8065
J. Phys. Chem. 1992,96, 8065-8069 (15) Hoshino, H.; Kogure, M. J. Phys. Chem. 1989,93,728. (16) Miyasaka, H.; Mataga, N. Bull. Chem. Soc. Jpn. 19% 63, 131. (17) Miyasaka, H.; Morita, K.; Kiri, M.; Mataga, N. Ultrafast Phenomena; Springer-Verlag: Berlin, 1990; Vol. VII, p 498. (18) Miyasah, H.; Morita, K.; Kamada, K.; Mataga, N. Bull. Chem. Soc. Jpn. 1990,63,3385. (19) Miyasaka, H.; Morita, K.; Kamada, K.; Mataga, N. Chem. Phys. Lett. 1991,178,504. (20) Miyasaka, H.; Morita, K.; Kamada, K.; Kiri, K.; Nagata, T.; Mataga, N.Bull. Chem. Soc. Jpn. 1991,178,504. (21) (a) Asahi,T.; Mataga, N. J. Phys. Chem. 1989,93,6575. (b) Asahi, T.;Mataga, N. Zbid. 1991,95, 1956. (22) Masuhara, H.; Ikeda, N.; Miyasaka, H.; Mataga, N. J . Spectros. Soc. Jpn. 1982,31, 19. (23) Miyasaka, H.; Masuhara, H.; Mataga, N. Loser Chem. 1985,I, 357.
(24) Miyasaka, H.; Ojima, S.; Mataga, N. J. Phys. Chem. 1989,93,3380. (25) Shida, T.; Iwata, S.; Imamura, M. J . Phys. Chem. 1974,78, 741. (26) Hiratsuka, H.; Yamazaki, T.; Maekawa, Y.; Hidaka, T.; Mori, Y . J. Phys. Chem. 1986, 90,774. (27) Land, E. J. Proc. R. Soc. London, Ser. A 1968,305,457. (28) Bensasson, R. V.; Gramain, J.-C.; J. Chem. Soc., Faraday Trans. 2 1980,76,1801. (29) Nishikawa, S.; Asahi, T.; Okada, T.; Mataga, N.; Kakitani; T. Chem. Phys. Lett. 1991,195,237. (30) Kakitani, T.; Yoshimori, A.; Mataga, N . Advances in Chemistry Series; Electron Transfer in Inorganic, Organic and Biological Systems; Bolton, J. R., Mataga, N., McLendon,G., Eds.;American Chemical Society: Washington, DC, 1991; Chapter 4; J. Phys. Chem. 1992,96,5385. (31) Yoshimori,A.; Kakitani, T.; Enomoto, Y.;Mataga, N. J. Phys. Chem. 1989, 93, 8316 and reference therein.
Theoretical Study of the Hydration Reaction of Ketene Per N. Skancke Department of Chemistry, The University of Tromss, N-9000 Tromss, Norway (Received: March 24, 1992)
The addition of monomeric and dimeric HzO to ketene has been studied by ab initio molecular orbital calculations using a 6-31G*basis set and energy calculations up to MP4(SDTQ). Complete geometry optimizations relaxing all internalmrdinatcp were performed. Our calculations indicate that a concerted addition of dimeric HzOto the carbon-oxygen double bond, yielding an enediol, is energetically slightly preferred to a similar addition to the carbon-xrbon double bond, yielding acetic were 17.4 and acid. The energy barriers obtained at the highest level of calculation, MP4(SDTQ)/6-3lG*//RHF/6-3lG*, 19.4 kcal/mol for the two additions, respectively. After a zero-point energy (ZPE) correction was introduced, the energies were 19.9 and 21.8 kcal/mol, respectively. The potential barriers to addition of monomeric HzO were found to be substantially higher at the same calculational level, the values being 37.8 and 41.4 kcal/mol for the addition to the carbon-xygen double bond and to the carbon-carbon double bond, respectively. The ZPE correction modified these values to 39.7 and 41.0 kcal/mol, respectively. The conversion of enediol to acetic acid through intramolecular hydrogen migration has an activation energy of 45.7 kcal/mol relative to the diol, calculated at the highest level and corrected for ZPE. A systematic search for stationary points representing noncyclic transient species for the system ketene and monomeric HzOrevealed only a weakly bound complex with a C,-O(water) distance of 3.02 A.
