RADIOLYSIS OF
LIQUID2,2,4-TRIMETHYLPENTANE
2381
Radiolysis of Liquid 2,2,4-Trimethylpentane.
Effect of Charge
Scavengers on Product Formation' by Krishan M. B a n d and Stefan J. Rzad* Radiation Research Laboratories and Center for Special Studies, Mellon Institute of Science, Carnegie-Mellon Universitu, Pittsburgh, Pennsylvania 16213 (Received January 2'7, 1972) Publication costs assisted by the U.S. Atomic Energy Commission
Electron scavengers are found to depress the hydrogen yield and have only a slight effect on the methane yield. The effects of these electron scavengers are quantitatively described by the empirical equations proposed earlier for cyclohexane. It is concluded that in 2,2,4-trimethylpentane only 30 and 5% of the ionic neutralization processes produce hydrogen and methane, respectively. Allhough some hydrogen atoms originate in ion recombination processes, this is not the case for methyl radicals, which originate exclusively from excited states produced directly by radiation. Methyl radicals are found to be scavenged by ethylene, and a value of 33 is obtained for the ratio of the rate conshant for the addition of methyl radicals to ethylene to that for abstraction of hydrogen from the solvent. From the effect of IYzOand NDs on the hydrogen yield in 2,2,4-trimethylpentane,values of C L N , ~= 6 M - 1 and O ~ N D=~ 0.15 M'-l are obtained.
Introduction
Irradiations and experimental procedure have been described previously.8
Recent interest in the radiation chemistry of branched hydrocarbons showed that, as in c y c l o h e ~ a n e , ~ - ~Results and Discussion charge scavenging has an effect on the products gen( A ) E f e c t of Charge Scavengers o n the Hydrogen erated by r a d i ~ l y s i s . ~ -However, ~ no systematic study Yield. (a) Electron Scavengers. The hydrogen yield has been made to account for the quantitative confrom the y-radiolysis of pure 2,2,4-trimethylpentane tribution of ionic processes in the radiation chemistry was found to be 2.44 and independent of dose over the of 2,2,4-trimethylpentane. Recently the kinetics of narrow range of 1.5 X 1018-6 X 1 O I 8 eV/cm3." As the charge scavenging processes have been studied in previously noted,I2 the hydrogen yield decreases a t 2,2,4-trimethylpentane8 and it has been shown that higher doses. An almost identical value of 2.45 has as in cyclohexane2~9the yield of a product P produced been reported for low doses of ?-irradiation of 2,2,4with unit efficiency in the charge scavenging process trimethy1pentane.l2 All the other values quoted are can be very well described by lower: G(H2) = 2.26?l3-lj and G(H2) = 2.1.16 These
where Gfi and G,i are the yields of free and geminate ions, respectively, S is the concentration of the scavenger, and a is a parameter which represents the reactivity of the solute toward the charged species relative to the recombination process. lo Parameters pertinent to eq I have been obtained for several scavengers in 2,2,4-trimethylpentane.8 With this information at hand, the present work has been undertaken in order to study the importance of the contribution of ionic processes to the formation of hydrogen and methane in the ?-radiolysis of 2,2,4-trimethylpentane. Experimental Section
Most materials and their purification have been described previously.8 Phillips research grade benzene and absolute ethanol from Rossville Gold Shield were used without further purification and were added with a syringe before outgassing the 2,2,4-trimethylpentane.
(1) Supported in part by the U. S. Atomic Energy Commission. (2) J. M. Warman, K.-D. Asmus, and R . H. Schuler, Advan. Chem. Ser., No. 82, 25 (1968). (3) K.-D. Asmus, J. 11.Warman, and R . H. Schuler, J . Phys. Chem., 74, 246 (1970). (4) K . M. Bansal and R. H. Schuler, ibid., 74, 3924 (1970). ( 5 ) M. Muratbekov, S. V. Zatonskii, and V. V. Saraeva, Khim. V y s . Energ., 5 , 134 (1971). (6) S. Iida, R . Yugeta, and S.Sato, Bull. Chem. SOC.Jup., 43, 2758 (1970). (7) K . Horacek and G. R. Freeman, J . Chem. Phus., 53,4486 (1970). ( 8 ) S. J. Rzad and K. M. Bansal, J . Phys. Chem., 76, 2374 (1972). (9) J. AI. Warman, K.-D. Asmus, and R . H Schuler, ibid., 73, 931 (1969). (10) S. J. Read, P. P. Infelta, J. M. Warman, and R . H. Schuler, J . Chem. Phys., 5 2 , 3971 (1970). (11) X. M. Bansal and S. J. Read, J . Phvs. Chem., 74, 3496 (1971). (12) R. H. Schuler and R . R . Kuntz, ibid.,67, 1004 (1963). (13) J . A. Knight, R . L. McDaniel, R . C. Palmer, and F. Sicilio, ibid., 67, 2273 (1963). (14) T. Kudo and S. Shida, ibid., 67, 2871 (1963). (15) T. Kudo, ibid.,71, 3681 (1967). (16) V. I. Pichuzhkin, V. V. Saraeva, and N. A. Bakh, Khim. V y s . Energ., 4, 317 (1970).
