3464
T. SMAIL, G. E. MILLER,AND F. S. ROWLAND
The Reactions of Energetic Fluorine-18 Atoms with Tetrafluoroethylenela by Thomas Smail, George E. Miller, and F. S. Rowlandlb Department of Chemistry, University of California, Irvine, California 98664
(Received February 6 , 1970)
Energetic lSFatoms react with CFz=CFz to form CFz=CF18F and CF2CF21sF*. More than 89% of CFz= CF1!F* molecules are excited enough to decompose to CF2 plus CFl6F(>76 kcal/mol excitation energy) at several atmospheres pressure; CF18Fis conveniently trapped by insertion in HI to form CHF1*FI. The decomposition of excited CF&F218F* by C-C bond rupture is followed by measurement of CHFzCF2l8F and radicals formed by thermal CHFPF yields in the presence of HI. The half-stabilization pressure for CF2CF2l8F '*F atom addition is 218 Torr of CZF4. The formation of 18F in gas ampoules by the 19F(n,2n)lsF reaction was monitored with a system utilizing a Teflon sleeve around the standard glass ampoule. The average recoil range of 18Ffrom the (n,2n)reaction is about 0.1 ern in 760 Torr of CzF4. Introduction Investigations of the reactions of thermal fluorine atoms with organic compounds have been mainly concerned with the abstraction of hydrogen atoms from hydrocarbons,* although some studies of the addition reaction have been made with olefin^.^,^ fJsually, the thermal F atoms are generated by reaction between Fz and the substrate,2,aalthough the photolysis of NOF has been the source in recent ~ t u d i e s . ~Additional reaction paths, especially the substitution otf F for atoms or groups in the substrate molecule, can readily be observed with energetic 18Fatoms formed by the nuclear reactions 160(T,n)18F,51eF(y,n)1*F,6-10or lgF(n,2n)l S F , 10
The inclusion of increasing amounts of an inert moderator (e.g., neon) in a system in which hot '*F atoms are formed leads to a continual reduction in the average kinetic energy of the atoms a t the time of reaction and permits the observation of the chemical reactions of F atoms with energies nearer and nearer to thermal energies. The nuclear methods of fluorine atom production have two strong advantages: (a) only tracer concentrations of fluorine atoms are involved; and (b) the fluorine atoms can be formed from stable, unreactive precursors such as CF4, SF6, etc. Consequently, the chemical difficulties of handling reactive precursors and macroscopic amounts of reactive products are avoided. Solids, liquids, and gases are all readily studied, and the reaction products can be identified and detected through radio gas chromatography. The present experiments form part of a generd investigation of ISF atom reactions with olefinic compounds and have been carried out with the intention of discovering the general features of both hot and thermal fluorine atom reactions with these substrates. Experimental Section Chemicals. Tetr fluoroethylene (Peninsular Chemical Co.) was distill! t d at -78" to free it from traces of The Jourml of Physical Chemistry, Vol. 74, No. 19, 1970
the polymerization inhibitor (bp 170") present in the original tank; it was then degassed at -196" prior to use and stored in a -78" bath. Hydrogen iodide (Research Chemical Go.) was degassed and stored at - 196". Sulfur hexafluoride (Matheson Co.) was degassed a t - 196" and used without further purification; a trace of CF4( 10. The effect on absolute product yields of var-
ISF
+ HI --+HIsF + I
(9)
iations in the C2F4/HI ratio at constant total pressure is shown in Table 111. The maximum, in absolute yields in this table identifies two separate mechanistic influences. At the lowest C2F4/HI ratios; direct reaction of 18Fwith HI is lowering the overall yields of all products which have precursors which originate at least partially from low-energy or thermal 18Fatoms. The formation of CFlSF via decomposition of an excited CF2=CFlsF* molecule has the greatest energy requirement of any of the observed reactions, and presumably is initiated by the highest energy I*F atoms on the average. As expected for the hottest process, the yield of CF1*F as CHFlSFI is least affected by a decrease in