2210
J . Phys. Chem. 1992, 96, 2210-2216
pentane from an 80-cm column packed with silica gel (2-250. B i c y ~ 4 . 2 . 0 ~ - 2 , 4 d i e n(Bo). e The synthesis and isolation of this compound have been described recently.20 Samples were purified by gas chromatography (GC) on &9'-oxydiproprionitrile (ODPN) before each experiment. !bVinyl-1,3-cyclohexadlene(SVCH). 5VCH was prepared by the method of von Doering and Roth'O and purified by GC. 1- and 2-Vinyl-1,fcyclohexadiene(1VCH and 2VCH). A mixture of the two isomers was obtained according to S ~ a n g l e r . ~ ' Their separation was achieved by GC on a column containing 0-xylyl dicyanide impregnated with Procedures. The techniques used to obtain radical ion spectra in Freon glasses and argon matrices have been outlined previO U S ~ Optical ~ . ~ spectra ~ ~ ~ were ~ ~recorded ~ using a Cary 171 (Kyoto) or a Perkin-Elmer Lambda 9 spectrometer (Fribourg). (30) von Doering, W.; Roth, W. R. Tetrahedron 1963, 19,715. (31) Spangler, C. W. Tetrahedron 1976, 32, 2681.
Background absorptionsbefore X- or y-irradiation were subtracted unless otherwise noted.
Acknowledgment. Y.F. and T.S. wish to thank Dr. K. Tanaka (Kyoto University) for various suggestions and assistance in synthesizing DHB and the three VCHs. The Kyoto work was supported by subsidies from Scientific Research of the Ministry of Education in Japan, Grants No. 62606006 and 63606005. T.B. and K.R. express their gratitude to Prof. M. Gross and Dr. C. Warner (University of Nebraska) for their gift of a sample of 5VCH. The efforts of Mr. Philippe Wrro (University of Fribourg) in the separation of lVCH and 2VCH are gratefully acknowledged. We also thank Prof. E. Haselbach for his continuing support and encouragement. The work in Fribourg was supported by the Swiss National Science Foundation, Grant No. 2028842.90. Registry NO. DHB", 95589-50-7; OT", 72257-39-7; BZ, 71-43-2; ET, 74-85-1.
An Experimental Estimate of the Threshold Barrier for the 1,P-Fluorine Atom Migration in l,l,l-Trifluoromethylcarbene Bert E.Holmes* Department of Chemistry, Arkansas College, Batesville, Arkansas 72501
and David J. Rakestraw? Department of Chemistry, Ohio Northern University, Ada, Ohio 4581 0 (Received: October 29, 1991)
A threshold energy barrier of 29 f 4 kcal/mol was estimated for the 1,2-fluorinemigration reaction converting l,l,l-trifluoromethylcarbene, CF3CH, into CF2==CHF in the gas phase. The CF3CH was formed by the 1,l-elimination of HCl from chemically activated CF3CH2C1containing 97.5 kcal/mol of internal energy. RRKM theory was used to calculate rate constants for the 1,2-fluorineshift that were fitted to the experimental pressure dependence to determine the threshold barrier.
Introduction Intramolecular 1,2-atom migrations converting carbenes to alkenes have been of interest to theoreticians and experimentalist for over three Carbene precursors' and new detection techniques monitoring short-lived speciese6 have recently been developed so that the richness of carbene reaction chemistry is now being unraveled at a remarkable pace. Current theoretical methodologies complement experiment by providing accurate 1,2-migration threshold energies for small, undetectable carbenes that cannot be probed spectroscopically and for carbenes whose 1,Zatom shift is too fast for current diagnostic techniques.' Little is known about methylcarbene, the simplest carbene that can rearrange, because the very small threshold barrier for 1,2-hydrogen shift, Eo( 1,2-H), leads to intramolecular reaction before the carbene can be detected or intercepted. The most sophisticated calculations predict an E,( 1,2-H) of 0.6 kcal/mol for CH3CH.' Recent work has shown that halogen substituents attached to the carbene carbon increase the Eo( 1,2-H) so that bimolecular reactions become competitivewith intramolecularrearrangement.8 Solution-phase Arrhenius parameters for the 1,Zhydrogen shift in methylchlorocarbene, CH3CCl, are E, = 4.9 f 0.5 kcal/mol and the A factor = (6.0 f 4) X lo9 s-I. Determinations of the Arrhenius parameters were based on time-resolved photoacoustic calorimetry in solution,8a and laser flash photolysis has confirmed the overall rate constant.8b The laser flash photolysis technique
'
Present address: Sandia National Laboratory, Combustion Research Division-8362, Livermore, CA 94551.
gave Arrhenius activation barriers of 4.5-4.8 and 4.7 kcal/mol for benzylchlorocarbene and benzylbromocarbene, respectively.8b*c Arrhenius A factors are between 0.8 X lo1*and 1.2 X 10l2s-' for the benzylhalocarbenes. A 2 order of magnitude difference between the Arrhenius A factors for CH3CCl and benzylhalocarbenes is unusual for similar reactions; a value of 6 X lo9 s-l is anomalously low and implies an extremely rigid transition state. If the A factor is closer to lo1*s-I for CH3CCl then the E, = 10 kcal/mol. Competitive rates for 1,Zhydrogen migration and addition to alkenes have been reported for difluoromethylfluorocarbene, CF,HCF, in the gas phases9 The Arrhenius activation energy (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) Moss, R. A. Acc. Chem. Res. 1989, 22, IS. (4) Platz, M. S.;Maloney, V. M. In Kinetics and Spectroscopy of Carbenes and Eiradicals; Platz, M. S.,Ed.; Plenum: New York, 1990; p 239 f. (5) Bley, U.; Koch, M.; Temps, F.; Wagner, H. Gg. Eer. Eunsenges. Phys. Chem. 1989, 93, 833. (6) Petek, H.; Nesbitt, D. J.; Moore, C. B. J . Chem. Phys. 1987,86, 1189. (7) Evanseck, K. D.; Houk, K. N. J . Phys. Chem. 1990, 94, 5518. (8) (a) LaVilla, J. A.; Goodman, J. L. J . Am. Chem. Soc. 1989,111,6877. (b) Liu, M. T. H.; Bonneau, R. J . Am. Chem. Soc. 1990, 112, 3915 and references therein. (c) Liu, M. T. H.; Subramanian, R. J. Phys. Chem. 1986, 90, 75. (d) Moss, R. A,; Mamantov, A. J . Am. Chem. Soc. 1970,92,6951. ( e ) Liu, M. T. H.; Subramanian, R. J . Chem. Soc., Chem. Commun. 1984, 1062. (9) Haszeldine, R. N.; Parkinson, C.; Robinson, P. J.; Williams, W. J. J . Chem. Soc., Perkin Trans. 2 1979, 954.
