EUWAULI P.RACKAND ADONA. GORDUS
944
Vol. 65
EFFECT OF MODERATORS ON THE (n,?) ACTIVATED REACTIOS OF Brso WITH CH41 BY EDWARD P. RACI? AKD ADONA. GORDUS Department of Chemistry, The University of Michigan, A n n Arbor, Michigan Received October d f , 1960
In an excess of gaseous methane, 13.3 0.5y0of Br80produced by the Br79(nlr)BrBOprocesswas found to become stabilized as organic activity. Varying amounts of inert gases, Brz, C2H&r, CF4 and C2F6 were added to determine the manner in
which these additives moderate the reaction of Br80 with CH,. These data suggest that the reaction with methane occurs principally, if not completely, as a result of the recoil kinetic energy acquired by the Brso in the (n,r) activation process.
We have investigated the reaction of B$O, Introduction formed by (n,-y) activation, with gaseous methane Willard, et u Z . , ~ have shown that I1Z8produced by the 112i(n,y)138 process is able to displace hydrogen in an attempt to determine whether the reaction from gaseous methane to form CH31128. In a lat'cr occurs principally because of the positive charge study, Levey and Willard4 observed that molecules or because of the y-recoil kinetic energy acquired with ionization potentials lower than that of an by the BrSo. If the positive charge is responsible for the iodine atom are more effective than inert gases as moderators for the IlZ8 CH, reaction. They reactivity of the BrS0, then different extents of interpreted these data as indicating that the reaction with CH, would be expected for (n,-y) positive charge5 associated with a t least 50% of the activated BPomand Brs2 since 12 and 25%, reIlZ8atoms is an important factor in the reaction spectively, of these isotopes are positively ~ h a r g e d . ~ A single experimentg indicated that both Br80 and with methane. react with CH, to produce the same per cent. Tritons produced by He3(n,p)H3activation have BrSom been found6q7to displace hydrogen from gaseous of organic activity. In gaseous mixtures of Br2 or n-CaH7Br, the per cent. organic methane. This reaction has been shown to occur and CsH5Br,CBHB as a result of the 192,000 e.v. recoil kinetic energy activity was also found to be the same for the three acquired by the tritons in the (n,p) act'ivation isotopes.1° The lack of an isotope effect in these latter experiments could be due, however, to the process.8 fact that the ionization potential of the main Bromine-80 formed by the isomeric transition component of each system is less than that of Br. of B$Om was found to react with gaseous methane.g Experimental Due to internal conversion, this activation process Quartz bulblets, 4-5 ml. in size, were filled with additive, results in a highly charged Brso. It was suggesteds and 2 mm. of Brz, so that the total pressure was that, following the isomeric transition in BFom CH4, (generally) about 1atm. containing molecules, charge transfer occurred The samples were irradiated in the University of Michigan and, due to coulomb repulsion, kinetic energy was megawatt reactor for approximately 2 sec. at a thermal acquired by the BrS0. The data appeared to indi- neutron flux of 1.5 X 10%/cm.2-sec. and an accompanying flux of 8000 r./min. Because of the presence cate that the excess kinetic energy of the BrSowas ?-radiation of scavenger Br2, there occurred negligible radiation-inof greater importance than positive charge. duced effects. Bromine-80 produc,ed by the Br7g(n,y)Br80 The samples were broken in a separatory funnel beneath process was also found to react with gaseous a two-phase mixture of CHCls and 1 2 and aqueous NazSOa. The two portions were counted to determine the activity in methane,g >95% of the organic activity being each phase and, after correcting for B P mcontent and countdue to CH3BrS0. Of the order of 18% of the BrS0 ing coincidence and density effects, the per cent. of the total was found in organic combination. For this activity present in the organic phase was determined. species, Wexler determined5 that approximately Results 18% of the neutron activation processes resulted The per ccnt. of the Brso present as organic acin a positively charged Br80. tivity for the various systems are listed in Table I. (1) Presented in p a r t a t t h e Symposium on t h e Chemical Effects To interpret correctly the data where the bromine of Nuclear Transformations a t Prague, Oct., 1960, sponsored by t h e was present as C2H5Rr,it is necessary to realize International Atomic Energy Agency. This work was supported in that a small fraction of the BrBO will not split from part b y a grant from T h e University of hlichigan-RZemorial Phoenix the C2HSBrSomolecule and thus will be recorded Project and by t h e U. S. Atoniic Energy Commission, Division of Research, Contract Eo. AT(l1-1)-912. as organic activity. This failure to bond rupture, (2) Based on work performed in partial fulfillment of the requirewhich amounts to 0 . 3 3 % , 1 1 must be subtracted inents of t h e Ph.D. degree of E. P. R. Receipt of Prooter and Gamble from these data of Table I. In addition, the Br80 fellowship is gratefully acknowledged. could react with the molecular additives and con(3) J. F. Hornig, G. Levey and J. E. n'illard. J . Chem. Phys.. 20, 1356 (1952). tribute to the observed organic activity. Thus, (4) G. Levey and J. E. Willard, ibid., 26, 904 (1956). it is necessary to correct further these data of (5) S. Wexler and H. Davies, ibid., 20, 1688 (1952). Table I for the fractional reactivity with the (6) (a) A. A. Gordus, hI. C. Sauer, J r . , and J. E. Willard, J . Am. molecular additives. To do this, me subtracted Chem. Soc., 79, 3284 (1957); (b) M. Bmr. El Saged and R. Wolfgang, ibid., 79, 3286 (1957). from the observed values the product of the mole (7) (a) M. Amr. E l Sayed, 1'. J. Estrup and R. Wolfgang, J . I'hyb. fraction of the additive, multiplied by the extent Chem., 62, 1356 (1958); (b) P. J. Estrup and R. Wolfgang, J. Am.
