~ ~ , ~ ~ ~ A ~Hot t i Iz8I v aReactions ~ed in Gaseous Methane an he Energy Degradation Factor' atthias Ysong, Y. 6. Pao,2 and E. P. Rack* DqmrtnwsC os Chemistry, University of Nebrash, Lincoln, Nebraska 68608 (Beceiued January $4, I H 2 ) Publicition costs asd8ted by The U . S. Atomic Eneryg Commission
It was found in a systematic study of lZ81 reactions activated by radiative neutron capture in various gaseous haiornethanes that the formation of 128I-labeled organic products proceeds entirely by hot (requiring excess kinetic energy to occur) reactions. This is unlike the reactions of lZsIwith CBd where the organic product CX31.i*l was formed not only by hot lZRI atoms but by thermal ion-molecule reactions imolving 1' in the states. I n the .various halomethane systems only two lz*I-labeled orga,nie products were 8Poo,Tr,and found, those resulting from halogen and hydrogen substitution. The limiting lZs1organic yields in gaseous GB3E', C€I&I, CWsBr, and CH31were 11.2, 4.2, 0.6'7, and 0.20%, respectively. The kinetic energy spectra for (n,y)-activated lZRX atoms or ions were calculated and the results showed that an itppreciable fraction of the lzsE species are born with low kinetic energies, in or near the reactive zone. The only physical or chemical parameter that explained the trend in organic yields was the energy degradation factor of the halomethane syF;tein, correhting well with the kinetic energy spectrum of MI.,
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I n the recent literature, there have been various systematic studies involving gas-phase fluorine and chlorine hot atoms (activated by nuclear reactions) with methane and various halomethanes, suggesting tbe importance of ~,iGeric,~ translational inertial,4a and bond energy effects.4b Previously there have been no such systemadtic studies for gas-phase iodine hot, atom reactions. Unlike chlorine and fluorine, which elieved to react as atoms, the reactions of iodine5 omplex, since nodine reacts not only as atoms or ions possessing excess kinetic energy, but also as atoms or ioris tylhich are ~ , r ~ t ~ s ~ a tthermalized ~ o ~ ~ l l yand elecironically excjted, especially as I+ ('D2) and 1" inns in "0, 3PI1and " y p states. By the use of rare-gas moderators, it is possible to distinguish the osga,nic yields or product yields resulting from '%o 1 atom ' processes (requiring excess kinetic enrergy to occur) from those yields which result from ~ ~ ion-molecule e r reactions. ~ ~I n this ~ paper ~ we ~ report how vsrious chemical and physical factors influence the psogrwa of (n, 7)-activated hot reactions of 12cIwith methane and halomethanes in the gas phase. ecaum l**Ij , born with B spectrum of kinetic energies ranging from 0 eV t o 8, maximum of 194 eV, quite low 8s compared to other nuclear activations, it was of interest to calculste the recoil energy spectrum of the (n,+y)-aetivated -7*X atoms. A substantial number of calculations i ~ p lhe idea ~ of a ~nuclear~ cascade ~ have been made rn the literature,6--lo all of which have ibwn performed uaing a Monte Carlo technique for various nuclcar activations, but; none for the (n,r) process. In our calculations of I 2 8 1 kinetic energy random-walk spectra w e used the three-di~ensiona~
~equations originally derived by Bsiung for the 35Cl(n,r)36C1 recoil energy spectra. Experimental Section A description of our sample-making techniques, irradiation procedures, and extraction techniques can ~ c in be found eIsewhere.5'12 The lZsI~ r g a ~yields CHaCl and CHaBr systems were determined from radio assay data employing a RIDL 400 channel pulse height analyzer with two 2 in. X 2 in. Nal(T1) crystals. Iodine-128-labeled products were separated and identiRed by a radio-gas chromatograph employing a Wolf window flow radiation detector. All irradiations were performed at the (1) This research was supported through an Atomic Energy Commission Contract No. AT(11-1)-1617. Thin is AEC Doournent No. (COO-1617-30). (2) Yen Ching Pao, Department of Engineering Mechanics and Mechanical Engineering, University of Nebraska, Lincoln, Nebr. (3) b. Bpicer and R. Wolfgang, J . Amar. G h m . Sac., 90, 2426 (1068). (4) (a) W. Colebourne, J. F. J. Todd, and R. ~ ~ ~ ~ J .g P a hw~ . g , Chem., 71, 2875 (1967); (b) T. Srnail, R. S. Pyzr, and F. 8. Rowland, ibid., T 5 , 1324 (1971). (5) (a) E. P. Rack and A, A. Gordus, J. C h m . Phys., 34, 1855 (1981); (b) E. P. Rack and A. A. Gordus, J Phys. Chern., 65, 944 (1961), (6) M. L. Goldberger, Phys. Rev.,'74, 1268 (1948). (7) 6. Bernardini, E. T.Booth, and 6 . J. Lindenbanrn, PhyY. Res., 88, 1017 (1952). (8) E.MeManus, W. T.Sharp, and W.Gellrnen, P h y a . Rev., 9.3, 924 ~ t ~ ~ ~ (1954) (9) J. W. Meadows, ibid., 98, 744 (1955). (10) N. Metropolis, R. Rivins, M. Storm, A. Turkevich, J. M . hIilSer, arid C . Frielander, ibid., 110, 185 (1958). (11) Chi-Hua Hsiung, Hsien-Chih Haiung, and A . A, Cordus, J. Chem. Phys., 34, 535 (1961). (12) J;. B. Nicholas and E. P. Rack, ibid.,48, 4085 (1968). I
The Journal of'Phgsica1 Chemistvy, Val. ?6,N o . 19,1978
M. YOONG,Y. C. PAO,AND E. P. RACK
2686 Hospital Triga nuclear reactor. The thermal neutron Aux was 1.1 X neutrons cm+' sec-I. The accompanying radiation flux was approximately 3 X 1017 eV g-l rnin-l
Nature of b2enctcoe lZsIin Halomethane Xystems. I n previous gas-phase studies5 of lZsIreactions activated by radiative neutron capture in CH4, it was found that IzsI reads not only by virtue of kinetic energy activation but by xhermal processes. Of the 54.4% organic lZ8%(:is CHg1181)about 18.4% forms as a result of hot l2&1reactions, 11% as a result of excited iodine atoms or I+ ions in the 3P2, 3P1,and/or 3P0states, and 25y0 as a reguli of reactions of If(lDD2)ions. From these experiments, 110 suggestion could be made as to whether the hot reactions were with lZsI atoms or lz*I+
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MOLE FRACTION ADDITIVE
Figure 1. Per cent hot organic yield us. t h e mole fraction additives for various rare-gas additives in the reactions of (n, ?)-activated 1281with CHaF: (0) helium; ( I ) argon; ( V ) neon; (0)krypton; ( 0 )xenon.
limiting hot organic yield at 0 mole fraction additives is 11.2 A 1.0%. The data when extrapolated t o 1 mol yo additives appear to give a value of 0% hot organic iOI& yield. Unlike the reactions of (n,?)-activated lZsI Three kinds of rcxctive lZsI species are possible. with CH4, the reactions of (n,y)-activated lZsIwith ( I ) A hot nc>uti.alatom receiving its kinetic energy CH3F occurred completely by hot processes. S o therby lrirtue of the radiative neutron capture process. mal contributions to the organic yields were observed. (2) A hot positive ion receiving its kinetic energy In the CIL the reactions of I+ in t h e lD2 by virtue of the radiative neutron capture process and state were shown by the consistency of xenon additives its positive charge by internal conversion of low nuclear being able to reduce thc formation of C1H31281to a energy states leading to Auger charging (occurring greater extent than either Xe, Ar, or EZr additives. during the "cooling down" of the hot IZsI). I n the CH3F systems the xenon additives like Ke, Ar, ( 3 ) k hot positive ion receiving its charge by into 0% inand Kr reduced the formation of CH3IZEI ternd conversion and Auger charging after the hot dicating the absence of the reactions of translationally 12*1 is chemically stabilized and its kinetic energy by thermalized and electronically excited I+ ions in ID2 thc coulombic repulsion between the lZsIion and the state and I-'-ions in 3P0,3P,, and VZstates. A similar molecular fragment charged by intramolecular eleceffect was observed for (n, y)-activated IzgIwith CNsC1. tron transfer, ' The limiting hot organic yield for @%GI system is It was originally suggested by Spicer and Gordus14 4.2 f 0.5%. The limiting hot organic yields for that the ET induced yields are lower than (n,?)