The Journal of Physical Chemistry, Vol. '76,X o . 17,1972
KRISHANM. BANSAL AND STEFAN J. RZAD
2382
Table I: Effect of Electron Scavengers on the Yields of Hydrogen and Methane from the ?-Radiolysis of 2,2,4-Trimethylpentane
...
. I .
CHaBr
2.7 8.4 36.8 137.0 139.0 187.0 317.0 127.0 304.0 136.0 250.0 1.2 3.6 20.6 43.4 83.3 100.0 312.0 328.0 680.0
2.44 2.07 1.92 1.69 1.47 1.47 1.42 1.35 1.34 1.24 1.73 1.63 2.24 2.18 2.00 1.89 1.79 1.76 1.56 1.55 1.44
2.44 2.09 1.97 1.77 1.50 1.48 1.53 1.32 1.46 1.26 1.65 1.44 2.39 2.25 2.04 1.92 1.79 1.73 1.45 1.49 1.32
... 2.33 2.93 3.48 4.33 4.26 4.46 4.45
1.21 1.16b 1.29 1.08 1.24 1.17 1.22 0.98 1.01 0.97 1.16 1.12 1.25 1.22 1.19 1.21 1.17 1.17 1.10 1.04 1.04
1.21 1.15 1.12 1.08 1.05 1.05 1.04 1.03 1.03 1.00 1.09 1.08 1.18 1.17 1.14 1.12 1.11 1.10 1.07 1.07 1.05
a Calculated using eq 11and the following parameters: e = 0.3, Gfi = 0.3, G,i = 4.54, CtCH8Br = 24.7 M-l, O~SF@ = 65 M-l, ~ U C ~ H=~ 5 M-1, O ~ N ~=O 6 M-1. b For methyl bromide solutions this yield was obtained by subtracting from the observed yield of methane the contribution of methane originating in electron scavenging w calculated by eq I with the pertinent parameters as given in footnote a. Calculated using eq 11, E = 0.05, and the other parameters as given in footnote a.
lower values were obtained at doses much greater ( 2lozo eV/cm3) than in the present work and, since the G(H2) is somewhat dose dependent, one would expect a lower yield. I n order to understand the mode of formation of hydrogen, the effect of various electron scavengers was investigated and the results are presented in Table I. One can see that the hydrogen decreases with increasing scavenger concentration. Since the four electron scavengers studied here neither react with hydrogen atoms nor undergo energy transfer other than electron c a p t ~ r e , ~ ,one ~ , ~can ' safely assume that some of the hydrogen has ionic precursors. I n 2,2,4-trimethylpentane such a decrease of the hydrogen yield with increasing electron scavenger concentration (SFs and NzO) has been already observed by Iida, et aL6 As previously shownJ3the hydrogen production by ionic processes can be represented by the following scheme
+ e- +e(H, Hz) e- + S SR H + + S- +e'(H, Hz) RH+
(1)
(2) (3)
where e and E' are efficiencies for production of hydrogen from reactions 1 and 3 because the hydrogen atoms produced finally give hydrogen by abstracting a hydrogen atom from the solvent. I n the case of cyclohexane, The Journal of Physical Chemistry, Vol. 76, No. 17, 1972
B ~
it has been shown that the efficiency of hydrogen production from reaction 3, i.e., e', is zero or nearly soS3 Since hydrogen production from 2,2,4-trimethylpentane is a much less efficient process than from cyclohexane (G(Hz) = 2.44 and 5.67, respectively), it should be expected that e ' would be equal to zero in 2,2,4-trimethylpentane. Therefore, one can calculate the hydrogen yield in the presence of various charge scavengers according to the following equation3
where G(H2)ois the yield of hydrogen in the absence of the scavenger (=2.44). Since all the parameters are known from electron scavenging studies8 (Gfi = 0.3, G,i = 4.54, a a H a ~= r 24.7 M-l), one can fit eq I1 t o the experimental yields of hydrogen observed in the presence of CH3Br in order to obtain E. The best fit was obtained for E = 0.3 (Table I). Using this value and the parameters pertinent to sulfur hexafluoride and CzHbBr as obtained previouslys SF^ = 65 M-', ~ c ~ H= ~ 5 BM~- l ) , one calculates the values given in the fourth column of Table I. The overall agreement is rather good. I n the case of NzO, however, one does not know the value of the parameter a, since the yield (17) The treatment of the hydrogen yields given in this work is essentially the one proposed for cyclohexane in ref 3.