0022-3654/92/2096-22 10$03.OO/O 0 1992 American Chemical Society
1,2-Fluorine Atom Migration in CF3CH for 1,2-H migration in CF2HCFwas approximately 11 kcal/mol higher than for carbene addition to C=C. An A factor of 2.5 x 1013s-I was assumed for isomerization and RRKM calculations of the thermal falloff rate constants were fitted to the data to estimate a threshold of 23 kcal/mol for 1,Zhydrogen migration. This gives a threshold energy of 12 kcal/mol for carbene addition to alkenes. If the unimolecular A factor was assumed to be 5 X 10" s-l the threshold energies would be 11.5 kcal/mol for 1,2-H isomerization and close to zero for carbene addition to C=C. Because recent measurements using laser flash photolysis favor 1,2-H migration A factors of 10"-1012 s-' for benzylhalocarbenessb and threshold energies close to zero for arylhalocarbene additions to alkenes,I0an Eo(1,2-H) = 1 1.5 kcal/mol for CF2HCF seems more appropriate. The 1,2-hydrogen atom migrations have received the most attention and consequently less is known about 1,2-halogen or 1,Zalkyl migrations to carbene centers." The threshold barrier for a fluorine shift must be larger than for hydrogen or alkyl groups because (a) hydrogen and not fluorine migrated9*12 for CHF2CF, (b) for CF3CF2CHthe product was CF2=CHCF3indication CF3 rather than F migration,I3 and (c) the CHF2 group rather than F migrated in the carbene CHF2CF2CH.9 These fluorinated carbenes added to the C = C of alkenes and inserted into C-H bonds of alkanes so the lifetimes are sufficiently long to allow interception by trapping agents. A chlorine rather than hydrogen or fluorine migrated'4p1sfor CHFClCCl, CH2ClCF, CH2C1CCl, and CF,ClCF carbenes. Insertion into Si-H bonds and addition to C=C also occurs for the CF2ClCF carbene, and thus, the threshold for chlorine migration in CF2ClCF must be at least a few kcal/mol.Is Trifluoromethylchlorocarbene,CF3CCl, adds to C=C but 1,Zfluorine migration was not observed.16 If the Arrhenius A factors for migration of atoms or alkyl groups are similar then these comparisons suggest that, in general, the 1,2-E,,'s increase according to C1 < H < alkyl F. Computational studies on fluorovinylidenes support this trend. Goddard17and Pople18 investigated 1,2-fluorine, 1,2-methyl, and 1,Zhydrogen shifts in fluorovinylidene, CHF=C:, difluorovinylidene, CF2=C:, and methylfluorovinylidene, CH3CF=C:. These calculations suggest that the barrier for 1,Zhydrogen migration is a few kcal/mol, while a methyl shift has an energy barrier near 20 kcal/mol and fluorine migration encounters a barrier of approximately 35 kcal/mol. Ab initio calculations on CH2=C: favor a very low barrier for 1,Zhydrogen migration.19 Experimental evidence supports an Eo(1,2-H) near 0 kcal/mol for vinylidene,20and a long-lived CF2=C:, which can abstract hydrogen from proton donors?' add to the C = C of alkene^,'^^^^,^^ and insert into C-H bonds of alkanes.24 However, there is no experimental evidence for 1,Zfluorine migration in CF2=C:.19-24 The only example of a 1,2-fluorine shift was reported by Haszeldine9for isomerization of CF3CH to CF2=CHF. Haszeldine (10) Moss, R. A.; Turro, N . J. In Kinetics and Spectroscopy of Curbenes and Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990; p 213 f. (11) Fields, R.; Haszeldine, R. N . J . Chem. SOC.1964, 1881. (12) Haszeldine, R. N.; Rowland, R.; Speight, J. G.;Tipping, A. E. J . Chem. Soc., Perkin Trans. 1 1979, 1943. (13) Atherton, J. H.; Fields, R.; Haszeldine, R. N . J . Chem. Soc. (01971, 366. ... (14) Bevan, W. I.; Haszeldine, R. N.; Middleton, J.: Tipping, A. E. J . Chem. SOC.,Dalton 1975, 620. (15) Haszeldine, R. N.; Pool, C. R.; Tipping, A. E.; Wats, R. O B . J . Chem. Soc.. Perkin Trans. 1 1976, 513. (16) Moss, R. A.; Guo,W.; Denney, D. Z.; Houk, K. N.; Rondan, N . G. J . Am. Chem. SOC.1981, 103, 6164. (17) (a) Goddard, J. D. J. Mol. Sfrucr. 1985, 133, 59. (b) Goddard, J. D. Chem. Phys. Leu. 1981,83, 312. (18) (a) Pople, J. A. Pure Appl. Chem. 1983,5.5, 343. (b) Frisch, M. J.; Krishnan, R.; Pople, J. A.; Schleyer, P. V. R. Chem. Phys. Lerr. 1981,81,421. (19) Gallo, M. M.; Hamilton, T. P.; Schaefer, H. F. J . Am. Chem. SOC. 1990, 112, 8714. (20) Skell, P. S.; Havel, J. J.; McGlinchey, J. J. Acc. Chem. Res. 1973, 6, 97. (21) Reiser, C.; Steinfeld, J. I . J . Chem. Phys. 1980, 84, 680. (22) Norstrom, R. J.; Gunning, H. E.; Strausz, 0. P. J . Am. Chem. SOC. 1976, 98, 1454. (23) Stachnik, R. A.: Pimentel, G. C. J . Chem. Phys. 1984, 88, 2205. (24) Brahms, J. C.; Dailey, W. P. J . Am. Chem. SOC.1990, 112, 4046.
