2888
KRISHAN11.BANSAL AND STEFAN J. RZAD
thermal velocity this corresponds to T S 7 X 10-14 see. This recombination time approximates the value deduced initially by Samuel and Magee, l 3 whose theory describes the behavior of dry electrons. The dielectric relaxation time a t constant charge has been shown by Mozumder to be 4 X see, but the microscopic relaxation time may be less.14 Theard and Burton15observed large effects of halides in methanolic solutions, with AG(glyco1) S -AG(CH,O) S -2.5 a t [CI-] = 1.5 M . Hole trapping is
considerable at this concentration and the effect may be due to neutralization of CH,OH+ prior to proton transfer which would decrease G(CH20H), and therefore G(glyco1). The increase in G(CH20) may be due to the reaction CHzOH Xz- -.+ CH20 H + 2X-, thereby accounting for the somewhat symmetric AG's of these two products.
+
+
+
(13) A. H.Samuel and J. L.,Magee, J . Chem. Phys., 21, 1080 (1953). (14) A. Mozumder, ibid., 50, 3153,3162 (1969). (15) L.M. Theard and M. Burton, J. Phys. Chem., 67, 53 (1963).
Electron Scavenging in the Y Radiolysis of Liquid Diethyl Ether' by Krishan M. Bansal and Stefan J. Rzad Radiation Research Laboratories, Mellon Institute, Carnegie-Mellon University, Pittsburgh, Pennsylvania 16229 (Received March SO, 1970)
The concentration dependence of electron capture in the y radiolysis of diethyl ether-methyl bromide solutions can be quantitatively accounted for by the empirical model previously proposed for hydrocarbons and extended to alcohols. The free and geminate ion pair yields are estimated as 0.15 and 3.8, respectively. Competition experiments allowed the determination of the reactivities of SF6and N20 towards electrons in the ether. From the study of the decrease in hydrogen yield upon addition of these electron scavengers, it is concluded that only 40y0 of the ion-neutralization processes yield hydrogen.
Introduction The y radiolysis of diethyl ether has, so far, received very little a t t e n t i ~ n . Our ~ ~ ~interest in the study of diethyl ether radiolysis originates from the fact that its dielectric constant (4.3) lies in the range between those of hydrocarbons (-2.0) and alcohols ( E C H ~ O H 32.6, Q H ~ O H 24.3). While several studies involving the use of electron scavengers in the radiolysis of hydrocarbons and alcohol^^^^ have been carried out, no such information is a,vailable for ethers. The present investigation was undertaken to understand the electron capture processes in diethyl ether by using (I4Cc) methyl bromide (electron scavenger) as a probe. Furthermore, the effect of various electron scavengers (CH3Br,NzO and SFe) on the hydrogen yield (a major radiolysis product) was also studied to obtain information concerning the importance of ion-electron neutralization processes leading to hydrogen formation. Experimental Section Sulfur hexafluoride, 14Cmethyl bromide, and nitrous oxide were purified by a method described elsewhere.6 hilatheson CFaBr was distilled a t -78" and stored. Spectrograde diethyl ether from Eastman Organic Chemicals Co. was first outgassed a t liquid nitrogen temperature. After that '/a of the Volume of ether Was The Journal of Physical Chemistry, VoE. 74,No. 16, 1870
pumped away at room temperature to remove dissolved carbon dioxide. The next '/3 fraction was distilled and stored into a glass reservoir on the vacuum line. This diethyl ether was used throughout the experiments since the same yields of methyl radicals were obtained whether or not the ether was dried over sodium and no impurities (e.g., ethanol) could be detected by gas chromatography using a 2.5-m column packed with 10% di-2-ethylhexylsebacate on diatoport WAW (6080 mesh) or a 10-m column packed with 2501, silicone grease on Chromosorb P. A known amount of ether (usually 1.2 cm3) was distilled into the irradiation cell and was degassed again by the freeze-pump-thaw cycles. The desired amount of the solute, as determined by pressurevolume measurements, was then distilled into the (1) Supported in part by the U.S. Atomic Energy Commission. (2) G. E. Adams, J. H. Baxendale, and R. D. Sedgwick, J . Phys. Chem., 63,854 (1959). (3) M. K. M.Ng and G. R. Freeman, J . Amer. Chem. Soc., 87, 1635 (1965). (4) J. M.Warman, K.-D. Asmus, and R. H. Schuler, Advances in Chemistry Series, No. 82,American Chemical Society, Washington, D. C., 1968, p 25, and references cited therein. (5) J. Teply, Radiat. Res. Rev., 1, 361 (1969),and references cited therein. (6) J. M. Warman and S. J. Rzad, J . Chem. Phys., 52, 485 (1970).
