J. Phys. Chem. 1981, 85, 2493-2497
2403
Atom-Transfer Reaction Rates for Thermal Fluorine Atoms with CH3X and CF3X (X = Br, I) R. Subramonla Iyer and F. S. Rowland" Department of Chemistry, University of California, Irvlne, California 92717 (Received: April 14, 1980; In Final Form: April 29, 1981)
The thermal reaction rates at 283 K of F atoms have been measured with CH3Br,CHJ, CF3Br,and CF31by use of radioactive 18Fatoms moderated to thermal energies by multiple collisions in gaseous sF6at a pressure of 2700-3000 torr. The relative rate constants for these reactions have been measured in competition with addition to C2H2and/or abstraction from HI, and have been converted to absolute rate Constants by comparison with the rate constant of (1.53 f 0.12) X cm3molecule-' s-l for the addition of F to C2H2.The thermal atom transfer to F of I from CF31,of I and/or H from CHJ, and of H from CH3Br are all extremely rapid. No reaction was observed with CF3Br. The absolute rate constants measured in competition with F addition to C2H2are (in units of cm3molecule-' s-l) as follows: (6) F + CH,Br HF + CH2Br,Ize = (6.1 f 0.7) X lo-"; (8) F + CH31 HF + CH21,(9) F + CH31 IF + CH3, k8 + k9 = (1.74 f 0.15) X 10-lo; (7) F + CF31 IF + CF3, Iz, = (1.62 f 0.16) X
-
-
Introduction Energetic halogen atoms formed in nuclear reactions can be thermalized by multiple collisions with nearly inert molecules, and then allowed to react with minor components in the gaseous mixture.'-12 Such thermalization has been applied both to energetic 18F atoms formed by the l?F(n,2n)'8F reaction in gaseous SF,'-7 or C2Fk9and to 38c1 atoms from the 37Cl(n,y)98C1 reaction in gaseous CC1F3or CC12F2.1@12The relative importance of the high kinetic energy reactions of "T or 38c1 atoms decreases steadily with decreasing mole fraction of the reactive minor components, and is of little importance for major component (e.g., SF6, CClF,) mole fractions > 0.95.1*7-10 The most obvious demonstration that such reactions lead to thermal atom distributions is the observed strong temperature dependence of the reaction of 38Clwith CH11,12while the thermalization of 18Fatoms has been shown by the lack of any further change in reaction yields as the mole fraction of SFJ or C2Fe9is extended from 0.95 toward 0.999. Thermal lsF atoms react with C2H2primarily by addition, as in (l),and the resulting C2H28F*radicals are deexcited by collisions with SF6,and converted to a stable, measurable product by reaction with HI, as in Abstraction of H from C2H2and from HI, as in (3) and (4), compete with addition to C2H2for the thermal 18Fatoms, yet as much as 85% of all the 18F atoms have been observed as C2H3l8F at low HI/C2H2 ratio^.^ The inclusion of an additional minor component can then reduce the yield of CzHJ8F to the extent that this other component competes with C2H2for the thermal 18Fatoms. lSF + C2H2 C2H2lSF* (1)
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(1)F. S.Rowland, F. Rust, and J. P. Frank, ACS Symp. Ser., 66, 26 (1978). (2)T.Smail, R. S. Iyer, and F. S. Rowland, J.Phys.Chen., 75,1324 (1971). (3)T.Smail, R. S. Iyer, and F. S. Rowland, J. Am. Chem. SOC.,94, 1041 (1972). (4)R. L. Williams and F. S. Rowland, J. Am. Chem. SOC.,94, 1047 (1972). (5)R. Milstein. R. L. Williams, and F. S. Rowland, J. Phys. Chem., 78,'857(1974). (6) R. L. Williams and F. S. Rowland, J.Phys.Chem., 77,301 (1973). 17)J. Cramer. R. S. her. - . and F. S. Rowland. J.Am. Chem. Soc.,. 95,. 643'ii973). (8)S.-H. Mo,E.R. Grant, F. E. Little, R. G. Manning, C. A. Mathis, G. S. Werre, and J. W. Root, ACS Syrnp.Ser., 66,59 (1978). (9)C. A. Mathis, K. D. Knierim, and J. W. Root, Chem. Phys.Lett., 72,368 (1980). (10)F. S . C.Lee and F. S. Rowland, J.Phys. Chem., 81,1229(1977). (11) F. S. C. Lee and F. S. Rowland, J. Phys.Chem., 81,86 (1977). (12)C.Yarbrough and F. S. Rowland, unpublished results.