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2211 formed CF$H by photolysis of 2,2,2-trifluorodiazoethaneand the photolytic energy partitioned to the carbene was sufficient to cause 1,2-fluorine isomerization. The goal of this work is to provide an experimental estimate of the barrier for 1,2-fluorine migration converting CF3CH to CF2=CHF. It appears this is the first measurement of a 1,2migration barrier for a species other than hydrogen or carbon. The CF3CH is generated by the threecentered elimination of HCl from chemically activated CF3CH2Cl**. Combination of trifluoromethyl and chloromethyl radicals, reaction 1, forms the CF3 + CH2Cl-
CF3CHzCl**
(1)
l,l,l-trifluoro-2-chloroethanewith 97.5 kcal/mol of internal energy.2s The asterisk denotes an energized molecule. Unimolecular decompositions paths for CF3CH2Cl**are known to i n c l ~ d e ~four-centered ~-~~ elimination of H F (reaction 2a), CF3CH2ClS* km
kw
CF,=CHCl
CF3CH*T
+ HCl
2CF3CH2+ C1
+ HF
(2a) (2b) (2c) (3)
three-centered elimination of HCl (reaction 2b) and rupture of the C-Cl bond (reaction 2c). Reaction 3 is collisional deactivation of CF3CH2Cl**. Reaction 2b forms singlet carbene, CF3CH*, which may react by 1,Zmigration of fluorine (reaction 4), or bimolecular events (reaction 5). The CF3CH*Trepresents the CF3CH*
kF +
CF2=CHF
(4)
total carbene formed and the CF3CH* is the portion of the carbene that has energy above the Eo(1,2-F). This mechanism assumes that a single collision will render singlet CF3CH* incapable of 1,2-fluorinetransfer; i.e., the "strong collision" assumption.2s The kM[M]is the collision frequency and the [MI is directly related to pressure, P. Reaction 5 includes all bimolecular processes; vibrational deactivation of CF3CH* within the singlet manifold, collisioninduced intersystem crossing to the triplet surface, reactive collisions between the carbene and other bath molecules, etc. These removal processes will be discussed next. It is well established that 1,2-atom migrations occur on the singlet surface29 and the barrier for rearrangement of triplet carbenes is very high, the Eo(1,2-H) for both CH3CH and CH2=C: exceeds 50 k c a l / m ~ l . ' ~ ~Trifluoromethylcarbene ~l has a triplet spin configuration in the lowest electronic energy ~ t a t e . ~ " ~ Thus, once CF3CH is collisionally quenched to the ground electronic energy surface, 12-fluorinemigration will not occur. Recent calculation^^^ place the singlet state 13 kcal/mol higher than the ground state. This level of theory also predicts the singlet-triplet (25) Rakestraw, D. J.; Holmes, B. E. J . Phys. Chem. 1991, 95, 3968. (26) Millward, G. C.; Tschuikow-Roux, E. Inr. J . Chem. Kinet. 1972,4, 559. (27) Setser, D. W.; Lee, T. S.; Danen, W. C. J . Phys. Chem. 1985, 89, 5799. (28) Robinson, P. J.; Holbrook, K. A. Unimolecular Reactions; Wiley: New York, 1972. (29) Schaefer, H. F. Acc. Chem. Res. 1979, 12, 288. (30) Harding, L. B. J . Am. Chem. SOC.1981, 103, 7469. (31) Sulzle, D.; Schwarz, H. Chem. Phys. Lerr. 1989, 156, 397. (32) Wasserman, E.; Barash, L.; Yager, W. A. J . Am. Chem. SOC.1965, 87, 4974. (33) Atherton, J . H.; Fields, R. J . Chem. SOC.(01968, 2276 and 1967, 1450. (34) Dixon, D. A. J . Phys. Chem. 1986, 90, 54.
2212 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
Holmes and Rakestraw I
120
100
1
I
Eo 30 - - - - - 2 9 3C
8
Ep
f
I
width
15
0.33
2.9
15
0.25
15
0.33
3.0 3.5
A-factor 5 x 10” 5 x 1011 5 x 10”
AH;’= 40
I
Progress o f R e a c t i o n
Figure 1. Schematic energy level diagram for the three-centered elimination of HC1 from CF3CH2CIand the subsequent 1,2-fluorine migration converting the CF$H into CF,=CHF. The range of values used for Eo(l,l-HC1),E,, and E, are shown in Table I. The dashed lines indicate that the thermochemistry associated with the carbene and the 1,2-fluorine isomerization is not known.
0
,”,/ 1
I I
I I
10
100
1 / Pressure ( atm x 1O 3 )
Figure 2. Experimental yield of [CF,=CHCl]/[CF,=CHF] versus the log of the reciprocal pressure (open circles). The lines are the calculated [CF,=CHCI]/[CF,=CHF] using eq IV and kHc,/kHF= 0.72. Other parameters, shown in the figure, are as follows: Eo,the threshold barrier for 1,2-fluorine migration in kcal/mol; E,, the exit channel potential energy in kcal/mol;f( E&, the fraction of E, partitioned to the carbene; the width of the Gaussian in kcal/mol; and the Arrhenius A factor in s-I.
splitting is 13 kcal/mol for CH2,which is 4 kcal/mol too high.35 If the splittings for CH2 and CF3CH are similar then singlet CF,CH may be approximately 9 kcal/mol above the triplet state. Because the singlet-triplet separation is similar for CH2 and in the threecentered elimination reaction. It is convenient to divide CF,CH, the collisional quenching pattern may also be similar. the carbene’s total energy into a statistical release of excess energy For singlet CH2 the branching ratio for collisional quenching to at the transition state, E,, and a release of potential energy as the the triplet surface versus total removal ranges from 0.20 for HCl departs from the CF3CH carbene. This latter quantity is ethene36to 0.28 for i~obutane.