Recently there has been a revival of the chemistry of ketenes.' Their ability to undergo concerted [2 21 cycloaddition reactions2 has renewed the theoretical interest in the mechanism and kinetics of their reactions. Theoretical calculations related to cycloadditions, including dimerization of ketene have very recently been publishedI3 and in some cases the relevance of a concerted mechanism has been q u e s t i ~ n e d . ~ ~ . ~ Additions, both electrophilic and nucleophilic, constitute an important class of ketene reactions.lb Nucleophilic additions, using a variety of nucleophiles, have been invoked in synthetic work.4 Among these additions the hydration reactions still present some intriguing questions regarding reaction mechanism. Hydration reactivities of ketene itself and of a series of substituted ketenes have been measured in aqueous and other solvents containing varying concentrations of water.5*6The observed reaction rates in neutral solutions have been interpreted in termsof a mechanism involving an interaction between a lone pair in HzOand the ketene LUMO.6 In ketene the LUMO is extended in the molecular plane and has a large contribution from C,. This atom is thus assumed to be a likely site for nucleophilic attack. The reaction mechanism is complicated by the possible role of both monomeric and dimeric water as active reagents in the process. The baseinduced reaction is claimed to occur via a mechanism that is analogous to the one in neutral solution, the OH-ion playing the role of water? Acid-catalyzed reactions are assumed to involve a ratelimiting proton transfer to C, of ketene. Observed moderate steric effects from bulky substituents have led to the conclusion that the protonation occurs perpendicular to the molecular plane? Theoretical studies of hydration reactions of ketene and ketene derivatives in the gas phase have previously been p~blished.~ In one of the published studies7aonly dimeric HzO was considered as attacking agent. Furthermore, the geometry optimizationswere
+
SCHEME I
-
-
A. Addition to the C=O Bond HpC=C=O 1
+
H20 2
H2C=C=O
I
11
H 5
B. Addition to the C=C Bond H*C=C=O 1
+ H20
---)
H2C=C=O
-C
2
OH2 4
6
made using a minimum basis, and correlation energies were not included in the estimates of energy differences. In addition, optimizations appeared to have been made without relaxation of all internal coordinates. In a later refined calc~lation'~ it was confirmed that one of the stationary points previously claimed to be a transition state had two imaginary vibrational frequencies. More recent calculation^^^^^ have made use of double-t basis sets augmented with polarization functions, but energy differences were estimated without inclusion of electronic "elation. Furthmore, transition structures were not characterized by using analytically
0022-3654/92/2096-8065$03.00/00 1992 American Chemical Society
Skancke
8066 The Journal of Physical Chemistry, Vol. 96, No. 20. 1992 SCHEME I1 A. Addition to the C=O Bond
+ (H20)2
H2C=C=0 1
3
-
fn
=0,.
HpC=,C
'H
W
7
H'
5
3
1
4
11
9
B. Addition to the C=C Bond i
H H A
4
;c=c.=o
* '
3.020
:
8 10 3
11 7
-,2.983
2.628 :
10
calculated second derivatives in combination with extended basis sets. Referring to this, we have found it appropriate to study these processes over again using more advanced computationalmethods. This implies optimizations using polarization functions in the basis set, analytically evaluated second derivaties of the energies, and inclusion of correlation energies in the estimates of energy differences. In our study we have included both monomeric and dimeric H 2 0 as active species in the hydration reaction, and we have considered attacks on both the carbon-carbon and the carbon-oxygen bonds. The reactions examined in the present study may conveniently be classified according to different reaction schemes. In Scheme I we describe the addition of monomeric water, whereas Scheme I1 presents the addition of dimeric water. The conversion between the enediol and acetic acid may proceed through the following intramolecular H-atom migration:
/
H
").;
9
1
13
12
11
Computations
Geometry optimizations within the restricted Hartree-Fock approximation and with use of a standard 6-31G* basis sets were carried through for all the species involved in the study. The optimizationswere performed without any symmetry constraints. Thus no planar structures were assumed, and all 3N - 6 coordinates were relaxed during the optimizations. In the stationary points, stability tests9 of the restricted Hartree-Fock solutions relative to unrestricted ones were performed. The restricted solutions turned out to be stable for all species involved. Analytically calculated vibrational frequencies were obtained in all stationary points using the same basis set. Thus all the transition states and local minima found were properly characterized. In the stationary points, energies were calculated up to the MP4(SDTQ) level using the same basis. The program system GAUSSIAN 90 was used throughout the calculations.1° Energy differences were corrected for zero-point vibrational energies calculated using the same basis.
