J. Phys. Chem. 1993, 97, 11908-1 1911
11908
1,2-Fluorine Shift Rearrangement of l,l,l-Trifluoromethylcarbene to Trifluoroethylene Suk Ping So Chemistry Department, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong Received: June 17, 1993"
The 1,Zfluorine shift rearrangement of CF3CH to CFz=CHF has been studied by a b initio molecular orbital calculations. The effects of basis functions, electron correlation and zero-point energy (ZPE) are considered. The reaction on the singlet potential energy surface has been computed to be exothermic with a vibrationally corrected MP4SDTQ/6-31++G** reaction energy of 60.1 kcal mol-' and a shift barrier of 23.5 kcal mol-' which compares well with the experimental estimate of 29 f 4 kcal mol-'. On the triplet surface, the reaction is, however, endothermic, and the corresponding energies are 6.05 and 16.0 kcal mol-' at the PMP4SDTQ/ 6-31++G** ZPE level of theory. This result for the barrier to the 1,2-fluorine shift in CF3CH differs from the reported prediction that the 1,2-hydrogen shift barriers in triplet CH3CH and CH2=C: are over 50 kcal mol-' and about 10 times larger than those for the singlet species.
+
Introduction
Calculation
Theoreticians and experimentalists have been interested over three decades in the intramolecular 1,2-atom shifts which convert carbenes to alkenes.lS2 Current theoretical methodologies complement experiment by providing accurate barriers to 1,2-atom shifts for small undetectable carbenes that cannot be probed spectroscopically and for carbenes whose 1,2-atom shift is too fast for current diagnostic technique^.^ Recent work has shown that halogen substituents attached to the carbenecarbon increase the barrier to the 1,2-hydrogenshift so that bimolecular reactions become competitive with intramolecular rearrangement.k7 The 1,2-hydrogenshifts have received the most attention and consequently less is known about 1,Zhalogen or 1,2-alkyl shifts to carbene centers.8 Experimental datae13 suggestthat, in general, the barriers to 1,2-atom shifts increase according to C1 < H < alkyl < F. Goddardl4 and Pople'5J6 investigated theoretically the 1,2-fluorine, 1,2-methyl, and 1,2-hydrogen shifts in fluorovinylidene CHF-C:, difluorovinylideneCF2=C:, and methylfluorovinylideneCH3CF=C:. Their results support the above trend in the barriers to 1,2-atom shifts. It has been well established17that 1,Zhydrogen shifts occur on the singlet potential energy surfaces of carbenes, vinylidenes and nitrenes and the barriers for the rearrangement of the triplet species are very high. For example, the barriers have been found theoretically to be 53 and 55 kcal mol-' for triplet methylcarbene CH3CH (with partially optimized geometry)ls and vinylidene CH2=C:,19 respectively,as contrasted to 0.6 and -3.0 kcal mol-' for the corresponding singlet specie^.^^^^ Recently, Holmes and Rakestrawz' studied the 1,I-elimination reaction of HCl from chemically activated l,l,l-trifluoro-2chloroethane CF3CH2Cl in the gaseous phase to form l , l , l trifluoromethylcarbene CF3CH, which may then undergo a 1,2fluorine shift to give trifluoroethylene CFz=CHF. A threshold energy barrier of 29 f 4 kcal mol-' was estimated for the rearrangement reaction. The ground state of CFJCH has been calculated22 to be triplet with the lowest singlet state lying 13 kcal mol-' higher in energy. Thus, the 1,Zfluorine shift rearrangement of CF3CH was taken21 to proceed on the singlet surface and not to occur when it was quenched to the triplet state. In this note, we would like to report our study on the 1,2fluorine shift on the singlet and triplet potential energy surfaces of CF3CH in order to produce some evidence for or against the conclusions of Holmes and Rakestraw.21
The equilibrium and the transition-state structures were optimized by the energy gradient method at the RHF and UHF levels for the singlet and triplet states, using the Gaussian 90 and 92 programs23implemented on our IBM RS/6000 Model 320H and 340 workstations, respectively. The basis sets used are the standard split-valence 4-31G, 6-31G**, and 6-31++G** ones. Theenergiesofthevariousstructuresattheoptimized6-31G** and 6-31++G** geometries were then recalculated by thefourthorder (MP4) perturbation theory with the Maller-Plesset partitioning of the Hamiltonian in order to take into account electron c o r r e l a t i ~ n .The ~ ~ fourth-order calculations (with frozencore) done in this work (MP4SDTQ) are complete in the sense that the effects of single, double, triple, and quadruple excitations are all included.2s Harmonic vibrational frequencies were computed by analytically differentiating the energies twice. They were determined to verify the nature of the stationary point structures and for the sake of zero-point energy corrections.