~’Absolute rates for reactive the energy associated with the threshold barrier for the reverse quenching of CH2 are nearly gas kinetic, N l O I 4 ~ m ~ / ( m o l . s ) . ~ ~reaction, CF3CH insertion into HCl, and E , will represent this Assuming CF,CH behaves similar to CH2, reactive quenching potential energy. Figure 1 shows the energetics for the consecutive and intersystem crossing should be competitive with vibrational reactions 2b and 4. The threshold energy barrier for 1,2-fluorine deactivation of the singlet CF3CH. All collisions may be equivalent migration converting CF,CH into CF2=CHF is Eo(1,2-F), the so that all bimolecular events would render the CF3CH incapable reaction enthalpy is AHo’, the ( E ) is the average energy of of unimolecular isomerization. chemically activated CF3CH2Cl**and the threshold energy for The reaction chemistry of CF3CH has been investigated by reaction 2b is Eo(l,l-HC1). The dotted lines for both CF3CH* photolyzing 2,2,2-trifluorodiazoethane and in the presence and the transition state for fluorine migration illustrate that E , of cis- or t r a n s - 2 - b ~ t e n e .Photolysis ~~ of pure, liquid trifluoroand Eo(1,2-F) are not known. diazoethane produced CF2=CHF (lo%), cis- and transThe CF2=CHF will only be observed if the Eo(1,2-F) is less CF3CH=CHCF3 (19%) from either reaction of the carbene with that the total energy available to CF3CH. The maximum possible the trifluorodiazoethane or direct dimerization of CF3CH, and energy of CF3CH is the sum of E , and E,. The E, = 25.5 47% of higher boiling products attributed to reactions involving kcal/mol, the difference between ( E ) and E,,(l,l-HCl), and this CF3CH addition to trifluorodiazoethane and polymerization reenergy should be statistically distributed among the quantum states actions between CF3CH and the hexafluorobut-2-enes. In the of the three-centered elimination activated complex. Assuming gas phase the CF2=CHF yield increased to 22% at 0.47 atm that E , I15 kcal/mol, Le., the threshold barrier for CF,CH pressure and to 32% at 0.1 1 atm. Gas-phase photolysis of triinsertion into HCl is less than 15 kcal/mol, an upper limit is fluorodiazoethane with a 4-fold excess of 2-butene gave addition Eo( 1,2-F) i= 40 kcal/mol. of carbene to the C-C (40-42 mol %), insertion of CF3CH into There are four distinct possibilities for the relationship between the C-H bonds of 2-butene (6-9%), trans-CF,CH=CHCF, the energy distribution of the carbene and Eo(1,2-F). (25%), and unimolecular isomerization of CF3CH (20 and 27%) Case 1. If all of the carbene’s energy distribution is above the a t pressures of 6.8 and 5.3 atm, respectively. Note that as the Eo(1,2-F) for reaction 4, and if Eo(1,2-F) is only a few kcal/mol, pressure declines the relative importance of 1,2-fluorine atom then for all pressures in our system the carbene will completely migration increases. Analysis of the isomeric dimethylcycloisomerize to CF,=CHF. In this case the [CF2=CHCl]/ propanes showed that about half of the cycloaddition was [ C F y C H F ] will be constant with pressure and equal to kHF/kHCI. nonstereospecific, suggesting that 20% of the CF3CH was colliIt will not be possible to estimate the Eo(1,2-F) for this case. sionally quenched to the triplet state. These product distributions Case 2. If the carbene’s energy distribution begins slightly above also support the suggestion that intersystem crossing, reactive the threshold barrier for 1,2-fluorine migration and if the size of collisions and vibrational deactivation are competitive. Thus, the Eo(1,2-F) causes kMi= kF, then at higher pressures bimolecular calculation of kMusing the “strong collision” assumption should reactions involving CF3CH* (reaction 5) will stop 1,2-fluorine be reliable because all collision cross sections should be comparable. migration for some of the trifluoromethylcarbene. In this case a plot of [CF2=CHCl]/[CF2=CHF] versus 1/P will decline with Experimental Results pressure, but at limiting low pressure all the CF,CH*T will The three-centered elimination of HCI from CF,CH,Cl, reisomerize. At low pressure the [CF2=CHC1]/ [CF,=CHF] will action 2b, will produce the carbene, CF3CH*,with a broad energy equal kHF/kHCI. distribution. The breadth and average energy of the carbene’s Case 3. If the range of the carbene’s energy distribution enenergy distribution will depend on the energy partitioning pattern compasses the threshold barrier for 1,2-fluorine migration, then a portion of the CF3CH*T cannot isomerize to CF2=CHF. The (35) MOSS, R. A.; Jones, M., Jr. In Reactiue Inrermediotes; Moss, R. A,; Eo( 1,2-F) could be near the high-energy tail of the carbene’s Jones, M., Jr., Eds.;Wiley: New York, 1985; Vol. 3, p 48. energy distribution so that little of the carbene will isomerize, or (36) Hack, W.; Koch, M.; Wagener, R.; Wagner, H. Gg. Eer. Eunsenges. the Eo( 1,2-F) could lie near the onset of the energy distribution Phys. Chem. 1989, 93, 165. in which case most of the CF3CH*Twill isomerize to CF2=CHF. (37) Bohland, T.; Temps, F.; Wagner, H. Gg. Ber. Bunsenges. Phys. Chem. 1985, 89, 1013. Collisional deactivation of the CF3CH* with energy above the
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2213
1,2-Fluorine Atom Migration in CF3CH 120 E,
Ep
f w i d t h
f
A-factor
100
F
19
Y
18
0.004
1 x 1013
f
width
A-factor
25
15
0.11
28
15
0.26
0.57 2.0
1 x 1Ol1 1 x 1011
33
18
0.51
1.0
1 x
E,
-___ ---
4
E,,
80 \&I '00
U"
B
.@:@--:-+.-.e* 1
10
100
1 I Pressure ( atm x 1 o - )~
Figure 5. Experimental yield of [CF2=CHC1]/[CF2=CHF] versus the log of the reciprocal pressure (open circles). The lines are the calculated [CF,=CHCI]/[CF,=CHF] using eq IV and kHcl/kHF = 0.72. Other parameters listed in the figure are described in the caption to Figure 2. I
Eo Ep
11 ',
100
F
9
.
80
UN
0
0
----
_ _ \ I
c
@,,
-:, 1
-
60 -
---
/,. \
27
15 15 5
width
A-factor
0.31 0.32 0.76
1.0
1 x 1011 5 x IO1' 5 x 10"
0.36 1.0
', \, ',
-
-
:__
9.
\
Lr
28 28
1
f
\
%\
-_
\\%\
because its energy is below the isomerization threshold barrier for fluorine migration. [CF,CH*T] = [CF3CH*] + [ C F ~ C H ] N
Mass balance for reactions 4 and 5 gives [CF3CH*] = [CF3CH] + [CF2=CHF]. Let 0 equal the fraction of the carbene's energy distribution, JTE),that is above the threshold barrier for isomerization so that [CF3CHIN= (1 - 0)[CF3CH*T]. The rate constant ratio for reactions 4 and 5 gives kF/kM[M] = [CF2= CHF]/[CF3CH]. Substitution of these last three expressions into eq I1 gives eq 111. Substitution of eq I11 into eq I and solving [CF~CH*T]= (l/Q)(k,[M]/kF
1
10
100
1 / Pressure ( atm x 1O 3 )
Figure 4. Experimental yield of [CF2==CHCI]/[CF2=CHF] versus the log of the reciprocal pressure (open circles). The lines are the calculated [CF,=CHCl]/[CF,=CHF] using eq IV and kHCl/kHF = 0.72. Other parameters listed in the figure are described in the caption to Figure 2.
