energy for the formation of trifluoroacetic acid and hydrogen fluoride is estimated to be 38.5
+
37.8 32.6 31.3 34.5 32.9 31.3
TABLE I V Total Energies (hartrees) for Optimized Species Involved in the Hydrolysis of CF3C(0)X to Trifluoroacetic Acid HF/ MP4SDQ/ HF/ species 3-21G 6-31G* 6-31G* Reactants and Products -1.122 96 -1.126 83 -1.150 82 H2 HF -99.460 22 -100.002 91 -100.1 86 01 HCl -457.86943 -460.059 98 -460.208 77 -75.585 96 -76.010 75 -76.204 59 H2O CF$(O)OH -521.49563 -524.37427 -525.495 81 CF$(O)H -447.023 36 -449.485 17 -450.431 17 -545.359 19 -548.357 35 -549.468 25 CFIC(0)F CFIC(0)Cl -903.736 55 -908.390 73 -909.477 83 CF3CH(OH)2 -522.66002 -525.525 14 -526.66604 CFICF(OH)2 -620.994 17 -624.391 82 -625.696 57 -979.367 92 -984.42068 -985.70231 CFjCCl(OH)2
-
CFqC(0)H + H,O CF,CH(OH), CF,C(O)F + H20 CF>CF(OH), CF$(O)Cl +'H*O CFjCCl(OH)2 CF$H(OH)2 CF$(O)OH + H2 CFjCF(OH)2 CF$(O)OH + HF CFSCCl(OH)2 CF$(O)OH + HCI +
-
+
+
+
Transition States -522.567 89 -525.422 12 -526.581 08 -620.907 80 -624.294 16 -625.617 63 -979.282 13 -984.323 21 -985.62401 -522.467 86 -525.355 25 -526.514 15 -620.914 15 -624.31037 -979.36263
-625.63055
-984.410 19 -985.68522
kcal mol-'. The activation energy compares well with that estimated for the four-center elimination of C F 2 0 and H F from trifluoromethanol, which is estimated as 47.9 kcal mol-' at the MP4SDQ/6-31G* level of theory.I4 It should be noted that the activation energy for 1,2 elimination of H F in CF3CF(OH)2is significantly lower than that for 1,2 elimination of H2 in CF3CH(OH)2. Because of the lower activation energy, the activation energy barrier for the water addition to CF3C(0)F is ca. 7.8 kcal mol-' above the barrier for CF3CF(OH)2decomposition to form CF3C(0)OH and HF. This suggests that the CF3C(0)F + H 2 0 reaction proceeds across the bamer for CF3CF(OH)2dissociation to form CF,C(O)OH and HF.
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The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
Francisco
TABLE V Heats of Reaction plld Activation Energies (krrl mol-') for CF,C(O)X Hydrolysis to Trifluoroacetic Acid HF/ MP4SDQf HFI system reaction 3-21G 6-31G* 6-31G* AZPE CFIC(0)H CF,C(O)H HzO CFICH(0H)Z -3 1.8 -30.8 -19.0 5.2 CF,CHIOH),l* 26.0 ICFiC(O)H + H,O 46.3 53.3 1.3 26.0 15.1 -8.4 CF3CH(OH)2 +kF,C(O)OH + Hi-12.2 [CF$H(OH)2 CF,C(O)OH + Hz] * 64.6 -7.2 120.6 83.1 CF$(O)F H20 CF,CF(OH), CF,C(O)F -14.9 -14.9 4.1 -30.8 46.4 [CF,C(O)F + HZ0 CF$F(OH),]* 23.4 34.6 0.9 CF,C(O)OH HF CF$F(OH)2 9.2 24.0 9.2 -3.8 51.1 [CF,CF(OH)Z CF,C(O)OH + HF] * 41.4 50.2 -2.9 CF,C(O)CI H20 CF,CCI(OH), CF,C(O)CI -12.0 -12.5 4.1 -28.5 CF3CCI(OH)2]* 25.3 [CF,C(O)CI H20 49.1 49.1 1 .o HCI 1.8 CF,CCI(OH)z 4 CF$(O)OH -8.5 -1.4 -4.1 [CF3CCI(OH)2 CF,C(O)OH + HCI]' 3.3 6.6 10.7 -3.1
+ + + +
4
+
+
+
-
+
MP4SDQ/ 6-31G* AZPE -13.8 54.6 3.8 75.9 -10.8 35.5 5.4 38.5 -8.4 50.1 -5.5 7.6
+
CF,C(O)Cl+ H20System. The calculated barrier height for CF3C(0)Cl H 2 0 CF3CCl(OH)2is 50.1 kcal mol-I. This barrier is only ca. 4.5 kcal mol-' lower than the barrier for addition of water to CF3C(0)H. However, it is ca. 15 kcal mol-' higher than the barrier for addition of water to CF,C(O)F. Consequently, for the hydrolysis process, one expects the hydrolysis rate of CF3C(0)Cl to be faster than that of CF3C(0)H. The present calculations predict that the 1,2-HC1elimination of CF3CCl(OH)2via
+
CF,CCl(OH)2
-
CF3C(O)OH + HCl
(5)
has a small activation energy barrier of 7.6 kcal mol-'. Once the energy barrier for the addition of water to CF,C(O)Cl is surmounted, little CF3Cl(OH)2is expected to form, and consequently a major fraction is expected to spontaneously decompose via reaction 5 to form trifluoroacetic acid. The CF3CI(OH)2hydrolysis product is predicted to be transient in the CF3C(0)Cl H 2 0 reaction. The energetic ordering of the addition of water to CF3C(0)X compounds shows that the activation energy for the process is