C2F4/HIratio and is essentially constant throughout Table 111. The other products show decreases in absolute yield as the relative concentration of HI increases. At the lowest HI concentration, C2F4 begins to compete effectively with HI as a monoradical scavenger and the yields of CHF21sF and CHFzCF21sFare markedly reduced. The yield of CHFlsFI is not affected by this ratio change, consistent with the unreactivity of C2F4 toward CF1*F.16 A ratio of C2F4/HI of 10 has been chosen for subsequent experiments in order to maximize low-energy I*F reaction with C2F4 rather than HI, while still preserving the role of HI as efficient scavenger of monoradicals. Hot vs. Thermal Reactions. One distinguishing test for hot and thermal reactions in a particular system involves the measurement of the effect of an inert moderator. A completely inert moderator, such as neon, will remove the kinetic energy of '*F atoms and reduce the probability that the ISFatom will possess high kinetic energy at the time of its reaction with C2F4 or other reactive substance in the system. In the present case, completely inert moderators are ruled out by the necessity that substantial quantities of ISFatoms are needed
30t
0.0
0.2
0.4
0.6
0.0
Mole Fraction SF,
Absolute yields of products from l8F plus CzF4us. mile fraction of c ~ F ~ - s F e,~CHF~CFPF; : Al CHF2''F; ,. CHF'EFI.
in the system as targets for the nuclear reaction forming ISF, Wolfgang, et al., have previously shown that CF4 and neon are about equally efficient as moderators for F atoms.6 However, since CF4 can also be a reactive source of CFlSFand CF21sF,we have substituted SF6 as a moderator in this work. To a first approximation, SF6 is inert toward reaction with ISF-the absolute yield of SFS1*F is only 1.3% and no other volatile products are readily observed. The absolute yields obtained from a series of experiments with varying CzF4/ SF6ratios are summarized in Figure 2. The three products in this data set are obviously ordered by their slope/intercept behavior in Figure 2 into decreasing order of the average energy of the primary l*F reaction involved: CFXsF > CF218F > CF2CF2'*F. The yield of CFISF extrapolates to zero for pure SF6 (complete moderation of kinetic energy of l8F) and is thereby identified as entirely of "hot" origin. The steady decrease in CHFZ1*Fyield to a nonzero intercept indicates both some loss of kinetic energy used in this reaction, as well as underlying yield of thermal '*F origin. The ISFcontent as SFsl*Fand CF2=CF18Fwas not resolved into its components in the ratio gas chromatograms, but the combined yields do not increase as would be required if the decrease in yield of CHFIsFI with increasing SFs concentration were matched by an increase in the yield of CF2=CF18F. The reaction forming CF18Fis clearly identified as initiated by energetic 18Fatoms, consistent with the minimum endothermicity of 76 kcal/mol for the dissociation of C2F4into 2CF2.17 (14) See D.M.Golden and 8. W. Benson, Chem. Rev., 69,125 (1969). (15) H. S. Johnston and C. Parr, J. Amer. Chem. Soc., 85, 2544 (1963). (16) W.J. Tyerman, Trans. Faraday Soc., 1188 (1969),and references therein. (17) H. F. Zmbov, 0. M. Uy, and J. L. Margrave, J . Amer. Chem. Soc., 90,5090 (1968). The J O U Tof ~Phyaical Chemistry, Val. 74,No. 10, 1070
3470
T. SMAIL, G. E. MILLER, AND F. S. ROWLAND
The rate of formation of CF218Fby the decomposition of CF2CF218Fradicals, as in reaction 6, certainly will decrease as the average energy of 18Fdecreases, but the nonzero moderated yield implies that the rate of decomposition at these pressures is not negligibly small for thermal 18Faddition to CzF4-both hot and thermal 18F atoms are involved in the formation of CF218Fin this system. This finding is consistent with the pressure studies described below. If a smaller fraction of CF&F218F radicals fails to decompose to CF2I8F, and if lower energy 18Fatoms are still scavenged by CzF4, then the yield of CHF2CF218F should show a complementary increase in yield with increasing mole fraction of SFs. Although the yield of CHF2CF2"F does increase with increasing SFe, the