+
C h e m . Sac., 82, 2661 (1960). (8) P. J. Estrup and R. Wolfgang, ibid., 82, 2665 (1960). (9) A. A. Cordus and J. E. Willard, i b i d . , 79, 4609 (1957).
(10) J. R.Evans, J. E. Quinlan, M. C . Sauor, .lr. and J. E. Willard, J . Phys. Chem., 62, 1351 (1958). (11) A. .4. Cordus, unpublished data.
June, 1961
PER CENT.
EFFECT O F n/lOL)ERilTORS ON ( n , y ) - A C T I V A T E D n E A C T I O N O F UR8' ON
TABLE I BrS0 STABILIZED I N ORGANIC COMBINATION VARIOUS GASEOUSMIXTURES~ Pressure
Additive
CHI, mm.
Mole fraction rtdditiveh 0
654 549 440 127 416 454 239 239 501 238 14s
8-15
IN
% BrsQa8 organicd
13.5(2) 13.6(4 ) 0.040(7) 12.6(1) 12.6(2) .131(2) 12.0(2) 12.0(2) .165( 5) 8.5(4) 9.2(4) .703( 123) 11.2(4) 12.5(3) Kc lll(12) 10.8(3) .22l( 17) 9.6(2) .476(52) 8.6(4) .478( 5 2 ) 12.1(5) 12.2(5) .202( 26) Ar 5.5(6) 6.7(4) .427(35) 4.8(6) .653(40) 11.5(3) 11.5(4) ti40 .059( 1) Kr 8.7(4) 8.9(3) 589 .141(2) 5.3(2) 5 . 6 ( 2 ) 283 .342( 3) 3.6(3) 3.6(1) 106 .462( 7) 11.8(3) 14.2(2) .029(2 ) SC' W3 11.3(4) 13.8(4) 662 .061(1) 9.8(3) 10.0(4) 568 .096(1) 6.9(3) 8.0(4) 335 .156(2) 5.1(5) 4.7(5) 150 .302( 4) 557 8.0(2) 8.4(3) .139(2) CF, 4.7(2) 5.0(3) 437 .356(3) 2.8(1) 3.4(1) .626(5 ) 262 49 8 6.3(3) C2F6 .223(2) 5.2(2) 6.6(2) .428(3) 368 714 13.1(7 ) 14.3(6) .0O3( 1) Brz 13.4(4) 658 .005(1) 11.3(3) 13.0(2) .034( 7 ) 656 10.4(1) 10.9(2) G29 .096( 7 ) 6.G(1) 7.2(1) 268 .195( 15) 3.2(1) 4.4(1) 112 .360(29) 12.8(4) 13.1(4) 699 .0l5( 4) C2H5Brf 12.4(4) 13.4(3) 657 .029(1) 11.1(3) 11.6(3) 467 .045(2) 10.5(2) 12.1(3) 323 .067(3) 9.0(7) 9.1(2) 213 .109(4) 7.2(1) 7.5(1) 3G0 .205(2) 5.8(1) 183 .356(5) 5.4(1) 5.5(1) 179 .454(5 ) 5 All samples, except where noted, contained 2 mm. BrZ. Calculated assuming additive pressures. Uncertainty in last figure or figures (given in parenthesis) is based on ostimates of determining individual pressures. Uncertainties (given in parenthesis) based on estimates of uncertainty in positioning 18.0 min. slope through decay data lor inorganic and organic fractions for each run. e Samples contained 2 mm. GEI,Br and 0.2 mm. Br2. 0.2 mm. Br2 scavenger present. He
c&
.
of reaction with essentially pure additive to pro-
duce organically bound Brso. These maximum extents of reaction to produce organic BrBOare, vorrecting for any failure to bond rupture: CF40.7, CzFe-3.0, and C2H6Br-2.2%. These corrected data of Table I are depicted graphically in Figs. 1 and 2 . The solid curves are calculated according to the Estrup-Wolfgang (E-W) theory,$ the method being described below. In order to avoid confusion, the uncertainties have been omitted from these figures. Extrapolating the data -for the systems where
MOLE FRACTION ADDITIVE.