RADIOLYSIS O F LIQUID2,2,4-TRIMETHYLPENTANE of nitrogen does not represent directly the yield of scavenged electrons. This is due to complications arising from secondary reactions of the anions produced in the electron scavenging process.2~8~1s One can, however, use eq I1 to obtain an estimate of this value, since one knows all the other parameters necessary for the calculation. The best fit (fourth column of Table I) was obtained for an ( Y N ~ O= 6 M-l. Again the overall description by eq I1 of the hydrogen yield in the presence of electron scavengers is very good. A value of e = 0.3 implies that a hydrogen yield of 1.45 G units (GiOns(e))originates from ionic processes and, hence, a G(Hz) = 0.99 comes from nonionic processes. It should be pointed out here that Horacek and Freeman7 found that 36% of ionic recombination processes lead to hydrogen formation in the y-radiolysis of neopentane-NzO solutions. Table 11: Effect of Electron Scavengers on the Yield of Ethyl Iodide from Ethylene-18112 Solutionsa S
[SI, mM
...
...
SFs
12 35 89 160 105 335
NzO
G(CzHslalI)ob,d
G(C2Hda1I),&hdb
0.81O 0.67 0.64 0.54 0.55 0.63 0.51
0.81 0.63 0.59 0.55 0.53 0.64 0.59
a [C2Ha] = 0.11 M , 13112 = 5.2 x 10-4 M in 2,2,4-trimethylpentane. b Calculated using eq I11 and the following parameters: Gri = 0.3, Ggi = 4.54, f = 0.09, (YSF~= 65 M-l, ( Y N ~ O = 6 M-l, and (kabs/ksdd)[RH] = 0.026 M . E Value interpolated from the data in Figure 1 of ref 11.
It is interesting to probe further the hydrogen originating from ionic processes in order to obtain information as to what extent these processes produce thermal hydrogen atoms and molecular hydrogen. It has been shown recently from the study of 2,2,4-trimethylpentane-1311z-ethylene solutions" that the yield of thermal hydrogen atoms is G(H)o = 1.0. Ethylene, which does not interfere with ionic processes in 2,2,4trimethylpentane,l' competes with the solvent for the hydrogen atoms to produce ethyl radicals, which in turn are scavenged by radioiodine to give ethyl iodide. A measure of the ethyl iodide yield gives a measure of the yield of scavenged hydrogen atoms. Extrapolation to infinite concentration of ethylene gives the yield of thermal hydrogen atoms produced in the y-radiolysis of 2,2,4-trimethylpentane. The study of such a system in the presence of electron scavengers, which interfere with reaction 1, should give information on the yield of hydrogen atoms produced in this reaction with an efficiencyf. The results of such experiments involving SFs and NzO as eleetron scavengers are presented in
2383 Table I1 for an ethylene concentration of 0.11 M . At this concentration 430% of the thermal hydrogen atoms are scavenged by ethylene. As can be seen in Table 11, G(C2Hs1311)decreases with increasing electron scavenger concentration, indicating that indeed reaction 1is the source of some thermal hydrogen atoms. One can extend to 2,2,4-trimethylpentane the kinetic model recently proposed for cy~lohexane.~I n this model, for a given ethylene concentration, the electron scavenger concentration dependence of the yield of ethyl radicals (measured as ethyl iodide) is given by G ( C Z H ~ . ) ~ C=~[HG~WI O
-
kadd
[Cz& 1
where G(H)o is the yield of thermal hydrogen atoms in the pure 2,2,4-trimethylpentane (= 1.0). f , as mentioned above, is the efficiency of producing a thermal hydrogen atom upon electron-ion neutralization (reaction 1) and k a b s l k a d d is the ratio of the rate constant for the abstraction of hydrogen atom from the solvent (kabs) to that for the addition to ethylene (kadd). The Value Of kabs/kadd Was found to be 0.0043.11 [RH] is the molarity of 2,2,4-trimethylpentane = 6.06 M . The other parameters have been defined previously. I n eq I11 the only unknown parameter is f. From the best fit of this equation to the experimental data, one obtains f N 0.09. The last column in Table I1 gives the values calculated with eq I11 and f = 0.09. The general agreement between the calculated and experimental values is fairly good. The value off 'v 0.09 shows that a yield of G = 0.44 [f(G(ions))] or 44% of the thermal hydrogen atoms come from ion-electron neutralization processes (reaction 1). The remaining yield of G(H) = 0.56 comes from direct excitation of 2,2,4-trimethylpentane. With the knowledge of the yields of hydrogen and thermal hydrogen atoms originating from ionic processes, one can estimate the contribution of various processes to the total hydrogen (Table 111). Table 111: Estimated Yields of Hydrogen and Methane According to the Different Sources from Which They Originate Atomic/rsdical