increase is not complementary to the other decreases. Finally, at the highest SFe concentration, the CHF2CF218Fyield decreases again, indicating that some other reaction is removing some 18Fwhen SFs is in almost 20fold excess 8s. the 1 O : l mixture of CzFh-HI. This other reaction path has not been identified, but could involve rather low energy processes in which the reaction of 18F with H I is favored in competition with C2F4. Addition of 18F to CF2=CF2. The addition of thermal 18Fto CF2=CF2 creates an excited CF2CF218F*radical with 70-75 kcal/mol excitation energy, while the initiation by a hot 18Fatom will provide still more excitation energy as well as a broad spectrum of excitation energies for the excited pentafluoroethyl-18F radicals. Pentafluoroethyl radicals from the pyrolysis of C2FsN= NC2F6have been observed by Lossing, et al., to decompose into CF3 CF2 above 950" ;18this observation was rationalized by the fact that the C-C split of reaction 6 is about 17 kcal/mol more exothermic than the loss of F as in reaction 7. The pathways for decomposition of excited radicals formed in this l*F system can also be determined by study of the effects of pressure upon the absolute and relative product yields. The data from such a study of C2F4-HI over the range from 110-4500 Torr is shown in Figure 3. The scatter in the magnitude of the yields is within the 10% accuracy attributed to the Teflon-sleeve monitor technique, and most of the scatter seems to arise in a systematic shift of all absolute yields from a particular sample and not in the relative ratio of product yields within a given sample. The steady decrease in the yields of all products below about 650 Torr is attributed to recoil loss of 18Fa t low pressures. By assuming that HI is a completely efficient scavenger for CF18F and noting that most of the excited CF2=CF 18Fmolecules have decomposed to CF18F a,t all pressures, the yield of CHF18FI can be assumed ,to be essentially independent of pressure and used as the standard for a recoil loss correction. The open triangles in Figure 3 indicate the expected yield of CHF218Fafter such a correction has been applied. Similar corrections could also be made for the other two products, but would not alter the qualitative conclusion
+
The Journal of Physical Chemistry, Vol. 74, N o . 19, 1970
-
CF2 = CFleF
I
I
I
2
3
4
-
I
5
Pressure (torr x
Figure 3. Pressure dependence of product yields from *SFplus CHF&Fz18F; A, CHFz'8F; B, CHF'SFI; 0 , CZFI: CFZ=CF18F; A, CHFz'sF corrected for recoil loss using CHF'SFI data.
+,
that CFzCFd8F yields decrease at low pressure; the yields of CF2=CF18F are discussed below. The decomposition/stabilization ( D / S ) ratio for C2F418Fradicals is plotted in Figure 4. The behavior of the graphs of Figure 4 suggests that there are two populations of CF2CF21sF*radicals-a roughly monoenergetic group formed by the exothermic addition of thermal 18Fto C2F4, and a more energetic group formed by the addition of hot 18Fatoms with a spread of kinetic energies. Assuming that those radicals which decompose at the highest pressures are chiefly the result of the addition of hot 18Fatoms (and that this 30/70 ratio of hot/thermal 18Fatom addition is independent of pressure), the observed D / S ratios can be corrected for the hot contribution, and a set of D / S values obtained which correspond to the behavior of the radicals formed by the addition of thermal 18Fatoms. These "corrected" values are also plotted in Figure 4, and give a straight line passing through the origin of the reciprocal pressure plot, as required for a monoenergetic group of radicals. The half-stabilization pressure for these pentafluoroethyl radicals formed by thermal addition of 1sF is 218 Torr, which corresponds 2 X lo9 sec-l to an average decomposition rate of assuming a strong-collision deexcitation mechanism. Recoil Losses for Energetic 18FAtoms Formed in the "JF(n,Wn)18F Reaction. The loss of energetic tritium atoms t o the walls of the vessel has been the source of a
-
(18) I. P. Fisher, J. B. Homer, and F. P.Lossing, J . Amer. Chem. Soc., 87, 967 (1965).