-induced CH3Br and CHd systems are 0.67 k 0.1 and 0.2 f yields because of the lower kinetic energy acquired 0.l%, respectively. Because these yields for CN3Br by the coulombic repukion in isomeric transition. In and GI-IJ systems are quite low, no attempt was made a study invol ving 130T'5 activated by radiative neutron t o study the effects of rare-gas moderators on these capture [12~I(n,y>139'l;"130T] and isomeric transition reactions. Since no thermal reac tions were observed [1301"(IT)1301]1 the IT hot organic yield (10.0 f 1.0%) for the CH3F and CH&l systems, we would not expect \vas much lo~verthan the (n,r)-induced organic yield any for thc CH& and CHJ systems. I n the halo(16.6 =t 2.0%) These results suggested that the methane systems, the lZsIatom or ion reacts completeiy (n, yj-activated laaIt 1301mhot reactions occur mainly by virtue of its kinetic activations to form the organic as the result of kinetic energy imparted to the recoil products. iodine atom or ions following y-ray cascade while the Calculations of Recoil Energy Spectra. Unlike other isomeric transition induced hot reactions occurred by nuclear transformations such as the (y,n) and (n,p) virtue of kinetic energy following coulombic repuIsions. processes where all the hot) atoms are born with a very Since the hot 3 organic yield"" (18.4%) is much high-kinetic energy (-lo5 eV), halogen atoms by virtue higher than the 13*1 IT-induced yield and we would of the radiative neutron capture process acquire a not expect a mass isotopc cffect between lZsIand IaoI, spectrum of energies ranging from (9 to -1 Oz eV. Since it is reasonable to assume that the hot l2*1atoms or there are no reported calculations of a kinetic energy ions acquire their kinetic energy mainly by the radia(13) A. R. Kazanjian and W. F. Libby, J . Chem. Phys., 42, 2778 tive neutron capture process. (1965). W-e employed rare-gas moderators to separate the (14) L. D. Spicer and A. A. Gordus, "Symposium on Chemical organic yields resulting from hot-atom processes from Effects Associated with Nuclear Reactions and Radioactive Transformations," Vol. 1, International Atomic Energy Agency, Vienna, those resultiiyg from thermal ion-molecule reactions. 1965, p 185. Figure 1 sbo.ils the effect of rare-gas additives on the (15) 31. Yoong, N. J. Nicholas, and E P. Rack, Radiochim. Acta, reactions of (n,:i)-aatnvated lZsT with CH3F. The in press.
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The Journal of Phvakal Chemistry, Vol. Y6,No. 19, 1978
spectrum for we calculated the lZsIspectrum beOYLUSL of Ihc ampoi taiice le may have to the underof Ibe I P e I hot resotion,r. et,ic energy spectra utilized d general solu.lionls of Lhc 01% r‘~~nctiam for l h three-dimen ~ reported hy ITsi~rigand Gordus. ‘The general course oC the calculations V X J S &30 Y~rt?rJ D I V v l l ~ K l 1 4 ) . pEVioUf3 ~ ( ~ m ~ ~ l a t lOof K this ls t y p ~and viill i i c + l I P discussed except where it ctiflerb from them, We eniployed an PBM-360 Model 651R ~onqauter. QUP pun po;r was t o improve the statiistics thatt caould be ~ ~ k ~ t t i i i ile~d yextending our e:alciilatiorru to ad1 pos~iblrdloxwd ~ o ~ ~ ~ nof ~, y - ,~ l ~t ;enwgies ~~r o ~ ~ s and not hmitinng our ~ ~ ~ ct o only ~ l a few ~ possibie ~ ~ o ~ eirerg1es as PIsiung and Gorcllxb Ihc three, and four randornrated ~~~~)~~ possible combhacornbinalions, only those alI used in our caid:r,bions ns of 7-ray cnergies me those whose sums are equal t o the skoeiated with thc 3271(n1y)3767
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activateti a”r,orxi ~h eq portant t o note that i hc random-walk equations reFigure 2. Probability distribution of ?-ray energies (in units qui?e thc use of on o luni vect ors. I n Clre particular of MeV-’) vs. t h e fraction of the maximum resultant 7-ray case of y-ray cniis~ion Lhe energy of a y-ray differs energy for two- (A), three- (a),and four-(E) slep processes. from Liae snomentum by only a rrluitiplicative constant(khe !,dority o l lig ‘b’hus the 7-ray energies were uecd in OUT calcmi fn? In ordcr to cornpwte a reing each y-ray energy by a degenerate factor equal t o coil energy s ~ ;or IzsI ~ atoms,~ both Ihe ~ high- r its intensity. ~ ~ Thus the random generation of possible and iow-energies 3E ihr. emitted y-rays c,f In, *{)-acti- allowed combinations of y-rays would not be affected vwtecl 1P8Imusi Iw k r o ~ ~ nThe . measuremerits of the significantly by the exclusion of Lhe t n o lines of low ys from ~ ~ ~ y ~ rs81 - reported B ~ ~by~Archer, ~ ~ a intensities. t ~ ~ et al.,18 ~ e r t uscd , since they reported b o ~ bthe high (n,y)-Activated lZ8I Recoil Spectrum. Our results in ‘ TIott’ever, the details of lhe Figure 2 show the probability distribution of y-ray enay spectrum are not well estabergies (in units of MeV-l) plotted against R/R”,,, (the ount for the existence of two fraction of the maximum resultant (y-ray energies) for EIUPJ (?