,--Molecular--
Ionic Nonionic Total
€12
CH4
H
CHs
1:Ol
-0.24 -0.24 0.48
0.44 0.56 1.00
0 0.73 0.73
0.43 1.44
--Total-Hz
CH4
1.45
0.24
0.99
0.97
2.446
1.21a
a These two values are the experimentally determined yields of hydrogen and methane.
(18) P. P. Infelta and R. H. Schuler, J. Phys. Chem., 76, 987 (1972).
The Journal of Physical Chemistry, Vol. 76, No. 17,1972
KRISHAN M. BANSAL AND STEFAN J. RZAD
2384
(6) ND3. From a study of ND3-cyclohexane solutions, Williams has shown that although G(H2) decreases with increase in E D 3 concentration, the total hydrogen yield remains ~ 0 n s t a n t . l ~Since in cyclohexane ion-electron recombination results in hydrogen formation with unit efficiency, this indicated that the neutralization of the secondary ions, produced by proton transfer from cyclohexane positive ion to ammonia, also results in hydrogen formation with unit efficiency. This has been further confirmed r e ~ e n t l y . On ~ ~ the ~~ other hand, in systems where the efficiency of hydrogen production from the electron-ion neutralization is less than unity, the total hydrogen should increase with added ammonia, since the efficiency of hydrogen production from the neutralization of the secondary ion in the ammonia system is higher than that in the pure solvent. As expected, the addition of NDa to 2,2,4trimethylpentane results in an increase in the hydrogen yield, as is illustrated in Figure 1. (The total yield of hydrogen is G(Hz)total = 3.00 at 0.3 M ND3.) This increase of the total hydrogen yield represents actually a constant G(H2) and a production of G(HD) (dashed circles in Figure 1.) The reaction scheme leading to these products consists of the proton-transfer reaction (4) in competition with reaction 1followed by the neutralization reaction (5)
+ NDa +NDsH+ + R . ND3H+ + e + (H or D + ammonia) RH+
~ N D ~
(4) (5)
The hydrogen or deuterium atoms produced in reaction 5 with the statistical weights of one-fourth and threefourths, respectively, will abstract a hydrogen atom from the solvent to give H2 or HD. Since only the yield G(HD) is measured, one obtains the yield of reaction 5 and, hence, the yield of ND3H+ produced by multiplying the G(HD) by 4/3 to correct for the hydrogen atoms p r o d ~ c e d . ~I ~ n *turn, ~ ~ one has to subtract from the yield of G(H2) a yield equal to ‘/sG(HD) to account for the hydrogen originating in the neutralization of ND3H+. These corrected yields of G(HD) and G(H2) are shown in Figure 1. Since the equations presented so far are also applicable to positive ion scavengers, the total hydrogen yield at any concentration of ND3 should be given by an equation such as eq I1 with - e being replaced by (eNDs - e), where END^ is the efficiency of producing hydrogen from NDaH + neutralization (reaction 5) and e is the efficiency of producing hydrogen upon ion-electron neutralization in the pure solvent. We will refer to this equation as eq 11’. Since ~ N = D ~1.0 and E = 0.3, one has ~ N D ~e = 0.7. I n eq 11‘ all the parameters are known but ( Y N D ~ and a fit of this equation to the total hydrogen yields should give an estimate of this value. However, total hydrogen changes from G(H2) = 2.44 to G(Hz) = 3.00 a t 0.3 M NDI, and one can very readily see that one cannot fit the yields of hydrogen with eq 11’unless Gfi = 0. The Journal of Phyaical Chemktry, Vol. 76, No. 17, 1972
[NO81
Figure 1. Hydrogen yields as a function of NDs concentration: ( 0 )G(tota1 hydrogen), (upper );: G(Ha), (lower S) G(HD); (0)G(Hs)(corr) (see text), ((3) G(HD)(corr) (see text). Lines are calculated using eq 11’, 11, and I for G(tota1 hydrogen), G(Hz), and C (HD), respectively, with the parameters given in the text. Dashed lines refer to the case where Gfi = 0.3 and solid lines to Gti = 0.
