N A L
O F
AL CHEMI Registered in ti. iT. Patent Ofice @ Copyright, 1975, by the American Chemical Society
VOLUME 77, NUMBER 3 FE
ydrogen Atom Abstraction by Fluorine Atoms' onaHd L. Williams and F. S. Rowland" Bepa! tmsnt of Chemistry, University of California, Irvine, California 92664 (Received August 4 , 1972) Publication costs assisted by the Division of Research, U.S. Atomic Energy C0"iSSiOn
The relative rates of abstraction of hydrogen atoms by 18F atoms have been measured for 132, Dz, CHI, CD4, C&, C H ~ C F BH2S, , and HI us. the rate of addition to acetylene. The measurements have involved the competitive diminution in yield of CH2=CH18F (from CHlsF=CH f HI) us. RH(or RD)/C2H2 ratio. The samples contained excess SFG which served both as the target for formation of ISF by the 19F(n,2n)18F nuclear reaction, and as the moderator for the excess recoil energy of these 18F atoms. The relative rates per H or D atom varied from 0.018 Z=! 0.004 for CH3CF3 to 0.65 0.05 for H2S. The abstraction of H was favored over D for both isotopic pairs of molecules: for Hz/D2 and for CH4/CD4. No reactmn was observed for 18Fwith xenon.
*
Pntroductioni Macroscopic studies of hydrogen atom abstraction by fluorine atoms have only infrequently been attempted, and have been complicated by several difficult experimental problems, including the high rates of reaction of fluorine atoms with many substrates, the great exothermicity of HF-forming reactions, the chemical reactivity of HF, and of many psetential fluorine atom sources, such as Fz, UF6, etc.2-12 Some of these problems can be avoided or ameliorated by experiments at tracer levels utilizing raatoms, a s described in our earlier communidioactive cation about this wnrk,13 and in related work by Root and We describe here the experimental details and more extensive conclusions from the data briefly summarized earlier.I3 In our experiments,, :I8F atoms are formed a t very high energies ( -IO5 eV) by the fast neutron induced nuclear reac%ion, 19F(n,2n)l-"F9 in gaseous SF6.15-19 These energetic 18F atom:; are then moderated to near-thermal energies by muitiple collisions with SFs, and finally react with minor components in the gaseous mixture by paths characteristic of thermal fluorine atoms. Approximately 1%of the ISF atoms react while still translationally hot by substitution into SFo with the formation of SF518FF,while other IsF storm also react while hot with the other components. Th'e hot reactions with these other molecules are roughly proportional in yield to the mole fraction of the substrate mixed with SF,s, and can be effectively mini-
mized in yield by performing experiments in a substantial excess of SFG. As discussed later, almost all of the abstraction reactions occurring in 95% SFs are initiated by essentially thermal 18Fatoms. (1) This research was supported by A.E.C. Contract No. AT-(04-3)-34, Agreement No. 126. (2) G. C. Fettis and J. H. Knox, Progr. ffeaction Kinet., 2, 1 (1964). (3) P. D. Mercer and H. 0. Pritchard, J. Phys. Chem., 63, 1468 (1959). (4) G. C. Fettis, J. H. Knox, and A. F. Trotman-Dickenson, J. Chem. SOC., 1064 (1960). (5) R. Foon and N. A. McAskill, Trans. Faraday Soc., 65,3005 (1969). (6) R. FoonandG. P. Reid, Trans. FaradaySoc., 67,3513 (1971). (7) K. H. Homann, W. C. Solomon, J. Warnatz, H . G. Wagner, and C. Zetzch, Ber. Bunsenges. Phys. Chem., 74, 585 (1970). (8) H. G. Wagner, J. Warnatz, and C. Zetzch, An. Assoc. Quim. Argent., 59, 169 (1971). (9) J. 6. Levy and 6. K. W. Copeland, J. Phys. Chem., 67, 2156 (1963). (IO) J. 6 .Levy and 6.K. W.Copeland, J. Phys. Chem., 69,408 (1965). (11) J. 9. Levy and 6. K. W. Copeland, J , Pbys. Chem., 72, 3168 (1968). (12) G. A. Kapralova, A. L. Margolin, and H . M. Chaikin, Kinef. Katal., 11,669 (1970). (13) R. L. Williams and F. S. Rowland, J. Phys. Chem., 75, 2709 (1971). (14) N. J. Parks and J. W. Root, presented at the 161st National Meeting of the American Chemical Society, Los Angeles, Calif., March 1971 (15) T. Smail and F. S. Rowland, J. Phys. Chem., 74, 1866 (1970). (16) T. Smail, G. Miller, and F. S. Rowland, J. Phys. Chem., 74, 3464 (1970). (17) T. Smail, R. S. lyer, and F. S. Rowland, J. Amer. Chem. Soc., 94, 1041 (1972). (18) R. L. Williams and F. S. Rowiand, J. Amer, Chem. Soc., 94, 1047 (1972). (19) R. L. Williamsand F. S. Rowland, J. Phys. Chem., 7 6 , 3509 (1972).
Ronald L. Williams and F. S. Rowland
302
When acetyiene is included as a minor component in SF6, two reactions account for >95% of the 18F atoms formed,18 abstraction from acetylene, as in (I), and addition to it, as -m (2). The excited CHlsF=CH* radical is
"F
dp
IaF
CzW2
--+
C2H2
+
Hl'F CZH CHi8F==CH*
(1) (2)
then stabilized by collision with SF6, and can be converted into CH2=43-PF by reaction with a hydrogen-donating scavenger such as HI, as in (3), or HzS. The inclusion of the scavenger permits another reaction path for 18F, abstraction from Hi, as in (4). The relative rate of (4) U S .