Results and Discussion Only a section of the parameters describing the fully optimized geometries are included in the paper. Figures 1 and 2 show bond
4
1.984
a 5
7 8
Figure 1. Optimized geometries of ketene (l),dimeric H 2 0 (3), weak complexes involving monomeric H 2 0 (4), dimeric H 2 0 (7) and (8), and the reaction products enediol (11) and acetic acid (12). Only key parameters are included in the figure. Basis set 6-31G*.
TABLE I: Dibedral Angles for the Cyclic Parts of the Fully O~timizedGeometries of the Transition States Studied" dihedral anale 5 6 9 10 13 2.4 165.9 14.4 -9.1 253.7 44.3 101.1 -5.3 9.5 a Basis set 6-3 1G*. Angles in degrees. For labeling of species and atoms, see Figures 1 and 2.
distances and valence angles that are key parameters describing the structural features of the transient species formed in the reactions studied. Dihedral angles essential for describing the nonplanarity of the transition states (TS's) 5,6,9,10, and 13 are given in Table I. 1. Addition of Monomeric H20. The initial interaction between ketene and H 2 0has the nature of a weak, attractive intermolecular force involving an oxygen lone pair and the ketene LUMO which has a dominant contribution from C,. The optimized geometry of this weakly bound complex, species 4, is shown in Figure 1. The H 2 0molecule attacks in the ketene plane, and the equilibrium distance between the H 2 0 oxygen and C, is found to be 3.02 A. The water molecule is found to be oriented in a plane orthogonal to the ketene plane and parallel to the ketene heavy atom framework. Due to the very weak interaction, amounting to a stabilization energy of 3.2 kcal/mol at the highest calculational
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8067
Hydration Reaction of Ketene
b 7 3 5
6
3
-
9 6 1.807
3
i 141.7 f 4 156ip;' 1.229
TABLE Ik Total Energies E(1+2) = E(ketene) + E(H20) and E(1+3) = E(ketene) + E(H,O,dimer) (au) and Energies (kcalhol) for Transient Species Relative to These at Geometries Optimized by HF/d31G*" RHF MP2 MP3 MP4(SDTQ) E(1+2) -227.735 419 -228.340440 -228.352 145 -228.385 183 AE(4) -3.3 -4.4 -4.2 -4.4 (-3.2)b AE(5) 51.5 36.2 38.7 37.8 (39.7) AE(6) 57.9 40.8 47.1 41.4 (41.0) AE(11) -11.8 -10.9 -14.2 -10.9 (-5.4) AE(12) -47.2 -47.1 -47.4 -46.4 (-40.3) AE(13) 47.8 36.8 38.6 38.1 (40.3) ZPE(1+2) 35.9 ZPE(4) 37.1 ZPE(5) 37.8 ZPE(6) 35.4 ZPE(11) 41.4 ZPE(12) 42.0 ZPE(13) 38.1 E(1+3) -303.755 129 -304.547 801 -304.564 952 -304.602 649 AE(7) -6.2 -8.0 -7.8 -8.0 (-5.6)b AE(8) -4.4 -6.9 -6.3 -6.8 (-5.1) AE(9) 31.9 14.9 18.2 17.4 (19.9) AE(10) 36.7 16.6 22.0 19.4 (21.8) ZPE(1+3) 52.5 ZPE(7) 54.9 ZPE(8) 54.2 ZPE(9) 54.9 ZPE(10) 54.8
" Energies calculated using the same basis. Zero-point vibrational energies (ZPE) (kcal/mol) obtained by analytically calculated vibrational frequencies using the same basis. Conversion factor used 1 au = 627.5 kcal/mol. Relative energies (kcal/mol) corrected for ZPE. 13 Figure 2. Optimized geometries of transition states for addition of monomeric H 2 0 to the C=O bond (5) and to the C=C bond (6) of ketene, for addition of dimeric H 2 0 to the same bonds, (9) and (lo), respectively, and for the H-atom migration converting diol to acid (13). Only key parameters are included in the figure. Additional parameters in Table I. Basis set 6-31G*.