If Gfi = 0.3, the free ion contribution to the total yield of hydrogen would be 0.21, and DlNDa should be close to zero in order to explain the yield of total hydrogen G(H2) = 2.71 at 0.073 M ND3. If free ions were contributing to the yield of HD, the best fit of eq 11’ to the total yield of hydrogen would be that illustrated as the uppermost dashed line in Figure 1 ( ( Y N D ~= 0.05 M-l). Assuming that there is no contribution of free ions to the yield of HD,Z1 the solid line drawn through the total hydrogen yields is calculated with eq 11’ and the following parameters: G(H2)o = 2.44, END^ = 1.0, E = 0.3, Gfi = 0, G,i = 4.54, and ( Y N D ~= 0.15 il4-l. One can calculate G(HD) and G(H2) by using eq I and 11. This is illustrated as the dashed (Gri = 0.3) and the solid lines (Gri = 0) drawn through the H D and H2 yields. The agreement between the calculated solid lines and the experimental data is gratifying, indicating again that ND3 does not react with the free ions. It should be pointed out here that the determination of C Y N Dis ~ independent of the correction of ‘ 1 3 applied to the H D yields. Therefore, the good agreement obtained between the calculated and corrected experimental H D yields supports the assumptions made in applying such a correction, i.e., (1) there is very little deuterium exchange between ND3 and ND3H+ to give ND4+, (2) there is no isotope effect in producing H or D in the neutralization of ND3H+ (and, therefore, they are produced according to their statistical weights). Muratbekov, et aL,6 reported a total yield of hydrogen G(Hz) = 3.2 from a solution of 1 M NH3 in 2,2,4-tri(19)F.Williams, J. Amer. Chem. Soc., 86, 3954 (1964). (20) K.-D. Asmus, I&. J. Radiat. Phys. Chem., 3 , 419 (1971). (21) One reason can be that free ions do not react with ND3. Another suggested by a referee is that traces of an electron scavenging impurity in the NDa would convert free electrons t o free anions and as a result no hydrogen would be produced upon neutralization.