+
CF3C(0)H > CF,C(O)Cl
> CF3C(0)F
(6)
All the addition reactions are thermodynamicallyexothermic, and the ordering follows a similar trend as in the activation energy with substituent substitution. The activation energy ordering of the 1,2-HX elimination of CF3CX(OH)2compounds follows a different trend than the addition process: CF3CH(OH)2 > CF3CF(OH)2 > CF,CCl(OH),
(7)
It is interesting to note that it is the 1,2-HX elimination process which controls the formation of the products CF3CX(OH), or CF,C(O)OH. If the activation energy for the forward dissociation via 1,2-HX elimination is greater than or equal to the activation energy for the reverse of the addition reactions, then CF3CX(OH)2 is the probable product of the hydrolysis. If the activation energy for the forward dissociation viz. 1,2-HX elimination is less than the activation energy for the reverse of the addition reaction, then CF3C(0)OHand HX are expected products from the hydrolysis. Figure 3 emphasizes the trends in the energy profile for the CF3C(0)H,CF,C(O)F, and CF3C(0)Cl reactions considered in this study. Viability of the Formation of Trifluoroacetic Acid from the Hydrolysis of CF3C(0)X Compounds. Of the CF3C(0)X compounds, gas-phase homogeneous hydrolysis is expected to yield trifluoroacetic acid for CF3C(0)F and CF,C(O)Cl. The formation of CF3CH(OH)2is expected to be the only product from the hydrolysis of CF,C(O)H. If formed, it should be removed readily by dissolving in cloud water droplets since it is stable to dissociation. Up to this point, only the relative reactivities of CF,C(O)H, CF3C(0)F, and CF3C(0)Cl are considered, in addition to the way substituent effects influence the choice of the product formation from the hydrolysis reaction. Results from this study show that the choice of substituent does influence the resulting hydrolysis products. If X = F or C1 in the CF,C(O)X reservoir species the resulting products will be trifluoroacetic acid.
L/-\ CFJ CClrOHh
" 0
cqc10wn~Ha
$ 5
Figure 3. Summary of the energy profile for the hydrolysis of CF3C(0)X
to trifluoroacetic acid (a) profile for CF,C(O)H, (b) profile for CF,C(O)F, and (c) profile for CF,C(O)CI.
Summary The present calculations show that gas-phase homogeneous hydrolysis of CF3C(0)X has substantially high activation energy barriers for the addition of water to CF,C(O)X. Substituent substitution has the most significant influence on the barriers for the 1,2-HX elimination reaction of the hydrolysis product CF3CX(OH)2in the formation of HX and CF3C(0)OH. However, the homogeneous gas-phase hydrolysis of CF,C(O)X compounds are expected to occur at a very slow rate in the atmosphere because of the high activation energy barriers for the addition of water. Consequently, homogeneous hydrolysis of CF3C(0)X
J. Phys. Chem. 1992,96,4899-4906 compounds in the troposphere and stratosphere are not major removal processes. Registry NO.Hi, 1333-74-0; HF, 7664-39-3; HCI, 7647-01-0; CF$(O)OH, 76-05-1; CF$(O)F, 354-34-7; CF$(O)CI, 354-32-5; CF3C(O)H, 75-90-1; CF$H(OH)2, 421-53-4; CF,CF(OH)2, 141293-73-4; CF3CCl(OH)2, 141293-74-5.
References and Notes (1) Chem. Brit. 1990, 217. (2) World Meteorological Organization Global Ozone Research and Monitoring Project, Report No. 10, Scientific Assessment of Stratospheric Ozone, 1989. (3) Francisco, J. S.; Ghoul, W. A,; Williams, I. H., to be published. (4) GAUSSIAN 88; Frisch, M. J.; Head-Gordon, M.; Schlegel, H. B.; Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; Defrees, D. J.; Fox, J. D.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R.; Kahn, L.