-
Abstract published in Aduance ACS Abstracts, October 1, 1993.
0022-365419312097-11908$04.00/0
Results and Discussion Geometry optimization yields a planar structure for singlet CF2=CHF but a nonplanar one for triplet CFz=CHF. The transition state structures for the 1,2-fluorineshiftrearrangement reaction of CF3CH to CF2=CHF have been found to be nonplanar on both the singlet and triplet potential energy surfaces. The optimized geometries are listed in Tables I and I1 for the various species studied, the numbering of whose atoms is depitched in Figure 1. Species 1S/1 T are respectively the singlet/triplet transition-state structures for the internal rotation of the CF3 group about the CC bond of CF3CH, while 2S/2T are those for the 1,Zfluorine shift reaction of CF3CH. It has been established26 that d polarization functionsoncarbon are required to give good C F bond lengths in fluorocarbons;their addition shortens these bond distances significantly. Hence, it is not surprising to find that the C F bond lengths of CF3CH and CFz=CHF computed with the 4-31G basis functions are much longer than thoseobtainedwith the6-31G** and the6-31++G** basis sets. The geometriesof CF3CH and CFz=CHF, optimized by Dixon and co-workers using a basis set of double zeta quality augmented with d polarization functions only on the carbene carbon atom of the former22and on both ethylene carbon atoms of the latter,26are reproduced in Table I for comparison purpose. The effect of polarizationfunctionson the C F bonds is pronounced as expected. 0 1993 American Chemical Society
Rearrangement of 1,1,1-Trifluoromethylcarbene
TABLE I: Optimized huilibrium Structures' singlet species 4-31G 6-31G**
The Journal of Physical Chemistry, Vol. 97, NO.46, 1993 11909
____
triplet 6-31++G**
4-31G
6-31G**
6-3 l++G**
1.504 1.090 1.362 1.347 106.3 109.6 114.3 59.6 180
1SO8 1.091 1.325 1.314 103.4 109.4 114.0 59.8 180
1.510 (1.512)c 1.090 (1.091) 1.327 (1.376) 1.315 (1.362) 103.3 (103.1) 109.3 (114.4) 114.2 (1 10.3) 59.7 180
1.463 1.066 1.362 1.356 129.8 112.1 111.5 59.8 180
1.479 1.071 1.324 1.319 129.3 111.7 11 1.0 59.8 180
1.480 (1 .474)c 1.071 (1,071) 1.327 (1.377) 1.321 (1.369) 129.2 (125.9) 111.8 (111.6) 111.2 (112.5) 59.8 180
1.299 1.062 1.360 1.339 1.333 124.0 120.4 122.5 125.9 180
1.304 1.068 1.327 1.303 1.298 123.3 120.4 122.9 125.5 180 0 180
1.306 (1.307)d 1.068 (1.070) 1.327 (1.331) 1.304 (1.310) 1.298 (1.304) 123.5 (123.1) 120.5 (121.2) 123.0 (122.8) 125.5 (126.0) 180 0 180
1.45411.460 1.06511.067 1.36611.361 1.35511.352 1.350/ 1.356 123.11124.1 117.1/115.6 117.211 15.3 116.9J116.4 152.8J152.0 87.7153.2 138.9176.8
1.47311.480 1.07311.076 1.32911.325 1.31911.320 1.31411.3 16 120.7p21.1 115.6/114.5 115.811 15.5 115.4/114.0 144.01142.9 78.8153.6 151.1174.4
1.47411.48 1 1.072/1.076 1.33011.326 1.320J1.321 1.31411.316 120.91121.3 115.5/114.4 115.911 15.4 115.711 14.2 144.31142.4 79.61533 150.1174.2
0 180
Bond lengths are in angstroms and bond angles in degrees. See Figure 1 for numbering of atoms. 7 and 7, are the FCCH and F,CCH dihedral angles, respectively. Basis set used is DZ + d function on carbene carbon atom.22 Basis set used is DZ + d function on the two carbon atoms.26 a