Eo(1,2-F) will still result in a reduction of the [CF,=CHCl]/ [CF2=CHF] with a reduction in pressure. The limiting value at low pressure of [CF,=CHCl]/[CF,=CHF] will be larger than kHF/ kHCl-
Case 4. If the threshold barrier for 1,2-fluorine migration is above the entire energy distribution of the CF3CH* then isomerization will not occur and CF2=CHF will not be detected. The experimental data, [CF,=CHCl]/[CF,=CHF] as a function of log of the inverse pressure, are in Figures 2-5. The ratio declines from 100 to a limiting low-pressure value of 18.0 as the pressure is reduced from 1.1 to 0.013 Torr. This must correspond to either case 2 or 3 just discussed. Details of the experimental procedure and unimolecular rate constants for each of the three channels are in ref 25. These unimolecular rate constants show that kHF/kHCIis in the range 0.72-3.0. This eliminates case 2 because the low-pressure limit of 18.0 for [CF2=CHCl]/[CF,=CHF] exceeds the kHF/kHcI. Calculated Results and Discussion Derivation of the Pressure Dependence of [CF2=CHCl]/ [CF2=CHFl. W e now derive the expression that will be used to calculate the pressure dependence of [CF,=CHCl] / [CF2=CHF]. The Eo(1,2-F) will be varied until the magnitude and pressure dependence of the calculated [CF,=CHCl]/ [CF,=CHF] matches the experimental data in Figures 2-5. Inspection of reactions 2a and 2b gives eq I. The total carbene, [CF,=CHCl] /[CF~CH*T]= kHF/kHCi (1) [CF3CHIT], is composed of a portion that can isomerize, [CF,CH*], plus some carbene, [CF3CHIN,that cannot isomerize
(11)
+ l)[CF,=CHF]
(111)
for the product ratio gives eq IV. The kF's were computed using [CF2=CHCl]
/ [CF,=CHF]
=
(l/Q)(kHF/kHCl)(kMIMl/kF
+ 1) (Iv)
eq V; the kps are 1,ZfluoMemigration rate constants at a specific energy, E . The kFs were calculated using RRKM theory and
Lil,2TF)
kMIMl/(kE + kMIMl)f(E) d E ( v )
a mcdel for the 1,2-fluorinemigration that will be developed later. Equation IV is used to calculate [CF2=CHCl]/[CF2=CHF] as a function of pressure; fitting this product ratio to the experimental data of Figures 2-5 determines the range of acceptable Eo(1,2-F)'s. Two parameters in eq IV, kHF/kHcIand kF, influence the calculated product ratio. The @ and kMare not adjustable parameters. Collision theory and a hard-sphere collision cross section of 5.7 A for the bath gas gives the kM. The is actually fixed by the choice of Eo(1,2-F) and its relationship to the calculatedJTE). The kF is calculated from eq V. The magnitude of kF is strongly influenced by the choice of Eo(1,2-F) and the computed values for the kE%, while theJTE) mainly effects the pressure dependence of kF. Selection of Upper and Lower Limit Parameters for the Calculation of [CF,=CHCI]/[CF,=CHF]. Our approach will be to determine the range of Eo(1,2-F)'s that match the experimental data. The selection of appropriate values for kHF/kHCI,f(E), and ki's is described in this subsection. Upper and lower limits will be established for these three parameters and then the range of E,( 1,2-F) values that satisfy the two extremes will be determined. Unimolecular rate constants for kHF and kHClwere determined25 for chemically activated CF3CH2Cland the range for kHF/kHCI is 0.72-3.0. The kHClwas estimated by subtraction of (ka + kHF) from the total rate constant. Be€ause the kHF is directly measured, the variation of kHF/kHCI corresponds to a change of kHClwhich
2214 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 TABLE I: Upper and Lower Limit Parameters for Calculation of
ICF,=CHCll/ICF~4HFl E,, E, A kHCl kcal/mol kcal/mol kcalfmol f(Ep)O factor, s-l upper 3.0 75 22.5 15 0.25 1.0X loi3 lower 0.72 72 25.5 5 0.0 5.0 X 10" kHF/ EdHCI),
" Fraction of E, released to internal energy of the carbene. TABLE 11: RRKM Models A . 1.2-Fluorine Migration Rate Constants carbene vibrational freq, cm-l (degeneracies)
rotor, amu A* moments of inertia I,; I,; lZ,amu A2 reaction path degeneracy
3035 (1) 1443 (1) 1248 (3) 1110 (1) 561 (3) 365 (2) 3.12 97.0;88.5;87.5
transition-state complex"
3035 (1) 1443 (1) 1233 (2) 1066 (2) 1050 (1)" 900 (2)" 541 (2)" 130.0;95.0;61.8 3.0
B. The Carbene's Statistical Energy Distribution HCI carbene vibrational freq, cm'l (degeneracies)
portion
portion
3044 (1) 1400 (1) 1201 (4) 851 (2) 541 (1) 377 (2) 180 (1)
1400 (1) 900 (1) 855 (1) 541 (1) 190 (1)
"Transition-state complex for Arrhenius A factor = 5.0 X 10" s d . For an A factor of 1.0 X l O I 3 s-l the 1055-cm-'frequency is changed to 200 cm-l and the 900-cm-l frequencies are changed to 344 cm-I. For an A factor of 1.0 X 10" s-I the two 541-cm-'frequencies rise to 1100 cm-I.
results in a different Eo(l,l-HC1) and hence a different E, (see Figure 1). Table I lists the two rate constant ratios and the corresponding Eo(l,l-HC1)'s and E;s. The energy distribution of the carbene results from two separate components: the statistical release of the excess energy, E,, and the release of the potential energy, E,. Details have appeared3* for calculation of the statistical portion and its convolution with the potential energy portion of the energy distribution. The vibrational frequencies of the HCl eliminated activated complex that were used to calculated thef(E) are in Table IIB. Disposal of E , is dictated by the potential energy surface leading from the transition state to the separated carbene and HC1. Recently, Setser and c o - ~ o r k e r determined s~~ that three-centered elimination of HX from halomethanes partitions -5% of E, to vibrational and rotational energy of the carbene. The precise fraction is not known because of uncertain thermochemistry but the small magnitude for the halomethanes is unmistakable. A fractional energy release to the carbene,f(E,), equal to 0.25 will be an upper limit and as a lower limit the carbene will not receive any energy from E . When thef(E ) = 0.25, a Gaussian with a variable width wih represent the &ape of the carbene's energy distribution resulting from E,. Several values of the width will be tested to determine the best fit to the pressure dependence of the data. Iff(E,) = 0 then the statistical distribution from E, is just shifted downward in energy by E . Two types of data can be used to estimate the magnitude of E,. Measurement of the threshold barrier for the addition of