RADIOLYSIS O F LIQUID2,2,4-TRIMETHYLPENTANE
2385
methylpentane. Using the above parameters and eq 11', one calculates a total hydrogen yield G(Hz) = 3.3 at this concentration. The agreement is again satisfactory. ( B ) liethane. ( a ) E$ect of Charge Scavengers. The methane yield from the 7-radiolysis of pure 2,2,4trimethylpentane was found to be 1.21 f 0.05 and independent of dose in the range of 1.5 X 1018-6 X lo1* eV/cm3. This value agrees with the yields of 1.12,12 l . l , 6 ~ 1 1 ~ 1 6l.2.!i,14and 1.2716 reported in the literature. Using the l3lI2scavenging technique in pure 2,2,4[SI trimethylpentane, a yield of 0.73 for methyl iodide was Figure 2. Yield of methane as a function of solute obtained in the present work. This indicates that the concentration: ( 0 ) [PI = C2H4,(0)= [SI = c-CaHB. Solid methane yield originates from methyl radicals, G(CH3) line calculated using eq IV and the parameters given in the text. = 0.73, and molecular methane, G(CH~)N= 1.21 The upper and lower dashed lines refer to the total ( = 1.21) 0.73 = 0.48. Similar values have been reported in and molecular (0.48) yield of met,hane from the radiolysis of pure 2,2,4-trimethylpentane. the literature for G(CH3) and G(CH4)br, respectively: 0.69 and 0.50,12 0.69 and 0.56,140.70 and 0.57.16 This methyl radical yield (G(CH3) = 0.73) remains unTable IV: Effect of Charge Scavengers on the Yield of Methyl affected in the presence of charge scavengers as inIodide from l311~-2,2,4-Trimethylpentane Solutionsa dicated in Table IV. These results indicate that the excited states produced directly in the radiolysis of 2,2,4S [SI, mM G(CH8lg'I) trimethylpentane are the precursors of methyl radicals. ... ... 0.73 However, the presence of electron scavengers decreases SFB 107 0.73 slightly the total yield of methane, as shown in 'Table 0.76 293 CFaBr 320 0.75 I . The yields of methane from CH3Br solutions when CpHrBr 300 0.72 corrected for the methane originating in the electron 0.77 CtHbOH 120 scavenging process as calculated by eq I are also preCeH6 135 0.75 sented in Table I and indicate a decrease of G(CH4) 1811~ = 5 . 4 x 10-4 M . with increasing CHIBr concentration. A similar decrease of the methane yield has been observed by Iida, et aL16 who used X2O and SF6 as scavengers. the value of 0.24 obtained in the present work (Table Equation 11, which has been used above in the case of 111)* hydrogen, can be extended to methane provided one (b) Efect of Ethylene. I n a series of experiments replaces G(H2)o by G(CH4)o = 1.21 and E by E", the effect of CzH4 on the yield of methane has been inthe efficiency of producing methane upon electron-ion vestigated, and the results are shown in Figure 2. The recombination. Since the effect of electron scavengers yield of methane decreases with increasing ethylene is relatively small, the calculation is rather insensitive it is unaffected by the presconcentration. However, to small variations of E". The last column of Table ence of cyclopropane (Figure 2). Since it has been I shows the yields of methane calculated by eq I1 with shown previously that ethylene as well as cyclopropane the parameters obtained previously and E" = 0.05. do not interfere with the reactions of positive ions in The agreement is sufficiently good, especially in view l1 the decrease in the methane 2,2,4-trimethylpentane1 of the experimental errors involved in the measurement yield should then be due to the reaction of CHI radicals of methane. Bearing in mind that the G(CH3) origwith ethylene in competition with the hydrogen atom inates only from direct excitation of the 2,2,4-trimethylabstraction from the solvent in a manner similar to pentane, one can now, as for hydrogen, estimate the that occurring with CF3 radicals in ethylene-cyclocontribution of various processes to the yield of methhexane solutions. 2 3 , 2 4 ane (Table 111). The effect of positive ion scavengers, If such competition represents the effect of ethylene, namely XD3 and ethanol, is smaller than the effect of then the yield of methane at any given concentration electron scavengers: G(CH4) = 1.15 and 1.13 a t of ethylene should be given by 0.14 and 0.32 M NDI and G(CH4) = 1.17 at 0.1 and 0.3 M ethanol, respectively. Such a smaller effect is (22! 5 . J. Raad, Abstracts, 158th Meeting of the American Chemical expected, since the value of O ~ N D=~ 0.15 M-I ( Y E ~ O H , ~ ~Society, New York, N. Y . , Sept 1969, p 239. It has been shown is much smaller than, for instance, ~ S ~= O6 M-I. that in cyclohexane ~ N =D 1.0 ~ M-1 and CZE~OH= 1.2 M-1. (23) P. .'I Infelta and R. H. Schuler, J . Phys. Chem., 73, 2083 It should be pointed out here that Iida, et u Z . , ~ found a (1969). yield of 0.2 of molecular methane originating from non(24) R. A . Weir, P. P. Infelta, and R. H. Schuler, ibid., 74, 2596 ionic processes. This value should be compared with (1970). Q