ELECTRON SCAVENGING IN THE y RADIOLYSIS OF LIQUIDDIETHYL ETHER irradiation cell containing the outgassed ether. The cell was sealed off in such a way that the vapor volume was approximately 0.2 cm3. For solutions containing less than M methyl bromide an approximate amount of methyl bromide was added initially and the actual concentration was determined from the methyl bromide activity as measured during the analysis. The concentration of the solute in diethyl ether was calculated on the assumption of the complete solubility of CH3Br,N20,SF6, and CF3Brin the ether. The radio-gas chromatographic method of analysis used has been described b e f ~ r e . ~ ~ ~ The volume of diethyl ether used for samples for which the analysis of noncondensable gases (Hz, CH4, and CO) a t liquid nitrogen temperature was carried out was usually 5.2 em3. The vapor volume after sealing off the sample tube was approximately 0.5 em3. After irradiation, the total amount of the products volatile at liquid nitrogen temperature was measured in a ToeplerJIcLeod apparatus and subsequently analyzed by mass spectrometry. The samples were irradiated in a tubular 6oCoy-ray source at a dose rate in the ether of 6.6 X 10le eV/cm3 min or 3.4 X 10'' eV/cm3 min.
Results and Discussion A . Diethyl Ether-Methyl Bromide Solutions. In the radiolysis of hydrocarbons or alcohols containing methyl bromide, it was shown that the methyl radicals are produced as a result of dissociative electron capture from methyl bromide (reaction 1).8-10 These methyl radicals e14CHsBr---f 14-CH3 Br(1)
+
+
can either abstract a hydrogen atom from the solvent (reaction 2) or react with other radicals R or R ' (reaction 3). The radio-gas chromatographic analysis of
+
14*CH3 (CzH6)zO ---f 14CH4
+
+ R*
14*CH3 R or R' +products
(2)
(3)
the products from the radiolysis of diethyl ethermethyl bromide solutions showed that reaction 3 amounts to about 1% of the total methyl radicals produced by reaction 1. Therefore, the yield of (1") methyl radicals" (or methane with a suitable correction for the occurrence of reaction 3) will give the yield of the scavenged electrons. The dependence of the methyl radicals yield on the concentration of methyl bromide was studied over the concentration range from 7X to 0.3 M and the results are reported in Figure 1. Over this entire concentration range the yield of methyl radicals changed from approximately 0.08' to 2.9 (Figure 1). Analysis sensitivity required higher doses to be given to samples containing higher concentrations of methyl bromide. Indeed the higher the concentration of the latter the more it has to be diluted with inactive methyl bromide for practical reasons.
I
I
10-5
I
1
1
1
1
2889
1
1
1
1 o'~
I
,
, , 1 1 , ,
I
,,,,,,,I
IO-^
10-2
,
,
,
I
,,,,, 10''
, , ,
[CH3Br] M
Figure 1. Yield of methyl radicals as a function of methyl bromide concentration in diethyl ether. Solid line calculated using eq I1 with Gfia = 0.15, G,iE = 3.8, OL = 18 M-1, 6 = 5.1 X l o 4 The dashed and dotted lines are calculated as described in the text.