Holmes and Rakestraw CF3CHto HCl is the mast direct method but data is not available. E, also equals Eo( 1,l-HCl) -AHoo(for carbene formation). Calculation of Woo requires enthalpies of formation for CF3CH2C1, CF3CH, and HCl; but the enthalpy of formation for the carbene is not known. However, data is available for the loss of HCl from CH3CHC1,; using AHf' = 50.7 f 4.7 kcal/mol for CH,CCl carbene,s"Eo(1,l-HCl) = 62 k c a l / m ~ l and , ~ ~the ~ known enthalpies of formation40for HC1 and CH3CHCI2gives E, = 4 f 8 kcal/mol as HC1 separates from CH3CCl. We can also estimate E, by analogy to the Eo for similar carbene plus HX reactions; Eo = 14.8 f 3.8 kcal/mol for CF2 HF, 13.6 f 2 kcal/mol for C H F + H2?' 9.6 kcal/mol for CF2 + HBr,428.9 kcal/mol for CFCl HCl,398.5 kcal/mol for CFH HCl,39and 6.2-15.5 kcal/mol for CF2 + HCl.42 The latter reaction is the most thoroughly studied threecentered HCl elimination but even here the uncertainty in the thermochemistry is larger than the 7-9 kcal/mol recommended for E,.39 It appears that lower and upper limits of 5 and 15 kcal/mol, respectively, are a reasonable guess for the E, when HCl is lost from CF3CH2C1. The krs were calculated in the usual manner using a direct count of states. Frequencies for CF$H were estimated from CF3CH3and CF3CH2C1by eliminating two bending and one stretching vibration for each atom removed.43," The rotational motion about the carbene's C-C bond was treated as a free rotor. Vibrational frequencies for the transition state and the free rotor for the carbene were estimated by analogy with the frequencies calculated for 1,2-H migration in the ethyl radical.4s Calculation of the moments of inertia for the carbene and the transition state used a geometry deduced from theoretical descriptions of 1,2-H and fluoromethylcarbene, CFmigration in dimethyl~arbene~~ H2CH.47 The transition-state structure is a nearly symmetric bridge with the fluorine slightly closer to the diradical carbon. The three-centered ring motion and two fluorine bends of the transition state were adjusted to reproduce the Arrhenius A factor expected for 1,2-F migration in carbenes. Identification of appropriate Arrhenius A factors is discussed next. Two models of the 1,2-F migration transition state corresponding to upper and lower limits will be tested. There are no experimental measurements of the A factor for 1,Zatom isomerizations of carbenes in the gas phase; however, theoretical descriptions of related isomerizations should assist selection of an upper limit. We assume that A factors are similar for 1,2-F and 1,2-H atom migrations to carbene carbons. Calculations for vinylidene conversion to ethyne give vibrational frequencies for the carbene and 1,2-H migration transition state.48 At 600 K these frequencies corresponding to AS+ = +0.60 cal/(mol.K). Vibrational frequencies ~alculated'~ for methylfluorovinylidene, CH3CF=C:, and the transition states for 1,2-CH3and for 1,2-F migration give ASt = -1.5 cal/(mol+K) and A$ = +1.6 cal(mo1.K) at 600 K, respectively. The 3.1 cal/(mol.K) lower AS+ for methyl versus fluorine migration reflects the loss of a CH3 torsion in the transition state. A molecular orbital c a l ~ u l a t i o n ~ ~ for 1,2-H migration in several alkylcarbenes gives A,!? = -3.5 f 0.5 cal/(mol.K), presumably at 300 K. Experimental Arrhenius
+
+
+
(40)JANAF Thermochemical Tables, erd ed.;Chase, Jr., M. W.,Davies, C. A,, Downey, Jr., J. R., Frurip, D. J., McDonald, R. A., Syverud, A. N., Eds.; The Dow Chemical Co.: Midland, MI, 1985. (41)Wagener, R.; Wagner, H. Gg. Ber. Bunsenges. Phys. Chem. 1990, 94, 1096. (42) Hsu, D.S.Y .; Umstead, M. E.; Lin, M. C. In Fluorine Containing Free Radicals; Root, J. W., Ed.; ACS Symp. Ser. No.66;American Chemical Society: Washington, DC, 1978;p 143. (43) (a) Harnich, D.F.; Hirschmann, R. P. Appl. Spectrosc. 1970, 24, 2835. (b) Crowder, G. A. J . Fluorine Chem. 1973174, 3, 125. (44)Edgell, W. F.;Miller, F. B.; Amy, J. W. J . Am. Chem. SOC.1957, 79, 2391. (45)(a) Hase, W. L.; Wolf, R. J.; Sloane, C. S. J . Chem. Phys. 1979, 71, 2911. (b) Hase, W.L.; Schlegel, H. B. J . Phys. Chem. 1982, 86, 3901. (46)Evanseck, J. D.; Houk, K. N. J . Am. Chem. SOC.1990, 112,9148. (47)Palma, A.; Semprini, E.; Stefani, F.; DiMartino, V. Chem. Phys. Lerr.
(38)(a) Kim, K. C.; Setser, D. W. J . Phys. Chem. 1974, 78, 2166. (b) Holmes, B.E.; Setser, D. W.; Pritchard, G. 0.I n t . J . Chem. Kine!. 1976, 8,
1990, 170, 549.
21s. (39)Arunan, E.;Wategaonkar, S. J.; Setser, D. W. J . Phys. Chem. 1991, 95, 1539.
1990, 112, 8714.
(48)Gallo, M. M.;Hamilton, T. P.; Schaefer, H. F. J . Am. Chem. SOC. (49)Bonneau, R.;Liu, M. T. H.; Rayex, M. T. J . Am. Chem. SOC.1989, 111, 5973.
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2215
1,2-FluorineAtom Migration in CF3CH
A factors for 1,2-H migration in alkyl radicalss0are generally near 5 X 10l2s-l. For our system the AS+ will be a larger negative value than any of these because the free rotation about the C-C of CF3CH is severely restricted in the transition state. Taken together, these gas-phase results suggest that A = 1.O X 10" s-l [A!?= -2.3 cal/(mol.K) at 600 K] is a generous upper limit. Solution-phase measurements of the Arrhenius A factor for 1,2-H migration are available for a limited number of carbenes: 1.2 X 10l2s-l for C6HsCH2CBr,8b8.0 X 10" s-' for C6HsCH2CCl," and 5.0 X lo9 s-l for CH3CCl [m= -16 cal/(m~l.K)].*~ Surprisingly, the A factors are 1-3 orders of magnitude lower than the gas-phase data just discussed. The solution-phaseA factor for CH3CC1seems unreasonably low and we believe A = 5.0 X 10" s-l is an objective lower limit. Finally, we note that an A factor = 5.0 X 1011s-l will be one of the smallest found for a gas-phase decomposition. Both the upper limit with A = 1.0 X 1013s-I [A!?= -2.3 cal/(mol.K) at 600 K] and the lower limit with A = 5.0 X 10" s-I [m= -8.4 cal/(mol.K) at 600 K] will be used to develop RRKM models for calculation of the kFs. At 300 K the A!? are -2.2 and -6.9 cal/(mol.K) for the two limiting cases. Table I summarizes the complete range of variables that will be tested and Table I1 contains the data for the RRKM models. Estimation of Upper and Lower Bounds for Eo(1,2-F) Using the Limiting Low-Pressure [CF2=CHCI]/[CF,=CHF]. An estimation of the upper and lower limit for Eo(1,2-F) can be determined using eq IV and just the product ratio at the low-pressure limit, [CF,=CHCI]/[CF,SCHF] = 18. Note that as k,[M] approaches zero the right-hand side of eq IV reduces to (1/ @')kHF/kHCl so that the kps are not needed to determine the threshold barriers. For example, with kHF/kHCI = 0.72 the @' = 0.04 and for kHF/kHCI = 3.0 the @ = 0.17. The (9 is the fraction of the carbene capable of isomerization and the value of Eo(1,2-F) must be chosen to divide the energy distribution in a manner that places 0.17 or 0.04 off(E) above the threshold for the lower and upper limits, respectively. The lowest possible E,,( 1,2-F) is with @' = 0.17 and none of E apportioned to the carbene. When f (E p ) = 0 the magnitude of E, does not influence the relationship between Eoand theflE). For this lower limit estimate, 17%of the calculated energy distribution is above the threshold barrier when