level with zero-point energy (ZPE) correction, the orientation of the H 2 0 molecule in this plane is not determined with high accuracy. With this weakly bound complex as the starting point, the reaction may proceed along two different paths, one representing an addition to the ketene C=O bond and the other to its C-C bond. The optimized structures of the transition states resulting from these additions, species 5 and 6, respectively, are shown in Figure 2 with additional structural data in Table I. The energies of these species relative to ,??(ketene)+ E(H20) are given in Table I1 at the different calculational levels applied. It is worth mentioning that these four-membered Ts's are not planar. Optimizations toward a TS under the constraint of C, symmetry led to final structures that displayed three imaginary frequencies each, two of which were of aff symmetry. The optimized structure reveals a significant bending of ketene at this stage of the reaction (the CCO angle being 144.0°). The H-atom transfer to the ketene oxygen giving the resulting enediol has also developed significantly at this stage. This is confirmed by optimized O(water)-H and O(ketene)-H distances of 1.15 and 1.35 A, respectively. The resulting new C-0 bond has also developed appreciably at the TS,the length being 1.53 A. For this TS we found one imaginary frequency of 2041i cm-'. The associated normal mode describes a transfer of the H atom between the water and the ketene oxygens. Thus this is a proper TS for the reaction examined. In the TS for addition to the C = C bond, 6, leading to acetic acid, the deformation of the attacking water molecule has proceeded to a large extent (the 0-H bond distance is 1.26 A), whereas the angular deformation of ketene is moderate (bending angle of 160.9'). One interesting structural aspect of 6 is that the formation of the new C-H bond has proceeded to a large extent, the distance being 1.34 A. Furthermore the C-C bond has lost an appreciable amount of its ?r-contribution,the bond distance being 1.43 A. This occurs at a C-O(water) distance of
1.97 A and indicates that the addition is initiated by an electrophilic attack on Cp The nonplanarity of the TS is consistent with this interpretation. The nature of this TS was confirmed by one imaginary vibrational frequency, the normal mode of which is virtually a pure migration of H from the attacking water to the @-carbon. The vibrational frequency for this mode was found to be 2 120i cm-' . The relative energies of TS's 5 and 6 given in Table I1 show that the concerted addition to the C 4 bond is slightly preferred at all calculational levels applied. The energy difference between these two species is largest at the SCF level and diminishes when correlation energy corrections are included. At the highest level, MP4(SDTQ), the difference is reduced to 3.6 kcal/mol. When this difference is corrected for zero-point vibrational energies, we arrive at an energy difference of only 1.3 kcal/mol in favor of 5. This implies that it is hard to discriminate between these two modes of attack on the basis of thermochemistry alone. 2. Addition of Dimeric H20. Also in this case the interaction between ketene and the water system is initiated by weak intermolecular attractions. The calculations gave two such six-membered ring complexes, one involving the ketene C=O bond and the other its C==C bond. They are labeled 7 and 8, respectively. These complexes were optimized with full relaxation of internal coordinates and converged toward equilibrium geometries having no symmetry. Imposing a symmetry plane, forcing the sixmembered rings to be planar, led to structures having three imaginary frequencies each. At the optimized nonplanar geometries, all vibrational frequencies were real. Thus species 7 and 8 are local minima. Some of the final geometry parameters are given in Figure 1 and in Table I. The geometries are characterized by a long C,-oxygen distance, 3.02 and 2.98 A in 7 and 8, respectively. In the latter ring the Cghydrogen distance is found to be 2.63 A. The geometry of this complex is influenced by the approach of the hydrogen atom orthogonal to the ketene plane. In complex 7 the closest oxygen-hydrogen intermolecular distance is found to be 2.22 A. Due to the weak interactions, the geometries of ketene itself and the dimeric water moiety, labeled 3, are virtually unperturbed. These results are not in agreement with previously published calculation^,^^ where only addition complex 7 was included. The main discrepancy between the present results and those obtained previously7cis our finding of a nonplanar cyclic complex.