-
+
C2HJ8F
18F3. C2H2 18F
-
+ HI
+ HI
-
C2HJ8F + I
H18F + C2H
-
HlaF
+I
(2)
(3) (4)
This system involving competition of a substrate with thermal reactions of C2H2and HI has been applied to the measurement of the rates of thermal reactions of 18Fwith certain classes of minor components. Earlier studies have given useful rate constants for H abstraction from a series of hydrogen-containing molecules,6while overall reaction rates have been measured with a variety of other gaseous molecules,5 including 02,NO, SO2, N2,CO, and Xe. In all of these experiments, the reaction of 18Fwith the substrate competitor as in (5) has not been observed directly but has
+
"F X-Y --* "F-X + Y (5) rather been followed through the diminution in the observed C2H3l8F yield from the competing reaction sequence of (1)plus (2). Experiments with the same substrate have shown that similar results can be obtained by direct observation8of the abstraction product HlsF to those found by our indirect diminution of the yield of a competitor product such as C2H318F. The direct method has been shown to be capable of high accuracy in kinetic measurements of H-atom abstraction rates by fluorine atoms.8 We have here applied the indirect C2H2/HIcompetitor technique to atom-transfer reactions involving CHJ, CH3Br,CFJ, and CF3Br. Reaction with thermal 18Fwas observed for CH3Br but not for CF3Br,and the measured reaction with CH3Br is attributed to H abstraction as in (6). Rapid thermal reaction was observed for lSF with CFJ, corresponding to the transfer of an I atom, as in (7). The fastest overall reaction rate for thermal lSFamong these compounds was found with CHJ, and can involve both H abstraction and I-atom transfer, as in (8) and (9).
18F+ CH3Br
--
H18F + CH2Br
18F + CF31 18F+ CHJ
18F+ CH31
I18F + CF,
H18F + CH21
-
I18F
+ CH3
(6)
(7) (8)
(9)
Experimental Section Neutron Irradiations. The procedures for irradiation of gaseous samples have been described in detail in earlier
0022-3654~ ~2085-2493$01.25/00 1981 American Chemical Society
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The Journal of Physical Chemistty, Vol. 85, No. 17, 198 1
Iyer and Rowland
TABLE I: Absolute Yields of Volatile I8F Products from Recoil I8F Atom Reactions in Mixtures of SF,, CH,Br, C,H,, and HI press., torr 2530 2460 2550 2610 2610 2590 2570 SF, CH,Br 128 128 127 128 127 129 127 0 19 64 64 64 128 127 gf-4 29 29 29 29 22 29 28 absolute yield, % total '*F product SF,18F 1.01 f 0.02 1.13 f 0.02 1.12f 0.02 1.08 f 0.02 1.05 f 0.02 1.17 f 0.02 1.22 f 0.03 CH,I8F 0.69 f 0.03 0.64 f 0.02 0.54 f 0.02 0.48f 0.02 0.53 f 0.02 0.55 f 0.02 0.55 r 0.03 CH,=CH18F none 22.4 f 0.1 50.5 f 0.2 47.2f0.2 60.9 f 0.2 65.2f 0.2 46.4 t 0.2 TABLE 11: Absolute Yields of Volatile I8F Products from Recoil 18F Atom Reactions in Mixtures of SF,, CHJ. C,H.. and HI press., torr SF, 2820 2750 2810 2740 2760 2880 2890 CHJ 142 137 136 137 137 141 142 36 70 70 137 138 288 288 18 19 19 26 26 43 44 absolute yield, % total I8F product SF.18F 1.11 f 0.03 1.17f 0.03 1.18 f 0.04 1.24 f 0.02 1.10 fi 0.03 1.26 f 0.03 1.20 f 0.02 CH;~~F 0.78 i 0.03 0.78 f 0.02 0.82 f 0.03 0.74 ?: 0.03 0.73 f 0.03 0.73 f 0.02 0.63?r 0.02 CH,=CH"F 16.8 f 0.1 29.2 f 0.2 29.4 f 0.2 43.1 f 0.2 41.7 f 0.2 57.3 ?r 0.2 54.2 f 0.2