the Eo(1,2-F) = 19 kcal/mol. These parameters give the narrowest possible carbene energy distribution. The higher Eo(1,2-F) would be for kHF/ kHa = 0.72, the @' = 0.04,f(E,) = 0.25, E, = 15 kcal/mol, and the Gaussian as wide as reasonable. These parameters result in a broad energy distribution and forces the Eo(1,2-F) to be very high because only 4% of theAE) can be above the Eo(1,2-F). For the upper limit estimate the Eo(1,2-F) is 29 kcal/mol when the Gaussian is centered at 3.75 kcal/mol and the width is 3.0 kcal/mol. We now refine the estimate of the 1,2fluorine isomerization barrier by fitting the calculated pressure dependenceof [CF2=CHCl]/ [CF2=CHF] to the experimental data in Figures 2-5. Calculated Fit to the Pressure Dependence of [CF2=CHCI]/ [CF2=CI-IF]. As a starting point the calculated variation of [CF,=CHCl] / [CF,=CHF] with pressure is shown in Figures 2 and 3 for the upper (Eo= 29 kcal/mol) and lower (Eo= 19 kcal/mol, the bottom curve in Figure 3) limit case8 discussed in the preceding section. It is obvious that the upper limit for Eo(l,2-F) is a much better fit, the dot-dash curve in Figure 2, but for both cases the calculated [CF,=CHCl]/[CF,SCHF] is too low at higher pressures; Le., isomerization of the carbene is too fast. We will first optimize the calculated fit using the upper case limit shown in Figure 2; kHF/kHCI = 0.72. Later we will investigate cases with kHF/kHcI= 3.0. Our approach will be to systematically test the upper and lower limits for each of the following parameters: (1) the kFs (the larger and smaller Arrhenius A factors), (2) the E,, and (3) the fraction of E, released to the carbene. The Gaussian's width will be adjusted to reproduce the curvature of the data. The experimental product ratio is more reliable at low ~
~~~
(SO) Gordon, A. 1400.
S.;Tardy, D.C.; Ireton, R. J . Phys. Chem.
1976, 80,
pressure because at higher pressures the yield of CF2==CHF is very small; thus, we first fit the low-pressure [CF2=CHCl]/ [CF2=€HF] and then adjust parameters to match, as closely as possible, the higher pressure data. To improve the fit of the upper limit case, the dot-dash curve of Figure 2, the rate of carbene isomerization needs to be slower; Le., the Eo(1,2-F) must be raised or the kFs must be reduced. An Arrhenius A factor of 5 X 10" s-l is the basis for the k r s used to calculate the product ratio in Figure 2. A smaller A factor will be tested later. If the Eo is raised then the fraction of E, released to the carbene must be increased a similar amount to shift thef(E) to higher energies; i.e., 4% of thef(E) must remain above Eo to ensure agreement in the low-pressure regime. An acceptable fit is shown in Figure 2 with Eo = 30 kcal/mol, f(E,) = 0.33 and a very broad Gaussian; the width is 2.9 kcal/mol. A release of 33% of the potential energy to the carbene is slightly more than we selected for our upper limit. However, the larger f(E,)is probably not significant becausef(E ) = 0.25 would give a similar fit to the data if the E, increasdby 4 kcal/mol and because the uncertainty in the experimental product ratio is greatest at high pressure. Figure 2 also illustrates the influence of the Gaussian's width on the calculated [CF2=CHCl]/ [CF,=CHF] with the other parameters constant. A wider Gaussian broadens the combined distribution function and places 5.76% off(,!?) above Eo. To restore the agreement at low pressure thef(E) must be lowered by reducing f(E ) or else the EOmust be raised. If the f(E,)is reduced then t i e calculated [CF2= CHCl]/[CF,=CHF] is too low at higher pressures whereas if the Eo is raised the 1,2-F isomerization rate is slower and the calculated [CF,=CHCl]/[CF24HF] is too high at higher pressures.' The effect of a narrower Gaussian will be shown later, in Figure 4. All of the calculated curves of Figure 3 are for kHF/kHcI= 3.0. The lowest curve is based on the parameters for the lower limit estimate of the threshold energy (Eo= 19 kcal/mol). An improved fit requires a much slower isomerization rate. Using the upper limit values for E, (1 5 kcal/mol), and an Arrhenius A factor of 5.0 X 10" s-', the calculated product ratio is too low at the highest pressures even for (E,) = 0.80 (the upper curve of Figure 3). Raising the Eoto match the high-pressure region removes the fit at low pressure even if all of E, is partitioned to the carbene. The release of 80% of the potential energy to the carbene is inconsistent with conclusions reached by Setser and c o - ~ o r k e r sfor ~ ~similar three-centered elimination reactions. Thus we conclude that kHF/kHCI is closer to 0.72 rather than 3.0. This implies that the threshold energy for three-centered loss of HCl from CF3CH2Cl is near 72 kcal/mol and this Eo( 1,l-HCl) supports the assignment of ref 25. The middle curve in Figure 3 illustrates the effect of the larger Arrhenius A factor with the other parameters the same as for the upper curve. As expected, the 1,2-F isomerization rate is faster so the yield of CF2=CHF is much larger at higher pressures. An acceptable fit is only achieved iff(E,) = 1.0, E > 15 kcal/mol, and Eo> 32 kcal/mol. Thus, we conclude that dermal activation Arrhenius A factors are nearer 5 X 10" PI rather than the larger value. This next section probes the effect of the lower E, limit listed in Table I. No acceptable fit to the data could be achieved with E, = 5 kcal/mol. For example, with Eo = 24 kcal/mol, kHF/kHcI = 0.72, andf(E,) = 0.20, agreement was good at low pressure but the calculated [CF2yCHCl]/[CF2=CHF] was only 45 at the highest pressure of Figure 4. If the Eo is increased to raise the high-pressure value of the [CFp=CHCl]/[CF,=CHF], a larger fraction of E, must be released to the carbene in order to maintain the agreement at low pressure. The closest fit, see Figure 4, is with Eo = 27 kcal/mol and f(E ) = 0.76. At higher threshold energies the calculated [CF,--ChCl]/ [CF,=CHF] will not match the low-pressure experimental data even when all of E, is partitioned to the carbene. As previously mentioned, a release of 0.76 of E, to the carbene is not consistent with results of Setser and co-workers on related systems; thus, our work favors an exit channel potential energy barrier closer to 15 kcal/mol. By con-
2216 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