8068 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
The energy lowering by complex formation, obtained at the highest computational level, is found to be 8.0 and 6.8 kcal/mol for 7 and 8, respectively. These values are definitely larger than the corresponding values previously obtained by using smaller basis sets.' After introduction of ZPE corrections, the values are modified to 5.6 and 5.1 kcal/mol, respectively. From each of these complexes the reaction proceeds through the TS's depicted in Scheme 11. These are labeled 9 and 10 for addition to the C=O and C = C bonds, respectively. Some of the final geometry parameters are given in Figure 2 and in Table I. The geometry of 10 indicates that the forming of the new carbon-oxygen bond precedes the establishing of the C-H bond at the C, carbon. Nevertheless, the geometric arrangement of the three H atoms around this carbon is roughly tetrahedral at this stage of the reaction. The electronic reorganization around the C, atom is also reflected by the elongation of the C=C double bond to 1.401 A in the TS. The H atom linking the two water molecules together has shifted its position and is approximately midway between the two neighboring oxygen atoms at this stage of the reaction. The TS was characterized by a single imaginary frequency of 1509i cm-I. The dominant contribution to the associated normal mode is a shift of the attacking H atom toward the C, atom, and a concomittant migration of the H atom linking the water molecules. The optimized geometry of TS 9, shown in Figure 2 with additional data in Table I, displays the same features as those in the case discussed above. The bond between C, and the approaching oxygen has a length of 1.505 A, and the distance between the ketene oxygen and the attacking hydrogen is 1.520 A. Also in this TS the hydrogen atom linking the two water molecules is roughly midway between the neighboring oxygen atoms. The bending of ketene itself has reached far in the TS, the CCO angle being 136.5O. This TS has one imaginary frequency of 1376i c d . The associated normal mode has dominant contributions describing a synchronous migration of one H atom toward the ketene oxygen and one shifting position between the two water molecules. The optimized geometries found for the TS's are in serious disagreement with previously obtained results7cusing the same basis set. We find nonplanar structures for both of the species. Furthermore there are several large discrepancies between our interatomic distances and those published previously. In species 9 we find the optimized distance between the ketene oxygen and the approaching hydrogen to be 1.52 A in contrast to the previously published value of 2.74 A. Also for the remaining bond distances and valence angles in both 9 and 10, there are several substantial differences between the two sets of results. The energies of these TS's relative to the sum of the energies of the ketene and water dimer are given in Table I1 for the different levels of calculation. As revealed by the table, the activation energies for these additions are substantially lowered when corrected for electronic correlation. At the highest level of calculation, MP4(SDTQ), the activation energies are 17.4 and 19.4 kcal/mol for the concerted addition to the C = O and the C=C bonds, respectively. When ZPE correctionsare added, these values are further changed to 19.9 and 21.8 kcal/mol, respectively. Thus the barriers toward addition to these bonds are virtually of the same magnitude. This finding is in sharp contrast to one of the previously published theoretical values7a of 23.6 and 4.1 kcal/mol for addition to the C=C and c10bonds, respectively. On the basis of our calculations, we may furthermore conclude that the reaction involving the water dimer is energetically significantly preferred to the addition of the monomer. This may be interpreted in terms of a catalytic effect of the second H 2 0 molecule in neutral environment. The discrepancies between our predicted geometries and those published in a similar recent calculation7care also reflected in the estimated energetics of the reactions studied. At the SCF level we find that the activation energy for addition to the C 4 bond is 4.8 kcal/mol lower than for addition to the C=C bond of ketene. This energy difference is found to be 13.9 kcal/mol at the same calculational level in the previous work.7c