REACTIONS OF ENERGETIC 18FATOMSWITH TETRAFLUOROETHYLENE
3
2
s I
IOOOIPressure (Torr'')
Figure 4. Decomposition (stabiIization ( D I S ) ratios for CF2"JFCFzfrom 1sF C2F4: 0 , ( D I S ) all data-CHFPF/ CHF2CF218F; ( D / S ) after subtraction of "high energy" CFzCF@F radicals.
+,
+
routine correction for many y e a r ~ in l ~the ~ studies ~ ~ of the hot atom chemistry of tritium. Several of the simplifications available in recoil tritium systems are absent in the study of 1SF recoils: the 18Fatoms are not monoenergetic, are not isotropically emitted, and are not produced homogeneously throughout the reaction vessel because of the severe inhomogeneity of the fast neutron flux. Consequently, only a very approximate estimate can be simply made of the recoil losses in 1SF systems. By making the unjustified but crudely applicable assumption that the fractional recoil losses of 18Fcan be converted into an average recoil range with the diagrams constructed for isotropic, monoenergetic homogeneously produced the average range of S ' F is calculated to be about 0.1 cm in 760 Torr of CzF4. Formation of Vibrationally Excited CF2=CF18F. The molecules of CF2=CF18F found in these experiments can conceivably be formed by either of two mechanisms: by the direct substitution of 18F for F in CzF4, in analogy with similar replacements for CF4 and other saturated molecules, and by the decomposition of excited CF2CFz18F* radicals formed by addition of hot lSF atoms to CzF4. From a pragmatic experimental point of view, these two mechanisms are essentially in-
3471
distinguishable, differing only in the time delay between the arrival of 18Fand the departure of the displaced F atom. Since the lower energy decomposition path of an excited pentafluoroethyl radical is through the C-C split discussed earlier, the loss of an F atom from pentafluoroethyl can only occur if the total excitation energy is very high. Experimentally, such higher energy paths are very difficult to stabilize at any gas pressure, so that the reaction (if it occurs) will proceed to completion without any distinguishable trace, nor will it be readily disclosed by a study of the pressure dependence of product yields even over a very wide range of pressures. We can certainly conclude that most CF2CF218F radicals formed by hot or thermal 18F atom addition decompose to CF218F and not by F loss; however, we are unable t o estimate the contribution of F atom loss t o the small yield of CF2=CF18F formed in this system. The yield of CHF"F1 greatly exceeds that of CF2= CFlsF at all pressures (Figure 3), with average yields for six determinations of 12.6% and 1.5%, respectively, corresponding to >S9% decomposition of CFt=CF lSF. Hence, we can conclude that the hot replacement of F in CzF4results in the deposition of >>76 kcal/mol of excitation energy in >89% of the cases. This conclusion is quite consistent with the very large energy depositions found for 18F-for-F in CF421and CHaCF3.22 The data of Figure 3 show little if any dependence of CF2=CF1sF yield with pressure-the apparent decrease a t low pressure is chiefly attributable t o the recoil loss of 18F, If a recoil loss correction is made, the yield of CF2=CF1*F is constant over this 110-4500 Torr pressure range within the not very stringent accuracy limits of this series of measurements. The ratios of CHF18FI/CFg=CF18F, in which all monitor fluctuations are canceled, also indicate no change in the fraction decomposition of CF2=CFlsF over this pressure range. (19) P. Estrup and R. Wolfgang, J . Amer. Chem. Soo., 82,2665 (1960). (20) J. W. Root and E'. S. Rowland, Radiochim. Acta, 10,104 (1968). (21) Y.-N. Tang, T.Smail, and F. 5 . Rowland, J . Amer. Chem. SOC., 91,2130 (1969). (22) C. McKnight, N. J. Parks, and J. W. Root, J . Phys. Chem., 74, 217 (1970).
The Journal of Physical Chemistry, Vol. 76, No. 19. 1970