trast, with E, = 15 kcal/mol an excellent fit is achieved with Eo = 28 kcal/mol, the middle curve in Figure 4. Compared to the upper curve in Figure 2, this case has a much sharper Gaussian and hence a narrowerflE), very similarf(E,)’s, and an Eoonly 2 kcal/mol lower. The middle curve in Figure 4 is a slightly better fit than the dot-dashed curve in Figure 2 because the agreement is improved at intermediate pressures. The upper curve in Figure 4 maintains the same parameters as the middle curve except that the kFs are for an RRKM model based on an A factor of 1.0 x 10” s-I. We investigated an A factor of 1.0 X 10” s-I because slower 1.2-F isomerization rate constants generally improved agreement at the high pressures. This calculated curve is slightly high at the highest pressures and a reduction in the threshold energy of 1 kcal/mol improves the agreement. This shows that a factor of 5 change in the A factor has minimal effect on the threshold barrier; thus, uncertainties associated with the A factor do not significantly alter the assignment of the Eo. The highest calculated curve in Figure 4 can also be improved if thef(E ) is reduced and the Gaussian is simultaneously broadened. %he dot-dashed curve in Figure 5 shows the result. This illustrates that more than one combination of thef(E,) and width can fit the data but there is no significant change in the Eo. The final section searches for the maximum and minimum Eo(1,2-F) that will give acceptable agreement with the pressure = dependence of the experimental product ratio using kHF/kHCI 0.72. Restrictions listed in Table I for the other parameters will be slightly relaxed; A factor = 1.0 X 10” s - l , f ( E , ) I0.5, and E, I 18 kcal/mol. The lowest threshold barrier will have the slowest 1,2-F isomerization rate constants; Le., A factor = 1.0 X 10” s-’. The highest Eo will have the fastest isomerization rate constants; Le., A factor = 1.0 X lOI3s-l, the largest possible E,, andf(E,) 0.5. Figure 5 shows data for the two extremes: Eo = 25 and 33 kcal/mol. With Eo = 25 kcal/mol the calculated product ratio is low at high pressure because the lower threshold allows too much isomerization. These data are forf(E,) = 0.1 1. If thef(Ep) is decreased so that less of thef(E) is above Eo then the rate will decrease but the calculated product ratio is too high at low pressure. No precise fit is possible with Eo = 25 kcal/mol and we consider this a lower limit. Calculated data for the upper limit threshold energy is shown as the upper curve in Figure 5. The calculated ratio with Eo = 33 kcal/mol andf(E,) = 0.54 fits the low- and high-pressure data but the magnitude at intermediate pressures and the shape are not correct. In conclusion, we find that we are able to reproduce the observed pressure dependence of the [CF2=CHCl]/[CF2=CHF] with a threshold energy for 1,2-fluorine migration equal to 28-30 kcal/mol. We believe that 25 and 33 kcal/mol represent reasonable upper and lower threshold energy limits. This appears to be the first measurement of a threshold barrier for 1,2-fluorine migration to a carbene center. Although Eo = 29 f 4 kcal/mol is much higher than the threshold energies measured for 1,2-H shifts in carbenes, recent theoretical calculations on analogous carbenes support a large Eofor 1,2-fluorine transfer. Goddard” estimates a barrier height for fluorine shift of 30 kcal/mol in methylfluorovinylidene and 35 kcal/mol for difluorovinylidene. Popleis reported similar threshold energies of 33 for CFH=C: and 36 kcal/mol for difluorovinylidene. The comparable Eo’s for trifluoromethylcarbene and fluoro-substituted vinylidenes is not surprising since the 1,2-H migration barriers are similar for methylcarbene and vinylidene. A range of parameters (Table I) were tested to determine upper
Holmes and Rakestraw and lower bounds for the Eo(1,2-F). We found the fitting process generally favored an E, closer to 15 rather than 5 kcal/mol, a rather small Arrhenius A factor (=loll) and af(E,) closer to 0.25 than zero. The larger E , indicates that the activation energy for CF3CH insertion into HC1 is fairly large, perhaps more than 10 kcal/mol. We are not able to judge the reliability of this suggestion because threshold barriers for insertion of methylcarbenes into hydrogen halides are not available. Arrhenius A factors between lo1’ and 10l2s-l generally agree with solution-phase data on 1,2-H migration but our results are not consistent with suggestionsgathat A factors are two orders of magnitude lower. Our results also imply that for CF3CH2Clthef(E,) is larger than zero, we favor anf(E ) close to 0.25. This is larger than the 0.05 recommended by detser and c o - ~ o r k e rfor s ~ HX ~ elimination from halomethanes; however, they noted that if revised thermochemistry increases the E, then thef(E,) would also increase. The three-centered HX elimination from the halomethanes is characterized by the repulsive release of E,. Consequently, these dynamics result in translationally and rotationally hot products. Assuming the E, = 8 kcal/mol for CFzHCl, then the Ep is mainly released into translation, UT)= 0.6, and vibration of HCl, Cr,) = 0.3, with the remainder going to rotational energy of HC1 and the carbene. The rotational energy of CFz exceeds the vibrational energy by 2.4 kcal/mol. The dynamics of three-centered HCl elimination may be the same for the halomethanes and haloethanes but the energy disposal pattern could release a larger fraction of E , to the carbene for the latter case. The HCl fragment may repulsively recoil from the carbene for both the halomethanes and ethanes. However, for the two carbon system and center-of-mass is near the CF3end. The recoil of the HCl from the carbene would push the C H portion of the CF3CH into the trifluoromethyl group causing more internal excitation for CF3CH (vibrationally excited C-C bond or rotationally hot carbene). However, the same repulsive motion for the halomethanes, e.g., CFzHCl, would lead to high translational and/or rotational energy for the CF2.
Conclusions 1. The pressure dependence of the unimolecular isomerization reaction converting CF3CH into CFz=CHF was modeled to estimate a threshold barrier of 29 f 4 kcal/mol for 1,2-fluorine migration. The small number of atoms makes CF3CH amenable to current computational methods which could result in considerable refinement of the Eo(1,2-F). The Arrhenius A factor for this process is probably between 1 X 1O1I and 5 X 10l2s-l. 2. The fitting parameters suggest that a significant threshold energy barrier exists for the addition of CF3CH to HC1, Eo= 10 kcal/mol. 3. The fraction of the exit channel barrier released to the carbene appears to be larger when HCl departs from CF3CH versus CF2. However, a repulsive release of the exit channel potential energy may dominate the energy disposal process for both the halomethanes and haloethanes. Acknowledgment. We are grateful for financial support provided by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by the Brown Chair Endowment. We thank Ms. Debbie Tipton and Yukari Jones for extensive help with the computer calculations. Registry No. CF,CH2C1, 75-88-7; CF3CH*, 2441-28-3; HCI, 76470 1-0.