TABLE II: O~timizedTransition-State Structures. singlet triplet species 6-31G** 6-31++G** 6-31G** 6-31++G** ~~
0
1.516 1.090 1.319 1.330 107.0 108.6 116.2 120.4 0
0
1.48 1 1.070 1.324 1.326 129.6 112.0 110.9 119.9 0
1.463 1.086 2.042 1.263 1.252 106.9 45.8 128.0 123.0 105.1 36.5 151.2
1.470 1.086 2.152 1.263 1.250 105.5 41.7 128.0 123.2 107.7 38.3 144.8
1.516 1.073 2.377 1.319 1.311 120.1 30.7 110.3 112.5 142.1 68.6 166.9
1.517 1.073 2.382 1.319 1.310 119.9 30.5 110.5 113.2 141.6 70.2 164.9
1.513 1.091 1.318 1.328 107.0 108.4 116.4 120.5
1.481 1.070 1.322 1.324 129.6 111.9 110.7 119.9
IS,IT
2s
2T
Figure 1. Equilibrium and transition-statestructures.
See footnotes of Table I. For the triplet states of both the equilibrium and the transition state structures, the S 2 values obtained here at the U H F level range from 2.01 1 to 2.015, almost identical to the value 2 of a true triplet. Hence, the unpredictable spin contamination effect due to U H F wave functions on molecular geometry2' may be neglected. The nonplanar triplet CF2=CHF has two rotomeric forms which have very similar molecular dimensions but different relative orientations of the CF2 and C H F groups. The rotomer with the CF bond of the C H F group staggered to the FCF bond angle (- 130°) of the CF2 group has a total energy somewhat lower than the one with the CH bond staggered to the FCF angle (Table 111). Thislatter rotomer was not considered when relative energies were calculated. It is interesting to note from Tables I and I1 that the geometrical structures of the transition state 2S/2T for the 1,2-fluorine shift
of CF$H to CFz=CHF resemble more closely those of CFz=CHF (Le., the product) than CF3CH (Le., the reactant). This is most easily seen by noting that the FcCCFtdihedral angles of singlet 2 s (176.9O at 6-31++G**) and triplet 2T(124.g0) are much more close to the corresponding angles ( 180° and 130.3O/ 128.0') of CFz=CHF than those (119.4O and 119.6O) of CF3CH, and that the migrating fluorine atom is closer to the carbon atom to which it is migrating than it is'to the carbon atom from which it originates. The near-planarity of the CF2 group of the transition-state structure 25 is consistent with the fact that singlet CF2=CHF is planar. Frequency calculations show that both 6-31G** and 6-31++G** basis sets yield the same number of imaginary vibrational frequencies for each of the structures investigated (viz. one for transition states and zero for equilibrium structures) except for singlet CF3CH and triplet 1T. The 6-31G** and the 6-31++G** bases give respectively one and no imaginary
11910 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
so
TABLE III: Total Energies (au). of Equilibrium and Transition-State Structures singlet species methodb 6-3 1 F** 6-31 ++G** CFaCH
HF MP4SDTQ PMP4SDTQ HF MP4SDTQ PMP4SDTQ HF MP4SDTQ PMP4SDTQ HF MP4SDTQ PMP4SDTQ
CFNHF
1S/lT 2S/2T
Total energy = -(370
+ value listed).