Fig. 1.-Effect of inert-gas moderators on the reaction of gaseous CHI with Brgo activated by the (n,?) process. Moderators: helium, V; neon, -e-; argon, D; krypton, A ; xenon, e.
12
3 8 8
3 8
6
&
* 4 2
0 0
0.2
0.4 0.6 0.0 MOLE FRACTION ADDITIVE.
1.0
Fig. 2.-Effect of molecular moderators on the reaction of gaseius CHI with BIgO activated by the (n,?) process. Moderators: CF,, 0 ;CzFs, V ; Brz, A; CzHaBr, 0.
the mole fraction of the additive was less than 0.1, it was found that a t zero mole fraction additive 13.3 0.5% of the Bflo reacts with CH4to become stabilized in organic combination. The only other determination of this quantity was made by Gordus and Willardg and indicated 18% BrBOas organic. The reason for this difference is probably due to the presence of radiation-induced reactions in their experiments. The y-dose received by their samples mas almost 100 times that received by our samples. Discussion It should be possible to interpret qualitatively the effects of the different moderators. There are various ways in which the BrS0 CH4 reaction can be moderated, depending upon the nature of the Brso and the moderator: (1) removal of the BrBOkinetic-energy; (2) neuiralisation of BrS0
*
+
946
EDWARD P. RACKAND ADONA. GORDUS
ions; (3) inelastic collisions resulting in the quenching of excited BrEOions or atoms; (4) reaction of Br80 with the additive, the Br80 becoming stabilized in chemical combination. Excess kinetic energy may be removed as a result of collisions of Brm with other atoms or molecules. Charge transfer between Brf and an additive is highly probable if the ionization potential of the additive is less than that of Br (11.84 e.v.). If the ionization potential of the additive is greater than that of Br, charge transfer would be of importance only if the relative velocities of the Br+ ion and the additive were large. l 2 A maximum Brsa velocity of 2.5 X lo6 cm./sec. results from the (n,?) activation. For a relative velocity of this magnitude, Gurnee and Magee l2 calculate that charge-transfer processes possessing an energy defect greater than about 0.15-0.20 e.v. have only a small probability of occurrence. Inert gases cannot moderate via process 4. In addition, inert gases are found to be inefficient in quenching excited species13 and, because of their high ionization potentials, are very inefficient in undergoing charge-transfer with Br + ions. Therefore, if moderatioii of the reaction by the inert gases occurs, it must be due mainly to process 1. We may next examine the experimental data for the samples containing inert gases. Referring to Fig. 1 and ignoring the solid curves, it is seen that each inert gas is capable of suppressing the extent of formation of organic Br80. If the inert gases moderate the reaction principally oia process 1, then the relative effectiveness of the additives would depend on the size of the inert-gas atoms and on the fractional energy transfer per Br*-inert gas collision. Thus, a plot such as Fig. 1 should indicate that the moderating efficiencies increase in the order: He, Ne, Ar, Kr-Xe. As seen, the data of Fig. 1 are in accord with that expected for kinetic energy moderation. We may also attempt to determine qualitatively whether the formation of organic Brsois due totally to processes requiring excess kinetic energy. This may be accomplished by extrapolating the data of Fig. 1 to zero mole fraction CH4. If, for example, the data extrapolated to 6%, such data would suggest that 6y0 of the organic Brso is formed via thermal processes and 13.3 - 6 = 7.3% via excess kinetic-energy processes. The data for Ar, K r and Xe, however, extrapolate to about 0 f 2%, and there is no reason to expect the helium and neon data to extrapolate to a value which differs from that of the other inert-gases. Therefore, it would appear that the organic Brso is formed principally cia reactions requiring excess kinetic energy. The molecular additives are capable of moderating according to all four processes. However, as with the inert-gases, we may eliminate certain processes from consideration. Molecular Brg and C2H5Br should be able to moderate efficiently via all four processes. For CF4we would expect charge-transfer to be a very inefficient process since the ionization potential (12) E F. Gurnee and J. L Magee, J . Chem Phvs , 26, 1237 (1057). (13) K. J Laidler, ”The Chemical Kinetics of Excited States,” Oxford L n i x r r b i t ~Pre