-rays) a t 1 4 7 04. ~aild150.18 IteV \~]nich.W ~ R the two -,three-, and four-unequal-step processes. While reiohred by J o ~ e s ef , a/.x7I s These lines arc of ~ e l a - Draperlg reported that the absorption of a neutron by tiveiy low ~ntonsitues (3.4 -+ 0.7 and 3.6 f 0.2, sean 1271 nucleus results promptly in a cascade of approxqmtively) w cornpared to line 133.31 iceV (cornimately four y-rays, the decay scheme of 1271t n in~ ~ p o i ~ l ilioi gline i32’ keV of Archer) whose inlertslty dicates that the recoil energy spectrum mvolve t w o y :s 42.0. Although these lines are important in deter. steps. 2o We have translated both the probability disriiinurg the exteni, of internal conversion leading t o tribution of the two-step process arid the sum probhriigca charging, ionization, mid excitation, the: exciuability distribution of the two-, three-, and four-step idon of these f w o 1itn.s does not significantly affeel, the processes in terms of recoil energy in eV. lrigure 5 computation o i t h er:oil energy spectrum since shows the recoil energy spectra for the 12“X atoms BCare of reiatiueiJ lo\ itensities. We are only inter 111 the distribuiioxi of the recoil energies imparted to (16) N. P. Archer, L.B. Hughes, T.J. Konott, and W~ V. Prcstwich, (n,-y)-actrvated izsli atoms 9s a result of y-ray cascades. Nuel. Phys., 83, 241 (1966). ’The solutions i a the probability function for the tt~rcc- (17) C. H.TV. Jones, J . Phys. Chem., 74, 3345 (1970). (18) R. G. Korteling, J. M. U’huria, and C, H. W, Jones, L\rucZ. dirnrmsimal ranclorn-n ail< processcs give the rlxstriPhys., A138, No. 2 , 392 (1969). bution of the recoil energy 8s a result of y-ray cascade. (19) J. E. Draper, Phys. Rev., 114, 268 (1959). ’They Lake into accoim L fbe relative jntensities (per (20) C. M. Lederer, J. M. Hollander, and I. Perlman, “Table of LOU captures) of t h e measured y-ray energies by weighIsotopes,” Wiley, New York, N. P.,1967, The Journal of Physical Chemistry, Vol. 7 6 , N o . 19, 1578
M. YOONG, Y. 6. PAB,AND E. P. RACK
2688
'7r-
Kinetic Energy Deyradation Factor. Presented in Table I are the various total hot organic yields for the reactions of (n, y)-activated IzSI with CH4, Gl&F, CH&I, CHa13r, and 6H8Lz3 The total bot organic yields are the per cent of the hot lzaIatoms stabilized I I as hot organic products. In all systems studied, the only hot organic products found by radio-gss ehromatography were tho halogen and h y ~ ~ r o gsubstitution e~~ products. Because of rapid exchange between 141 and Iz we could not determine the hot yid& for the in&vidual abstraction products. Oiir results in Table I show w, progressive decrease in Erecoil hot organic yields for CHBF > @I-IIC1 > CRaBr > Figure 3. Probability distribution of y-ray energies (in units of CHJ. By using the metpliodsuggested by Bpicer and M e V 1 ) us. t h e recoil energies (in units of eV of t h e la1 atoms: W ~ l f g a n g ,we ~ , ~calculated the areas available for rem( . ) indicates spectrum obtained using t h e two-step process; tiona and the resulting expected yields i f steric factor (----) indicates spectrum calculated summing t h e two-, three-, were opwating for substitution reactions. Our results and four-step processes. as seen in Table I indicate that there is no systematic or simple correlation between the hot "*I organic yieldfi and steric factors, although it has been reported that tivated by radiative ncutron capture. The dotted line steric and translational inertial factor infiineneed hot indicates the recoil energy spectrum obtained using the (y,p)-activated 39Clreactions wirh CH4 molecules. two-step process and the dash line indicates the recoil