GM'sF---CH
+ HI
--+
CHz=CHIBF -t I
(3) (4)
(1) plus ( 2 ) shouid be proportional to the HI/C2H2 ratio in an experiment if the three reactions are directly competing for the same pool of 1 8 F atoms. If and when this system is well behaved, the inclusion of a fourth component, RM, permits additional reactions, including especially abstraction from this additional substrate, as in ]SF'
-t. RH. (or RD)
----t
H18F(or IF")-t R
15)
We have carried O U , ~such competitive measurements us. acetylene with several hydrogen -containing molecules, including HzS, CH4, CD4, €32, D2, CzH6, and CHsCF3. Several experiments with Xe are also reported. Most of these experiments have been conducted at pressures between 2500 and 3000 Torr to maximize the number of fluorine atoms as targets In the fast neutron flux, and therefore At such pressures, there is no evithe total yield of dence for decomposition of CH18F=CH* before stabilization.'" ~ ~ ~ e r ~Saction ~ e n t a ~ Fluorine-18 atoms were formed by the nuclear reaction 19F(n,2n)18%occurring in SF6 exposed to the fast neutron flux from a Kaman A711 neutron generator. Most samples contained >90% SF'e as weli as CzHz, HI, and usually one other minor component in a 15-ml bulb at about 3000 wm. After irradiation, the gaseous contents by radio gas chromatography. The experimental details %re essentially the same as those described in a detailed study of 18F reactions with acetylene itself.ls N e direct analysis was usually performed for the Hl8F molecules formed. by abstraction reactions 1, 4, or 5 . Instead, the 'summed yield of such reactions was indirectly estimated from the diminution in CH2=CHlsF yield, the other major fate for near-thermal IsF atoms in these systems. The indirect measurement procedure was adopted because we are able to obtain much better reproducibility for the deter.ainatinn of product compounds which are both volatile and relatively nonreactive, such as GH2-=CM1W, than we are for reactive species such as I T . One cojlselquenee of this technique, however, is that we are unable 'io distinguish among reaction paths that do not produce a readily identifiable lsF-containing species. Among such uncertainties are the possible reactions 6 and 7,20 whose products might not pass through our normal gas chromatographic analytical procedure, even if otherwise nonreactive under our experimental conditions. The substitution of IsF for H in hydrocarbons as in ( 8 ) , although usually also exothermic, has been shown experimentally not f o be important by the negligible yields (in The Journal oi Physical Chemistry, Voi. 77, No. 3, 1973
95% SFa) of the corresponding RlsF product. (With i m e r SF6 concentrations, appreciable yields of R18F molecules can be observed.) The abstraction of F from CHsCFs is highly endothermic because of the weakness of the 18F-F bond, and has a negligible yield in 95% SFe mixtures.
-
+ HI + H,S --laF + RH 1BF
l8F
18Ff f E4 H P F 4R'aF
--+
(6)
(7)
+- H
(8)
The eventual accuracy of the quantitative evaluation of these experiments thus depends upon the reproducibility of absolute measurements of CH2=CH1SF yieid. The most important error here involves the general monitoring of the total 18F production in the system, estimated to be k6-89'0 in these experiments. This error in the total production is proportionately smaller for yields less than 1009'0,i.e., k3-4% for yields of about 50%. Most of the chemicals used in these experiments were Research grade (Matheson Co.): C2H2, &S, HI (96%), SF6 (98%), CH4, C2&, D2. The sources of the other gases were (CD4) Merck Sharpe and Dohme, (CHaCFs) Peninsular Chem Research, and (Xe) Air Reduction eo. Results and Discussion
Kinetic Competition in C2N2-HI Mixtures. After moderation of the initial extra kinetic energy, the I8F atoms react competitively among the available reaction paths. With C2H2 as the only other component (in addition to SFe), the distribution between H18F and CH18F-CM is in the ratio of the respective rate constants, kl*/kz*. (The rate constants have been marked with asterisks as a reminder that these "constants" are averaged over whatever energetic distribution is present in the particular experimental system. The similarity of these distributions to the Maxwell-Boltzmann distribution, and of kl*/k2* to kllk2, the thermal rate constants, is discussed later.) No direct measurement of the yield of (2) is possible until a hydrogen donor, usually HI, has been added and the ratio of yields of HlsF to C&=CHlsF is then giveri by (9), and the ratio of total available 18F ( i e . , all 18F atoms that did not, while still possessing extra kinetic energy, react to form a stable chemical bond) to the yield of C H Z = C H ~ ~isF given by (10). 'This equation predicts a straight line for a graph of (Y(CH2=CH1sF))-1 us. [HI]/ CzHz] with an intercept >1.0 and a slope reflecting the relative rates of reaction 4 us. 2 . Y(H"F> / Y(CH,====CH"F)= (h*[C&I Y(total)/Y(GH,=CH'8F) k2*[C.HJ =(( h*
f ~,*[HIl)lh*[C2H?] (9)
= ((kl" f
h,*)[C,M,]
+ k,*)/ k,*J + h,*[Hi]/
+ k,*[HI]I /
h2*[C,H,j (10)
The data for a series of CzH2/HI competition experiments are given in Table I. In each case, yields were also observed for SF518F (about 19'0), CH=C18F (