The doses which correspond to the data presented in Figure 1 are in the range of 1.3 X 10'' to 1 X 1019eV/ em3. Since no dose effect on the methyl radical formation is observed, the intercomparison of the different data is warranted. It was shown that in the case of hydrocarbons, the concentration dependence of methyl radical yield could be described by the empirical eq 18,10,12 G(e-)
G f i + Ggi
I+*
where Gri and Ggi represent the yield of the free and geminate electrons, respectively, S is the concentration of the scavenger, and a is a parameter which represents the reactivity of the solute towards the electrons relative to their recombination with the positive ions. In the present study of the radiolysis of diethyl ether-methyl bromide solutions, it was observed that eq I, together with suitable parameters (Gri = 0.15, Ggi = 3.8, a = 18) is a good description of the experimental data (Figure 1) at scavenger concentrations above 3 X M. Below this concentration, the experimental points fall below the calculated values (dashed line in Figure 1). A similar behavior for methyl radical production was found in the radiolysis of solutions of methyl bromide in methanol and ethanol in which case eq I1 was found to describe the data quite welL9 (7) S. J. Rzad and R. H. Schuler, J. Phys. Chem., 72, 228 (1968). (8) S. J. Rzad and J. M. Warman, J. Chem. Phgs., 49, 2861 (1968). (9) S. J. Raad and J. H. Fendler, ibid., 52, 5395 (1970). (10) J. M. Warman, K.-D. Aamus, and R. H. Schuler, J. Phys. Chem., 73, 931 (1969). (11) Throughout the text methyl radical yield refers only to the ("C) methyl radicals. (12) K.-D. Asmus, J. M. Warman, and R. H. Schuler, J. Phys. Chem., 74, 246 (1970). The Journal of Physical Chemistry, Vol. '74, No. 16,19'70
2890
I n this equation, 6 = k/kd, where k d represents the first-order rate constant for the deacy of the solvated electron and k is the second-order rate constant for its scavenging by the solute. Gfisand Ggis represent the solvated free and geminate electrons, respectively. The solid curve drawn through the points in Figure 1 represents the best fit of eq I1 with the following parameters: Gfis = 0.15; Ggis = 3.8; a = 18 M-l and 6 = 5.1 X lo4 M-l. Although in alcohols the pulse radiolysis studies have shown the existence of solvated electrons,13no such direct evidence for their existence in diethyl ether at room temperature is available. However, from the observation of the solvated electrons in pulse-irradiated diethyl ether at -110' (rlla = 2 psec) l 4 and the similarities in electron scavenging curves for diethyl ether and alcohol^,^ it appears that the electrons may also be solvated in diethyl ether at room temperature and the lack of their observation in the pulse-radiolysis study14 might be due to the time-scale limitations of the technique used. Assuming that these electrons are solvated one can obtain the value of their first-order decay rate constant ( k d ) from the parameter 6 provided the value of the rate constant k is known. The rate constant IC is expected t o have a value between 3 X loll M-l sec-', l5 reported for electron scavenging by biphenyl in cyclohexane and 1.4 X 1O'O M-l sec-l reported for electron scavenging by methyl bromide in methanol and ethanol.9 Using a value of 3 X l o l l M-l sec-l, an estimate of the value of ka = 5.9 X lo6 sec-l is obtained. This corresponds to n/, of 0.1 psec for solvated free electrons in diethyl ether. Assuming no rise time of the equipment and using k d = 5.9 X lo6 sec-l, the fraction of solvated free electrons remaining at the end of a 0.4 and 0.1psec pulse16 is 0.38 and 0.17, respectively. Since Gfis = 0.15, these values correspond to yields of G = 0.06 and 0.03, respectively. On the other hand, a value of 1.4 X 1010 M-l sec-l would lead to the following parameters: k d = 2.8 X 105sec'l, rl/, = 2.5 psec, and fractions of 0.95 and 0.87 remaining a t the end of a 0.4 and l-psec pulse, respectively. These fractions correspond t o G = 0.14 and 0.13. The lack of observation of any optical absorption for solvated electron in pulseirradiated diethyl ether at room temperature14 would be more consistent with a half-life of T I / , = 0.1 psec indicating that the rate constant for electron scavenging in diethyl ether is similar t o that in cyclohexane. It should be pointed out here that a value of -3 X lo1' M-' sec-l for electron scavenging by methyl bromide in diethyl ether is not unreasonable in view of the following. A value of 3 X 10" M-l sec-' is a lower limitl6 in cyclohexane and, bearing in mind the recent measurement of a high electronic mobility in cyclohexane" (0.35 emz V-' sec-l), may actually be higher The Journal of Physical Chemistry, Vol. 74, No. 16, 1070