The Journal of Physical Chemistry, Vol. 76,No. 17, 1979
K. M. BANSAL, L. K. PATTERSON, AND R. H. SCHULER
2386 G(CH4)
=
G(CHJM
+ G(CHa)
1 klsdd
1+--
krahs
[CZH~I [RH]
(IV)
where G(CH4)M and G(CH3) are the yields of molecular methane (=0.48) and of methyl radicals (=0.73), respectively, in pure 2,2,4-trimethylpentane. k'add/ k'abs is the ratio of the rate constant for the addition of methyl radicals to ethylene to that for hydrogen abstraction from the solvent. According to this equation a plot of G(CH3)/(G(CH4) - G(CH~)M) vs. [CzHk] should be a straight line with an intercept of one and slope of k'sdd/klehs[RH]. Such a linear relationship is indeed observed and a slope of 5.5 M-l is obtained. Since [RH] = 6.06 M , k l a d d / k l a b s = 33.3. With k l a b s N 20 M-' sec-',12 k l s d d 'V 665 M-' sec-l. The
value of 33.3 agrees very well with the value of k'add/ k l a h s = 34 obtained at 65" in 2,2,4-trimethylpentane by Szwarc and ~ o w o r k e r s . ~The ~ order of magnitude of k'add 21 665 M-" sec-' is in accord with the results obtained in the present work that the yield of methyl iodide is unaffected by the addition of ethylene (in the range of 3.53 X 10-3-0.22 M ) to solutions of 5 X M 1 2 in 2,2,4-trimethylpentane (for example, G(CH31) = 0.74 at 0.22 M C2H4).2e Using eq IV and the rate constants determined above, one calculates the solid line through the points in Figure 2.
Acknowledgment. The authors wish to thank Mr. G. K. Buzzard for the mass spectrometric analyses. (25) R. P. Buckley and M . Szwarc, Proc. Roy. Sac., Ser. A , 240, 396 (1957). (26) The rate of reaction of CHI I2 to give CHsI has been estimated to be 3 X 108 M-1 sec-1.lZ
+
The Production of Halide Ion in the Radiolysis of Aqueous Solutions of the 5-Halouracils]
by Krishan M. Bansal, Larry K. Patterson, and Robert H.Schuler" Radiation Research Laboratories, Center for Special Studies, and Department of Chemistry, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 26218 (Received March 2, 1972) Publication costs assisted by Carnegie-Mellon University and the U . S . Atomic Energy Commission
The radiation chemical production of halide ion from aqueous solutions of 5-fluoro-, 5-chloro-, and 5-bromouracil has been examined by conductometric pulse radiolysis and ion-selective electrode methods. The conductometric studies indicate that on the time scale of 10-6 to lov2sec approximately 15, 50, and 80% of the hydrated electrons produced in the irradiation react to form F-, C1-, and Br- from the respective halouracils while attack by .OH radicals gives hydrogen halide with corresponding efficiencies of 75, 65, and 55%. I n these latter cases the yields of hydrogen halide presumably result from addition of OH a t the 5 position followed by dehydrohalogenation of the resulting radical. About 50% of the H atoms produce Br- from bromouracil but the contribution is small in the case of the fluoro and chloro compounds. Steady-state experiments in which the halide ion was determined after y radiolysis show that long-term processes are responsible for significant additional yields of halide ion. I n particular it is shown t h a t each of the halouracils is subject t o attack by organic radicals arising from the uracil itself or produced from other solutes present in the system. I n 5-chloro- and 5-bromouracil solutions containing added isopropyl alcohol a chain mechanism leads to the production of large yields of C1- and Br-.
I n recent years there has been considerable interest in the radiation chemistry of 5-bromouracil (BrUr) and related compounds as a result of the observed increase in the radiation sensitivity of DNA in which thymine has been partially replaced by BrUra2 The mechanism of this effect is not wholly understood, but it seems likely that dehalogenation following attack on DNABrUr by radicals produced in the irradiation is imThe Journal of Physical Chemistry, Val. 76, N o . 17, 1978
p~rtant.~ A number of pulse radiolysis studies have been carried out on aqueous solutions of BrUr4 and (1) Supported in part by the U. S. Atomic Energy Commission. (2) See J. E. Zimbrick, J. F. Ward, and L. S. Myers, Jr., Int. J. Radiat. Biol., 16, 505 (1969), for a summary of previous work on this subject. (3) G. E. Adams, "Current Topics in Radiation Research," Vol. 111, M . Ebert and A. Howard, Ed., Wiley, Kew York, N. Y., 1967, p 35.