4.496 05 5.276 35
4.512 59 5.311 67
4.574 20 5.376 28
4.591 1 1 5.411 17
4.493 91 5.274 01
4.510 00 5.308 68
4.436 18 5.239 88
4.454 84 5.274 98
singlet
49, 360,420, 519, 525,577, 813, 1036, 1157,1246.1294.2875
15.5
triplet
157,365,410,.518,550,597,829,886,
15.7
1155,1201,1287,3046 CF24HF
singlet triplet
1s 1T 2s 2T
225, 305,468, 583,603,814,914, 1133, 1246,1347,1804,3078 124,212, 344, 509,641,725,840, 1105, 1173,1262,1368,3023 225i, 361,443, 523, 527, 580,811, 1021, 1188,1216,1274,2871 171i, 363, 379, 523, 545, 598,832, 883, 1169,1188,1283,3051 3361, 127, 388, 569,735,777, 860,977, 1190,1305,1389,2865 129i, 226, 388,491, 528,740, 917, 1142, 1175,1261,1357,3015
17.9 16.2 15.5
15.5 16.0 16.1
* Frequencies have been uniformly scaled by a factor of 0.89 and imaginary frequencies neglected in calculating zero-point energies. frequency for singlet CFjCH, while the opposite result is found for triplet 1T. However, it is seen from Tables I and I1 that the geometries of all the species studied remain practically constant (possibly except the CF bond and the FCCh angle of 2s) when the basis set is extended from 6-31G** to 6-31++G**. This illustrates the well-known fact that molecular geometries are generally much less wave-function-dependent. Accordingly, only the 6-3 1++G**results will be considered in the discussionbelow. Total energies, relative energies with respect to CF3CH vibrational frequencies (uniformly scaled by a factor of 0.89 to allow both for SCF approximation and for anharmonicity) and zero-point energies (ZPE) of the species under investigation are given in Tables 111-V. CF3CH is calculated here to have a triplet ground state with its lowest singlet state lying 13.0 kcal mol-' (triplet at PMP4SDTQ/6-31++G** + ZPE and singlet at MP4SDTQ/6-31++G** + ZPE)higher in energy. This tripletsinglet energy difference has also been obtained by Dixon.22 The barrier to internal rotation about the CC bond is calculated to be 1.88 kcal mol-' for singlet CF3CH but 0.126 kcal mol-' for triplet CF3CH at the highest level of theory considered. The former value falls within the range of barrier heights (e.g., from
TABLE V
6-31++**
4.545 02 5.299 00 5.300 1 1 4.520 8414.520 21 5.284 8615.283 74 5.285 8615.284 64 4.544 30 5.298 13 5.299 27 4.506 58 5.269 5 1 5.270 48
4.559 35 5.331 67 5.332 77 4.540 0614.538 96 5.322 9015.321 18 5.323 9315.322 12 4.558 68 5.331 1 1 5.332 25 4.525 30 5.306 86 5.307 87
RHF and RMP4SDTQ for singlets and UHF and UMP4SDTQ for triplets.