Skancke 3. -nI Migration of Hydrogem. The two alternative reaction paths described above lead to two diMrent products, viz., enediol and acetic acid, labeled 11 and 12, respectively. The optimized geometries of these are given in Figure 1, where only some of the structural parameters are included. Energetically the reaction leading to 12 is far more exothermic. The reaction energies are, relative to ketene and monomeric water, -10.9 and -46.4 kcal/mol, respectively. These values are modified to -5.4 and -40.3 kcal/mol, respectively, after introduction of ZPE corrections. The reaction leading from the most stable form of 11, shown in Figure 1, to 12 goes through a TS, labeled 13, shown in Figure 2. As shown in this figure, a significant change of the structure around C, has taken place as the reaction passes through the TS. The carbon atom has changed from a planar to a tetrahedral arrangement of bonds, and the carbon-carbon bond has changed from 1.32 A in 11 to 1.43 A in 13. This TS has one imaginary frequency of 2394i cm-I, the normal mode of which is a pure H-atom migration from the OH group in the enediol to the methyl carbon in acetic acid. The energy of this TS relative to the enediol is found to be 49.0 kcal/mol at the highest calculational level, MP4(SDTQ). This value changes to 45.7 kcal/mol by correction for ZPE. Our results indicate that a conversion from enediol to acetic acid cannot compete energetically with the direct production of acetic acid by addition of water in its dimer form. 4. Cyclic v e m Noacyclic Tramition S t a h In previous work it has been stressed that a distinction between a concerted hydration process and a stepwise one is still not reliably established.Ib It has been concluded that structural effects indicate the presence of a transition state with considerable polar character. In a stepwise process the reaction may have to go through a transient species of the form6 0-
H%, 4C-c' H
.P
O ' H', iI.,A
where the oxygen atom of the approaching water has transferrd one lone pair to the in-plain ketene LUMO which has its largest contribution from C,. In a subsequent step a hydrogen atom from water would migrate either to the ketene oxygen or to the C, atom depending on whether the addition is to the C=O or to the C-C bond of ketene. A transient species of this form, which is highly polar, has been invoked in the interpretation of the observed interaction with substituents.6 In a discussion of open versus cyclic transient species, the monomer H 2 0 may serve as an adequate model also for the dimer reaction as the interaction is initiated by an oxygen attack on C, in both cases. Systematic searches for a stationary point on the potential surface describing an open structure of the kind shown above failed. Irrespective of the starting point chosen, we ended up with a local minimum describing the weak complex 4, given in Figure 1. The optimized structures of the two cyclic transition states, 5 and 6, also give some support for the assumption that open transition states are rather unlikely in gas-phase hydrations of unsubstituted ketene. See the discussion of these structures given above. Thus the active role of the water H atom in an early stage of the reaction may indicate that a transient species like 14 need not be invoked in the mechanism. It should however be emphasized that our study is contiined to reactions in the gas p h w . In solutions there are certainly additional interactions generated by extensive hydrogen-bonded solvation shells. These interactions may in turn modify the potential surface for the hydration reaction. This problem has not been addressed in our study. It may be added that the electronic distribution in all the transition states gives rise to significant dipole moments, viz., 3.78, 2.63, 4.97, and 5.53 D for species 5, 6, 9 and 10, respectively. Acknowledgment. I thank the Research Council for Science and Humanities for financial support, making it possible to obtain access to the CRAY supercomputer at NTH/SINTEF, Trondheim. I also thank one of the reviewers for thorough and valuable comments.