KRISHAN M. BANSAL AND STEFAN J. RZAD by at least one order of magnitude. On the other hand, in polar media such as alcoholsg~18 and waterI8 the rate constant of reaction of the solvated electron with a solute is of the order of magnitude of the diffusioncontrolled rate constant as calculated by the Debye equation when using for the solvated electron a mobility comparable t o that of a negative Diethyl ether with a dielectric constant of 4.3 is closer to hydrocarbons and one would expect that the electron be less solvated (bound) in this medium than in more polar liquids. This in turn would give a higher mobility to the solvated electron leading t o a higher rate constant for its scavenging. The departure of the yields from the dashed line (Figure 1) at low methyl bromide concentrations could result from small amounts of impurity present in diethyl ether. For example, if the rate of electron scavenging by methyl bromide and the impurity were similar (3 X 10" M-l sec-l), the concentration of the latter would be 2 X M . Such a concentration should have been detected by gas chromatography. However, a definite proof of the existence of the solvated electron in diethyl ether will eventually come from nanosecond pulse-radiolysis experiments. The yield of Gfis obtained in the present study agrees well with the value of 0.19 obtained by conductivity measurements in pure diethyl etherz0 but disagrees with that obtained by Schmidt and Allenz1(Gfi = 0.35) using the Clearing Field technique. By using this latter value of Gfis = 0.35, the best fit of equation I1 to the experimental data at high solute concentrations M ) , is obtained with Gfis = 4.0 and a = 9.3 M-l. These parameters together with a value of 6 = 1.6 X lo4 M-' also give a good fit to the experimental M ) ; however, data a t low solute concentrations ( to M the fit in the solute concentration range is poor (dotted line in Figure 1). The present study shows that the total ion-pair yield is Gi = Gfis Ggie = 3.95 which agrees reasonably well with the value of 4.2 calculated on the basis of gas phase W value of 23.6 eV for diethyl ether.zz B. Competition Studies between Methyl Bromide and Other Solutes in Diethyl Ether. Studies involving the
+
(13) For example, I. A. Taub, D. A. Harter, M. C. Sauer, Jr., and L. -M.Dorfman, J . Chem. Phya., 41, 979 (1964). (14) S. Arai and M. C. Sauer, Jr., ibid., 44, 2297 (1966). (15) S. J. Raad, P. P. Infelta, J. M. Warman, and R. H. Schuler, ibid., 50, 5034 (1969). (16) This fraction can be calculated using eq 17 of ref 9. (17) W. F. Schmidt and A. 0. Allen, J . Chem. Phya., 52,4788 (1970). (18) See, for example, J. K. Thomas, Radiat. Rea. Rev.,1, 183 (1968). (19) E.g., the diffusion of the solvated electron in water has been measured to be 4.75 X om2 sec-1, K. H. Schmidt and W. L. Buck, Science, 151, 70 (1966). (20) G. R. Freeman and J. M. Fayadh, J . Chem. Phys., 43, 86 (1965). (21) W. F. Schmidt and A. 0. Allen, J . Phya. Chem., 72, 3730 (1968). (22) P.Adler and H. K. Bothe, 2.Naturforsch. A , 20, 1700 (1965).
ELECTRON SCAVENGING IN THE y RADIOLYSIS OF LIQUID DIETHYL ETHER competitive electron capture by two solutes in diethyl ether were also carried out. The scavenger combinations used were methyl bromide-nitrous oxide and methyl bromide-sulfur hexafluoride. The results are presented in Table I. Assuming ideal behavior, one Table 1 : l4CCH3Yields from Competitive Studies between Methyl Bromide and Other Solutes in Diethyl Ether" Solute, M x 108
[CHaBrl, M X 108
NsO solute, 0.89 2.24 2.27 2.50 4.98 5.09 8.80 2.30 4.80 10.8 21.4 31.7 30.0 21 .o 36.0 75.9 80.0 116.0
1.49 1.69 1.69 5.35 5.35 5.60 5.35 9.30 9.00 9.00 9.40 9 -40 9.90 10.36 83.0 84.5 84.6 81.7
Gobsd(CHa) CY
Gcaiod(CHa) f~
= 16
0.46 0.42 0.39 0.79 0.54 0.67 0.55 1.03 1.05 0.80 0.71 0.58 0.53 0.69
1.81 1.36 1.31 1.28
0.52 0.43 0.43 0.84 0.70 0.72 0.57 1.10 0.96 0.76 0.58 0.47 0.51 0.63 1.74 1.42 1.39 1.16