TABLE I V Calculated 6-31++C** Vibrational Frequencies (cm-l) and Zero-Point Vibrational Energies (kcal mol-')' for Equilibrium and Transition-State Structures species frequencies ZPE CF&H
triplet
6-31G**
0.78 kcal mol-' for CH3COCH3 to 4.7 kcal mol-' for CH3C( C H S ) ~ ~to* )internal rotation about the CC bond of various molecules. The 1,Zfluorineshift reaction of CF3CH is predicted here to be exothermic on the singlet potential energy surface but endothermic on the triplet one. Thus, the above finding of a late singlet transition state contradicts Hammond's postulateZ9which states that. for an exothermic reaction. the transition state should more closely resemble the reactant than the product. Results in Table V reveal that on the singlet potential energy surface the effect of electron correlation increases the exothermicity of the 1,2-fluorine shift reaction of CF3CH by 13.1 kcal mol-' but decreases the activation energy of this reaction by 13.2 kcal mol-'. However, on the triplet surface, it decreases both the endothermicity and the activation energy by 6.60 and 5.80 kcal mol-', respectively. The electron correlation effect on the barrier to internal rotation about the CC bond of CF3CH is much smaller probably because there is no bond breaking/forming in this process. The approximate spin-projected%PMPAsDTQ energies of the triplet equilibrium and transition state structures are seen to differ only slightly (by -0.001 au for total energies and by 60.05 kcal mol-' for relative energies) from the corresponding spin-unprojected UMP4SDTQ values in line with the negligible difference between their computed Szvalues of 2.01 1-2.015 and that of 2 for a true triplet state. As noted above, the 1,Zhydrogen shifts of carbenes, vinylidines and nitrenes have been foundI7to occur on the singlet rather than the triplet potential energy surface owing to energy barrier reasons. For example, the theoretical barriers3J8to the 1,Zhydrogen shifts of triplet CH3CH and CHz'EC: exceed 50 kcal mol-', being about 10 times larger than those of the singlet species. In view of these reported barriers and a triplet ground state predictedZZ for CFjCH, Holmes and Rabestrawzl concluded that the 1,2fluorine shift of CF3CH proceeds on the singlet potential energy surface and that once excited CF&H is collisionally quenched to its triplet ground state, the 1,Zfluorine shift will not occur. In addition, they estimated experimentally a threshold barrier of 29 f 4 kcal mol-' for the 1,2-fluorine shift of singlet CF3CH. Theoretically, the reaction energy and barrier of the 1,2-fluorine shift reaction of CF3CH on the singlet potential energy surface have been computed in this work to be -60.1 and 24.0 kcal mol-' respectively at the MP4SDTQ/6-31++G** + ZPE level of theory. The correspondingvibrationallycorrected PMP4SDTQ/ 6-31++G** energies of this reaction on the triplet surface are 6.05 and 16.4 kcal mol-'. Hence, the small difference (7.6 kcal
Relative Energies (kcal mol-') with Reswct to CFqCH Calculated at 6-31++G** Geometries singlet triplet ~~
species
HF
CFaCH CF24HF 1S/lT 2S/2T
0 -49.3 1.63 36.2
MP4SDTQ 0 -62.4 1.88 23.0
MP4SDTQ + ZPE 0
-60.1 1.88 23.5
HF 0 12.1 0.420 21.4
MP4SDTQ 0 5.50 0.351 15.6
PMP4SDTQ 0 5.55 0.326 15.6
PMP4SDTQ t ZPE 0 6.05 0.126 16.0
Rearrangement of 1,1, l -Trifluoromethylcarbene mol-') of the barriers predicted here for the 1,2-fluorine shifts in singlet and triplet CF3CH and the barrier for the triplet species being lower are interesting and represent a first finding of its kind for the 1,2-atom or 1,2-group shifts in carbenes, vinylidenes, and nitrenes. Obviously, although the experimentally estimated barrier of Holmes and Rakestraw*' agreesvery well with our best calculated value for the 1,2-fluorine shift of singlet CF3CH, their concluded inability of triplet CF3CH to undergo this shift reaction due to a high barrier contradicts the present result. The 1,2-fluorine shift on the triplet surface being not observed by Hohmes and Rakestraw is most likely due to one or more of the following: (i) thesinglet shift barrier lay near theonset of theenergy distribution of the chemically excited singlet CF3CH molecules produced so that most molecules have sufficient energy to overcome the barrier, (ii) very few singlet CF3CH molecules were quenched to the triplet ground state due to inefficient intersystem crossing under their experimental conditions and in the absence of heavy atoms in the molecules, and (iii) the reaction is endothermic. It is thus hoped that this calculation will stimulate further and more experimental as well as theoretical works along this line on the 1,Zatom shifts in various singlet and triplet carbenes.