J. Phys. Chem. 1992,96,8069-8073
Refereaces and Notes (1) (a) Tidwell, T. T. Acc. Chem. Res. 1990,23,273. (b) Seikaly, H. R.; Tidwell, T. T. Tetrahedron 1986, 42, 2587. (2) Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1969, 8, 781. (3) (a) Heinrich, N.; Koch, W.; Morrow, J. C.; Schwarz, H. J . Am. Chem. Soc. 1988,110,6332. (b) Wang, X.; Houk, K. N . J. Am. Chem. Soc. 1990, 112,1754. (c) Valenti, E.;Pcricas, M.A.; Moyano, A. J. Org. Chem. 1990, 55,3582. (d) Bernardi, F.;Bottoni, A.; Robb, M. A,; Venturini, A. J . Am. Chem. Soc. 1990, fI2,2106. (e) Scidl, E.T.; Schaefer,H. F. J . Am. Chem. Soc. 1991, 113, 5195. (f) Schaad, L. J.; Gutman, I.; Hess, B. A.; Hu, J. J. Am. Chem. Soc. 1991,113,5200. (4) (a) Baigrie, L. M.; Seikaly, H. R.; Tidwell, T. T. J . Am. Chem. Soc. 1985,107,5391. (b) Baigrie, L. M.; Lenoir, D.; Seikaly, H. R.; Tidwell, T. T. J. Org. Chem. 1985,50,2105. (c) Haner, R.; Laubc, T.; Seebach, D. J . Am. Chem. Soc. 1985,107,5396. (d) Naef, F.;Dcconant, R. Tetrahedron 1986,42,3245. (e) Fehr, C.; Galindo, J. J. Org. Chem. 1988,53, 1828. (f) Baigrie, L. M.; Leung-Toung, R.; Tidwell, T. T. Tetrahedton Lett. 1988,29, 1673. (g) Gong, L.; Leung-Toung, R.; Tidwell, T. T. J . Org. Chem. 1990,
8069
55,3634. (h) Arjona, 0.; de la Radilla, R. F.; Mallo, A.; Perez, S.; Plumet, J. J. Org. Chem. 1989, 54,4158. (i) Cieplak, A. S.; Tait, B. D.; Johnson, C. R. 1. Am. Chem. Soc. 1909,111, 8441. (5) A list of references to these experiments are found in ref la, (6) Allen, A. D.;Tidwell, T. T. J . Am. Chem. Soc. 1987, 109, 2774. (7) (a) Nguyen, M. T.; Hegarty, A. F. J. Am. Chem. Soc. 1984,106,1552. (b) Nguym, M.T.; Ruelle, P. Chem. Phys. Lett. 1987,138,486. (c) hhos, J.; Kresge, A. J.; Peterson, M. R.; Csizmadia, I. G. J . Mol. Struct. ( T H E 0 CHEW 1991, 232, 155. (d) Andraos, J.; Kresge, A. J. J. Mol. Struct. (THEOCHEM) 1991, 233, 165. (8) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972,56, 2257. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973,28, 213. (9) Seeger, R.; Pople, J. A. J . Chem. Phys. 1977, 66, 3045. (10) Frisch, M.J.; Head-Gordon, M.; Trucks, G. W.; Foreman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M. A.; Binklcy, J. S.;Gonmla, C.; Dcfm, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.;Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.;Pople, J. A. GAUSSIAN 90,Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1991.