publications, and are only summarized here.'-'J3 The samples were placed in ll-mL cylindrical flat-bottomed bulbs made from Corning Pyrex 7740 glass and equipped with break-tip seals. The gas pressures were measured on a calibrated vacuum line, and the gases were frozen into the sample bulb at 77 K. All samples were irradiated at a pressure of several atmospheres, sufficient that almost all C2H218F*radicals were stabilized before reverse decomposition to 18F + C2H2could O C C U ~ . ~ The samples were irradiated with the 14-MeV neutron flux from a Kaman A-711 fast neutron generator. The fast neutron flux to which each sample was exposed was monitored by the I8F activity induced in a Teflon sleeve encasing the glass bulb during irradiation. The temperature of the gases during irradiation was effectively controlled by the temperature of the cooling water for the titanium tritide fast neutron target, Le., approximately 283 f 2 K. Radiogas Chromatographic Analysis. A complete radiochemical analysis of these samples could have been carried out by separate determinations of (a) nonexchangeable, nonreactive volatile gaseous components by radiogas chromatography; plus (b) appropriate additional experiments for measurement of other species (e.g., H18F, P F ) which might have deposited on or reacted with the glass surface of the irradiated ampule. Since comparable measurements of the yields for the same reaction product (such as the formation of H18F) have been obtained both by methods (a) and (b): we have only applied the former in these experiments. A "sandwich" proportional counter was used for the detection of the radioactivity of "'F after chromatographic ~eparati0n.l~ The quantitative estimates of the relative rates of the various chemical reactions are based upon the measured percentage yield of CzH318Fand its variation with the changing relative concentrations of various components of the irradiated gas mixtures. A radioactive peak for the hot reaction product SF618Fis always observed, together with a minor peak for CH:8F from 18F reactions with CH3Br or CH31. Separate experiments involving twocomponent mixtures of SF6with CF31or CF3Br have es(13)F.S.Rowland and R.S.Iyer, Atomic Energy Commission Report
NO.UCI-1973-1.
tablished that the yield of CF318Ffrom either is negligibly ~4rnall.l~Consequently, our samples of four-component systems containing SF6, CF&, C2H2,and HI were analyzed with a 50-ft dibutylphthalate column operated a t 273 K. In these experiments the radioactivity peaks of SFtV and CF318Fwere not separated, but were assumed to be composed almost entirely of the former. Samples containing CH3Br were analyzed with a 125-ft dimethylsulfolane column operated at 297 K, which cleanly separates S F t T , CH3I8F,and C2H318F. No other radioactive peaks were observed from the radiogas chromatographic analysis of samples containing CF31, CF3Br, or CH3Br. The contents of the ampules containing CH31were analyzed by using a combination of a 50-ft silicone oil column at 341 K and a 75-ft dibutylphthalate column at 270 K in series. The macroscopic and radioactivity peaks for (SF6 + SF618F),CH318F,C2H3l8F,and C2H2were all eluted in that order from this combination within 80 min, and the dibutylphthalate column was then removed from the flow stream. The mass peak for CH31was subsequently eluted and measured with continued flow through the silicone oil column alone. Four other radioactive peaks were often, but not always, observed during this elution period involving flow solely through the silicone column. The fist, emerging before the CH31peak, varied erratically in yield between
