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The Decomposition of Vibrationally Excited 1,1,1-Trideuterio-2,2-difuoroethane1 M. J. Perona, J. T. Bryant, and G . 0. Pritchard Contribution f r o m the Department of Chemistry, University of California, Santa Barbara, California 93106. Received March 6 , 1968 Abstract: Evidence is presented that the vibrationally “hot” molecule CF2HCD3*, formed by the combination DF, of CFpHand CDI radicals, may decompose by three competing unimolecular processes to form CFH=CD, CFD=CD2 HF, and CF2=CD2 HD. The second process, a three-center a,a elimination of HF from the same carbon atom, has not previously been observed. The evidence for the third process, HD elimination, is more tentative. An estimation of the Arrhenius parameters for the decompositions using classical Rice-RamspergerKassel (RRK) theory is made, and the effect of collisional quenching of the “hot” molecules is discussed in terms of the critical energies for the eliminations. A similar experiment with CFH2CD8* showed n o a,a elimination of
+
+
+
L
HF.
The
dehydrohalogenation of haloethanes is generally accepted to proceed oia a four-center transition state (a$ elimination) in both thermal? and chemical activation3 systems. However, in some preliminary experiments on the vibrationally “hot” molecule CF2HCD3*, formed by the chemical activation techn i q ~ e we , ~ observed the formation of both d3- and dzvinyl fluoride, leading us to suggest that a three-center elimination may occur in which both the hydrogen and halogen atom come from the same carbon atom, a,a e l i m i n a t i ~ n . ~We have 0bserved5,~that our interpretation of the elimination of HF from the “hot” molecules CF2HCFH2* and CF2HCF2H* may be complicated by the occurrence of such a process. Herein we give a detailed report of the decomposition of the “hot” CF2HCD3*molecule.
Experimental Section Mixtures of chromatographically pure 1,1,3,3-tetrafluoroacetone (TFA) and d6-acetonejd6A),which was 99.5% D, were photolyzed together in the 3130-A region. Experiments were generally conducted on a mercury and grease-free apparatus at low pressures6 and on a convential apparatus’ at higher pressures. None of our reported products can be due to Hg-sensitized processes. Product identification and analysis was performed by vpc and mass spectrometry. The methanes and ethane CF2HZrCF,HD, and CiD6 were observed but no analysis was carried out.* The following ethanes and ethylenes were identified: CFzHCD3, CFzHCF2H, CFH=CD,, CFD=CD2, and CFz=CD2. At small percentage conversions no other volatile products were observed. Vpc calibrations for the deuterated products were made with their nondeuterated analogs. The vinyl fluoride peak was collected off the chromatograph in each experiment and the ratio of d3- to &-vinyl fluoride was determined by the 49/48 parent mass ratio on the mass spectrometer. The mass spectra of each of the product fractions were compared carefully with standard samples of CF2HCH3, CFZHCF2H, CFH=CHp, and CFz=CH2, and the identification and deuterium composition of the products that we report are un(1) This work was supported by a grant from the National Science Foundation, GP-4090. (2) See H. E. O’Neal and S. W. Benson, J . PhJ)S. Chem., 71, 2903 (1967), for a very recent review. (3) J. C. Hassler and D . W. Setser, J . Chem. Phys., 45, 3246 (1966); R . L. Johnson and D. W. Setser, J . Phys. Chem., 71, 4366 (1967). (4) B. S. Ravinovitch and M. C. Flowers, Quarr. Reo. (London), 18, 122 (1964). ( 5 ) J. T. Bryant, B. Kirtman, and G. 0. Pritchard, J . Phys. Chem., 71, 1960 (1967). (6) G. 0. Pritchard and J. T. Bryant, ibid., 72, 1603 (1968). (7) G . 0. Pritchard, M. Venugopalan, and T. F. Graham, ibid., 68, 1786 (1964). (8) The fraction volatile at - 195”, CO, CDIH, and CD4, was not analyzed.
equivocal.$ The major peaks in the mass spectrum of CF2HCDs were at m / e 51 (base peak), 69 (parent), and 50 in the ratio of 1 :0.5 :0.15. The probable positive ions are CF2HC, CF2HCD3+, and CFHCD3+ (the CF2+ ion peak, which is also at m / e 50, is negligible in the mass spectrum of CF2HCH3). Small ion peaks, -1 % of the base peak, occurred at m / e 52 (after C13isotope correction), 35, and 34; the probable ions are CF2D+, CFD2+, and CFHD+, respectively, and may be assumed to be rearrangement (transference) peaks. If the contribution from the CF2CD3+ion was assumed to be zero, the sample possibly contained a maximum of E,>,*. (23) The value of the intercept may be adjusted by assuming different relative quenching efficiencies for the bath molecules. To be consistent we have retained the relative efficiencies used for the discussion of the a,a elimination; to do otherwise would cause our argument to appear to be contrived. Note in Figure 2 that the total elimination, and therefore the a,/3 elimination, has zero intercept. Since there is no reason to presuppose anything other than a “hot” molecule mechanism for the CFH=CDz formation, our choice of relative quenching efficiencies would appear to be justified.
Perona, Bryant, Pritchard
/ Decomposition of I,I,I-Trideuterio-2,2-difluoroethane
4786 0.2
lip, cm-l. 0.4
0.6
preliminary form, and Professors F. S. Rowland and D. W. Setser for prepublication copies of their manuscripts.
0.8
(24) A referee has observed that the disproportionation between any radical (CFZH, CD3, CDZCOCDI, CFZCOCFZH)in the system and CFzH will yield CFZand that the reaction sequence
CF2
+ CDCOCD, +(CFZCD~COCD~)* +
+ COCDB
CFZCDI
accounts for the CFZCDZ. We make the following points. (a) Probably only the self-disproportionation us. recombination of 0.1 r I I I I CFzH's ( k d / k , ) is important, and is equal to 0.19;e values of k,i/k, 0.1 0.2 0.3 (to give C R ) are for CFHz f CFZH = 0.06,o for CF3 CFzH = 0.09 1 fl, cm-1. (M. J. Perona and G. 0. Pritchard, to be published), and for CH3 CF?H = 0.'' The fate of the CFZhas been difficult to establish unFigure 5 . Plot of Re1imimarirn/Rstai,iiiration us. 116 (see Figure 2): equivocally,s but fluoropropanes, from combinatiorr with monoradicals, been observed (M. G. Bcllas, 0. P. Strausz, and H . E. 0, RCF~-CD~/RCF~HCD~ at 240"; 0 and 0 , R c F ~ = c ~ ~ / atR 145 c F ~ ~ have c ~ ~recently ~ Gunning, Can. J . Chem., 43, 1022 (1965); J. B. Hynes, R. C. Price, and 188", respectively. W. S. Brey, Jr., M. J. Perona, and G. 0. Pritchard, ibid., 45,2278 (1967); and to be published). (b) A pathway involving CFz cannot be ruled out, and, where we conThe evidence that we have presented strongly suggests ducted analysis for CzF CFzCD?. Simi~ the participation of acetonyl larly 0.19C?F,H? > C F I C H Z . ' ~However, that the "hot" molecule CF2HCD3*may decompose by radicals cannot be assumed under all of our reaction conditions, altwo, and probably three, competing unimolecular prothough CFKDz is formed under all conditions. I n our previous lowcesses. 2 4 A complete understanding of this complex pressure experimcnts,G abstraction from the fluoroacetones was negligible up to 500"I