A bond energy effect was reported by Rowland, energy spectrum calculated as the sum of the two-, et aZ.,4bfor FF' substitution for X r e a ~ t in ~ ohalornet)h~ three-, and four-step processes. It is of interest to anes highly moderated by the major component SFe observe thc general features of these spectra. I n the in the gaseous mixture. The ~ n ~ ~ reactions ~ c ~ ~ ~ Lwo-step process, the probability distribution of y-ray occur in the near thermal range, Jn OXIF jn,yl-acti2nergies increases from zero a t low kinetic energies and vated 12*1 reaction with halomethanes, the major comrises as a logarithmic function. The spectrum obtained ponent was the halomcthanes, dlowkig t h hot le8I 6s a sum of the two-, three-, and four-step processes atoms to be moderated internadly by collisions mainly behaved in the same manner as that of the two-step with halomethane molecules. We observed no thermal process a t low-recoil energy range but reached a maxcontribution to the organic yields. The organic yields imum at 152 eV. Aside from this maximum, both spectra show that a substantial fraction of the lZsI were the result of hot processes. While it i s true that the hot, IZ8T atorris or ion&by virtue uf their kinetic enatoms or ions are born in or near the reaotive zone ergy can form organic as well as ~norganieproducts Ez-E121 (roughly estimated to be 80-10 eV for iodine (remltiag from substitution or a b s t ~ a c t ~ oreactions, n reactions). Those hot atoms born with recoil energies respectively) , the lypr of product8 hhould depend on in the range &-El can either react on the first collision the mode of reaction (or collision). Jf ~ u ~ s t ~and t ~ t ~ o ~ or on subsequent coll ialons while possessing kinetic enabstractiorr occur by the same type of collision, then ergy En the range Er-E, forming stable organic prodbond energy may be an iniportnnt parameter for the ucts, or be reduced in kinetic energy by cooling collisions formation ol' inorganic products, which would be in with the internal moderator. I n the (n, 7)-activated competition Kith the formation of organic products. system, moderation of the hot lZsI atoms to energies However, W ~ ~ f g a nsuggested g ~ ~ that the formation of below the reactive zone can be affected in one or two ways: either by rare-gas additives introduced into the (21) R . Wolfgang, .I. @hem. Phys., 39, 2983 (1963). eystern or by the efficiency of the target organic mole(22) Employing the Wolfgang-Estrup kinetic theory, we calculated cules as internal moderators. the 1 (integral probabilitj) and the Ti (correction term for 1) values for the reactions of (n,y)-activated 1 2 8 1 with CHI and CKh. If the The recoil energies imparted t o hot lzSIatoms as a kinetic theory was applicable t o these systems, the K values should result of yray cascade (194 eV) is low compared with be equa1 to ( 1 / 2 ) 1 ' The correlations were extremely poor supporting &heidea that the (n,r)-activations does not oroduce hot atoms of (n,p)-activated tritiuin atoms (in the order of lo5 eV). sufliciunt energies t o obey assumption Is of the Wo1fg;ang-Estrup I n the (n,p)-activated system it was possible to obtain kinetic theory. by successive collisions of hot tritium atoms, a statis(23) T w o different kinds of experiments were performed: (a) those F, C1, and Br) systems containing the 2ialometthanes CIhX (X t i d l y we1l-defined distribution of energies in the chemancl pHax (the g hot organic yields reported in Table I are ically reactive zone l&-131, However in the (n,r)found hv extrap the data to zero mole fraction CHJI additives . the 1.1% failure to bond riipture of CHYI activated lZ*I system, we do not obtain a statistically induced reactions) ; and (b) those systems ronvycL'c iy radiation corrc C11& CHJ, XZ, and rare gas additrves Except for the CHII well-defined distributkon of energies in the &--E1 zone, , all wstems cont*jl?ed 700 Torr total pressure and 2 mm of assumption Ia the Wolfgang-Estrup kind 0.1 mm of LZ as scavenge1. netic theory.21,%? (24) R . Wolfgang, Progr. Reuet. Kinet., 3 , 99 (1965).
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