Acknowledgment. I would like to thank the Hong Kong Research Grants Council for its financial support in the acquisition of the IBM RS/6000 Model 340 workstation. References and Notes (1) Kistiakowsky, G. B.; Mahan, B. H. J. Am. Chem. SOC.1957, 79, 2412. (2) Chong, D. P.; Kistiakowsky, G. B. J . Phys. Chem. 1964,68, 1793. (3) Evanseck, K. D.; Houk, K. N. J . Phys. Chem. 1990, 94, 5518. (4) Moss, R. A.; Mamantov, A. J. Am. Chem. Soc. 1970, 92, 6951. (5) Liu, M. T. H.; Subramanian, R. J. Chem. SOC.,Chem. Commun. 1984, 1062; J . Phys. Chem. 1986, 90, 75. (6) LaVilla, J. A.; Goodman, J. L. J . Am. Chem. SOC.1989,111,6877. (7) Liu, M. T. H.; Bonneau, R. J. Am. Chem. SOC.1990,112,3915 and
references therein.
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 11911 (8) Fields, R.;Haszeldine, R. N. J. Chem. Soc. 1964, 1881. (9) Haszeldine, R. N.; Parkinson, C.; Robinson, P. J.; Williams, W. J. J . Chem. Soc., Perkin Trans. 1979, 2, 954. (10) Haszeldine, R. N.; Rowland, G. J.; Speight, G. J.; Tipping, A. E. J. Chem. Soc., Perkin Trans. 1979, 1, 1943. (1 1) Atherton, J. H.; Fields, R.; Haszeldine, R.N. J . Chem. Soc. C 1971, 366. (12) Bevan, W. I.; Haszeldine, R. N.; Middleton, J.; Tipping, A. E. J. Chem. Soc., Dalton Trans. 1975, 620. (13) Haszeldine, R. N.; Pool, C. R.; Tipping, A. E.; Wats, R. OB. J. Chem. Soc., Perkin Trans. 1976, 1, 513. (14) Goddard, J. D. J. Mol. Strucr. 1985, 133, 59; Chem. Phys. Lett. 1981, 83, 312. (15) Pople, J. A. Pure Auul. Chem. 1983.55, 343. (16) Frisch, M. J.; Krishnan, R.; Pople, J. A.; Schleyer, P. v. R. Chem. Phys. Lett. 1981, 81, 421. (17) Schaefer 111, H. F. Acc. Chem. Res. 1979, 12, 288. (18) Harding, L. B. J. Am. Chem. Soc. 1981, 103, 7469. (19) Conrad, M. P.; Schaefer 111, H. F. J . Am. Chem. Soc. 1978, 100, 7820. (20) Gallo, M. M.; Hamilton, T. P.; Schaefer 111, H. F. J. Am. Chem. Soc. 1990, 112,8174. (21) Holmes, B. E.; Rakestraw, D. J. J. Phys. Chem. 1992, 96, 2210. (22) Dixon, D. A. J. Phys. Chem. 1986.90, 54. (23) (a) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J.
B.; Schlegel, H. B.; Raghavachari, K.; Robb, M.; Binkley, J. S.; Gonurlez, C.; DeFrees, 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, Revision F; Gaussian, Inc.: Pittsburgh, PA, 1990. (b) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, 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. (24) Mprller, C.; Plesset, P. S.Phys. Rev. 1934, 46, 618. (25) Krishnan, R.; Pople, J. A. Int. J. Quantum Chem. 1978, 14, 91. (26) Dixon, D. A.; Fukunaga, T.; Smart, B. E. J. Am. Chem. SOC.1986, 108, 1585. (27) McDouall, J. J. W.; Schlegel, H. B. J. Chem. Phys. 1989, 90,2363,
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