Atmospheric Cryochemlstry: Oxygen Atom Reaction with the Fluorocarbon Freon 11 in Matrices. FTIR Spectra of Isolated COFCl and COFCI:Ci, Complex in Solid Argon L.Schriver,* 0. Abdelaoui, and A. Schriver Laboratoire de Physique Moleculaire et Applications, CNRS, UniversitC Pierre et Marie C u r i e T o u r 13-Bte 76, 4 Place Jussieu, 75252 Paris Cedex 05, France (Received: March 30, 1992; In Final Form: May 28, 1992)
Fourier transform infrared spectroscopy has been employed to investigate the reaction of CFCl3 (Freon 11) with atomic oxygen in argon and oxygen matries at 12 K. Ozone was employed as a photolytic oxygen atom source. The results indicate that the only primary reaction product is COFCl when the oxygen atom is in an excited state. No intermediate was observed. Upon thermal annealing after photolysis, diffusionthrough the matrix leads to the eficient formation of COFCI:C12aggregates. The spectra obtained after irradiation in various experimental series (isotopic ozone, wavelength dependence, product growth curves) are analyzed and have been interpreted from vibrational spectra of normal and isotopically substituted COFCl isolated in argon and from vibrational spectra of their molecular complexes with CIz embedded in argon. Fermi resonance doublets in the vM stretching region are observed mainly for the CI8OFCl species and the C'60FCl...C1z complex.
Introduction The fluorocarbons F-11 (CC13F) and F-12 (CCl,F,) are the two most abundant species of the anthropogenically produced halocarbons in the atmosphere.' Both species may influence the equilibrium concentration of stratospheric ozone2 and the radiative budget of the earth?s4 There has been speculation about oxidation of halogenated compounds in the troposphere that can remove the halocarbons before they can reach the stratosphere. The fluorocarbonsare considered inert, but since their lifetime is very long.5 a combination of many very slow processes can have a significant effect on the global budgets and hence on reducing the potential of these compounds for destroying the stratospheric ozone. In the present work, the matrix-isolation technique has been used in conjunction with FTIR spectroscopy to identify the products for the photolysis of ozone in the presence of CFC13. In the troposphere (A > 290 nm), ozone production occurs either via the photodissociation of NO2 into NO and 0 followed by the recombination of 0 with 026-7 or via carbon monoxide and hydrocarbon oxidation with NO, species (NO NO,) acting as a catalyst.* The reaction presently studied in a matrix between CFC13 and atomic oxygen differs from that known in the stratosphere where CFC13is photolyzed by short-wavelength ultraviolet radiation and then can be removed by reaction with O(lD) according to eq 1 in a first step?
+
CFxCIy+ hv(0'D)
+
+ C10
CFxCI,l
(1)
The irradiation domain used in our experiments for inducing in situ the O3photodissociation cannot release the chlorine atom from UV photolysis of CFC13.
Experimental Section The studies were conducted using a closed-cycle helium refrigerator (AirProducts Model 202A). Modification of the device allowed us to place the matrix sample in the beam path of the spectrophotometer through a CsI window or situate in the perpendicular direction the matrix sample in front of the photolysis source through a quartz window. Mixtures of CFC13/Ar (typically M/R = 1/500) and ozone/argon (typically M/R = 1/200) were deposited from separate inlets onto a gold mirror cooled at 12 K. Deposition time and rate were approximately 1 h and 5 "01-h-I, respectively. Infrared spectra were recorded by reflection after deposition and also after each photolysis on a Bruker IRF 113V in the 4000-400-cm-'spectral region, at a resolution of 0.5 cm-I (200 scans) with a frequency accuracy of fO.l cm-l. Samples were irradiated with a 90-Wmedium-pressure mercury lamp (Philip 93136) in combination with Schott and lO-cm water filters which exposed the samples to the following wavelengtb ranges: 330-800, 260-800, and full arc 240-800 nm. CFC13(from Merck Schurhardt) were degassed by freezethaw cycles and pumping at 77 K before matrix samples were prepared. Ozone was synthesized by Tesla coil discharge of Ozgas in a Pyrex finger immersed in liquid nitrogen. Diluted ozone samples were immediately used. Argon and oxygen were commercially obtained with a purity of greater than 99% and also used without further purification. The l 8 0 content of the enriched oxygen obtained from CEA was 50%. COFCl was prepared from a gas mixture of CFC13and ozone in a quartz bulb which was irradiated by the medium-pressure mercury lamp for 12 h. Oxygen was removed by vacuum pumping at 77 K, and then COFCl was vacuum distillated several times at 190 K.
0022-3654/92/2096-8069S03.00/0 ~,- 1 0 1992 American Chemical Society I
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