499
R.; Stewart, J. J. P.; Fluder, E. M.; Topial, S.;Pople, J. A. Gaussian, Inc.: Pittsburgh, PA, 1988. ( 5 ) Binkley, J. S.;Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980,102, 939. (6) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 66, 217. (7) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214. (8) Krishnan, R.; Pople, J. A. Znt. J. Quantum Chem. Symp. 1980,14,91. (9) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Znt. J . Quantum Chem. Symp. 1979, 13, 225. (10) Pople, J. A. In Modern Theoretical Chemistry; Schaefer, H. F., Ed.; Plenum: New York, 1977; Vol. 3. (1 1) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. In Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (12) Spangler, D.; Williams, I. H.; Maggiora, G. M. J . Compur. Chem. 1982. 4. 524. (lj)'Harding, L. B.; Schlegel, H. B.; Krishnan, R.; Pople, J. A. J . Phys. Chem. 1980.84, 3394. (14) Francisco, J. S.Chem. Phys. 1991, 150, 19.
H/D Isotope Effects in Water Radiolysis. 4. The Mechanism of (H)aqs (e-)aq Interconversion Ping Han and D.M. Bartels* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: December 9, 1991; In Final Form: February 11, 1992)
We report new measurementsof the reaction rates of H with OH- in light water and D with OD- in heavy water. Measurements performed by optical transient absorption and by EPR using the free induction decay attenuation method agree quantitatively. Arrhenius parameters for the H + OH- reaction are A = (1.33 f 0.16) X l O I 4 M-' s-' and E, = 38.38 f 0.31 kJ/mol. For the D + OD-reaction corresponding parameters are A = (1.19 f 0.22) X 1014M-'s-I and E, = 38.16 f 0.47 kJ/mol. The reaction rate data for the forward and reverse reactions, taken together with our current understanding of the electron and H atom solvation structure and dynamics, lead us to conclude that the reaction mechanism is best described as a proton transfer from the weak acid (H) to the strong base (OH-)aq. The H atom effectively disproportionates, leaving the electron to become hydrated within the atom solvent cage. By extension, numerous reactions of (e-)aqwith protonated aqueous species and water itself can best be understood in terms of a proton transfer to the weak base (e-)aq.
I. Introduction In an earlier paper of this series,' we found it essential to investigate the isotope effect in the highly unusual reaction of H or D with OH-/OD- in mixtures of light and heavy water:
(e-)aq
+ A H + (AH-) + A - + (H)aq
(2)
Reaction -1 of the hydrated electron with H20posed a particular problem for this conceptual framework because there is no thermally accessible unoccupied molecular orbital in the H 2 0 molecule into which the electron could reasonably be expected to "transfer". Moreover, the activation energy for reaction -1 is quite the slowness of the reaction clearly due to a large A new EPR-based method of reaction rate measurement, free "entropy barrier". Hart and Anbar were forced to postulate that induction decay (FID) attenuation, was invented for the purpose? a major solvent rearrangement somehow made possible the electron Investigation of the temperature dependence of the light water reaction rate revealed an unusual positive activation e n t r ~ p y . ~ transfer "onto" a single water molecule but bluntly admitted' "The mechanism of the H 2 0reaction is not understood." As we will Taken together with literature values for the reverse reaction show, data collected and theoretical simulations performed over rate,4Vs kl,these data lead to significant revisions of the tabulated the past two decades regarding the solvation structure of the thermodynamic properties of the hydrated electron p r o d ~ c t .In~ reactants and products now allow us to envision the approach and the present paper we complete our study of reaction 1 with separation of the species involved in this equilibrium. The measurements of the D OD- rate as a function of temperature properties of the transition state revealed by the reaction rate and confirm the EPR measurements with an alternative optical measurements then make it clear that proton transfer is the most transient absorption method. reasonable mechanistic description of the transformation. On the As indicated by the title of this paper, our goal in the further basis of this conclusion, we then speculate on the mechanisms of investigation of equilibrium (1) has been to deduce the mechanism other unusual reactions of the hydrated electron. of interconversion of hydrated electrons and H atoms. Little work on this subject can be found in the literature subsequent to the 11. Experimental Section 1970 publication of the book The Hydrated Electron by Hart and Anbar.6 These authors argued persuasively that all hydrated Pulsed EPR measurements of reaction rate 1 were carried out electron reactions should be interpreted in terms of an electronas described in previous ~ o r k . H ~ .and ~ D atoms were generated transfer mechanism. Those reactions that produce H atom as a by pulse radiolysis of aqueous solutions with 3-MeV electrons. product, it was argued, must proceed by way of a reduced inSolutions, containing 0.1 M t-BuOH to scavenge OH radicals, termediate or transition state which happens to contain a proton, were circulated continuously through a flat cell in the EPR cavity. as in Temperature was measured with thermocouples inserted into the
+
0022-365419212096-4899%03.00/0 0 1992 American Chemical Society