parameter a2. For this reason, the methyl bromide concentration used in competition experiments was always greater than M . The best fit of eq I11 to the experimental data, neglecting the solvated electron decay, allowed the determination of ~ X , O = 16 and C Y S F= ~ 42. The values of Goalod(CH3) are presented in the last column of Table I and the agreement with Gobsd(CH3)is fairly good. C. Hydrogen, Methane, and Carbon Monoxide Yields. The hydrogen yield changed from 3.35 to 3.20 as the total dose absorbed was increased from 1.7 X 10'8 to 1.2 X 1019 eV/cm3. The extrapolated G(HJ value t o zero dose was found to be 3.4, which is slightly lower than the value of 3.7 found by Ng and Freeman3 Over this dose range studied, the yields of methane and carbon monoxide (G(CH4) = 0.37, G(C0) = 0.08) remained unchanged with dose. Eflect of Electron Scavengers. The dependence of the yield of hydrogen on the concentration of various electron scavengers (CHsBr, NzO, SFe) was studied and the results are presented in Table 11. Furthermore, it
Table I1 : Hydrogen Yields from the Radiolysis of Diethyl Ether-Scavenger Solutions" [SI,
mM
9.0 9.0 9.0 9.0 74.3 76.8 76.8 76.6
0.93 0.73 0.54 0.41 1.79 1.57 1.06 0.73
G(H2)obsd
Scavenger, NzO
S F ~ s o l u t e ,CY = 42 2.28 4.98 11.0 21.5 12.7 20.4 39.3 78.7
289 1
0.90 0.70 0.48 0.32 1.69 1.51 1.18 0.82
The dose was 3.4 x 1018 eV/cmg but for solutions containing Calculated [CHaBr] > 0.01 M it was 1 X 1019 eV/cm3. using eq 111, the values of CY and the following parameters: Grin = 0.15, G,P = 3.8, CYOH~B~= 18 M-l. a
can use eq I11 which gives the yield of electrons scavenged by a solute a t concentration SI,in the presence of a second solute at concentration 82. The various symbols
0.30 2.76 9.41 35.5 106 289
3.47 3.06 2.92 2.74 2.61 2.40
3.30 3.14 2.98 2.75 2.54 2.36
Scavenger, CHsBr 0.36 3.08 9.90 32.5 35.5 114 115 336
3.40 3.03 2.86 2.70 2.65 2.50 2.54 2.39
3.29 3.11 2.95 2.74 2.72 2.50 2.50 2.32
Scavenger, SFs 115 335
2.51 2.30
2.36 2.20
a The dose was 3.4 X 10'8 eV/cm8. b Calculated using eq I V and the following parameters: G(Hz)o = 3.4, G g i ~= 3.8, CYN~O = 16, a C H a B r = 18, C Y S F= ~ 42, E 0.4.
in this equation have the same meaning as in case of eq I1 and the subscripts 1 and 2 refer to methyl bromide and the second solute, respectively. At methyl bromide concentrations above 3 X M , the decay of the solvated electron is negligible and, therefore, eq I11 will reduce to a form which has only one unknown
was also observed that the addition of 0.11 M CFaBr reduced the hydrogen yield t o 2.60. The yields of methane and carbon monoxide remained unaffected by the addition of electron scavengers (NzO, SF6, CF3Br) indicating that excited states proThe Journal of Physical Chemistry,Vol. 74, No.16,1970
2892
KRISHAN M. BANSAL AND STEFAN J. RZAD
duced directly in the ether are the precursors for their formation. The various electron scavengers are expected to affectthe hydrogen yield by interfering with the positive ion electron neutralization processes. The overall hydrogen production by ionic processes can then be represented by the following general scheme positive ion
+ e- -+ eHZ
e - + S d S positive ion
+ S- -+ no H or Hz
(4) (5)
(6)
where the positive ion can be a primary or a secondary positive ion and E is the efficiency for production of hydrogen upon neutralization. I t was assumed that the solvated free electrons decay according to reaction 7 and hence do not contribute to any hydrogen produc-
+
(e-solv)ether +CZH~O- C Z H ~
(7)
tion. This reaction is analogous t o the decay of the solvated free electrons in alcohols.2~ (eso1v)ROH --3 RO-
(8) According to this general scheme, the yield of hydrogen a t any electron scavenger concentration S is given by eq IV, where G(H& (= 3.4) is the hydrogen yield in the H
absence of the scavenger. The best fit of eq IV to the experimental data was obtained with E 0.4. The calculated values of the hydrogen yields are given in the last column of Table 11. The overall agreement with the observed values is fairly good. From the above it follows that a yield of hydrogen of 1-52 (G(H2)ion = Ggi X e) comes from ionic precursors and 1.88 (G(H2)non ionic = G(Hz)totRl - G(H2)ion) from nonionic processes. D. Nitrogen Yield from Diethyl Ether-NzO Solutions. The yields of nitrogen from the radiolysis of diethyl ether-NzO solutions are shown in Table I11 together with the yields of electrons scavenged by NzO (G(e8-)) calculated by using eq 11. As previously noted in hydrocarbons,24,26 the yield of nitrogen is in excess of the yield of scavenged electrons at m y NzO concentra-
The Journal of Physical Chemistry, Vol. 74, No. 16, 1970
Table I11 : Nitrogen Yield from the Radiolysis of Diethyl Ether-NiO Solutionsa [NaOl,
miM
0.30 2.76 9.41 35.5 106 289
G(Nd/ G(Nz)
G(eS-lb
Q(e7
0.60 1.20 1.75 2.59 3.25 4.04
0.38 0.81 1.21 1.78 2.30 2.74
1.58 1.48 1.45 1.46 1.41 1.47
a The dose was 5 X 1018 eV/cms. Calculated using eq I1 with the following parameters: Giis = 0.15, G,,n = 3.8, ~ N = ~ i 6 , 6 = 4.4 x 104 ~ - 1 .
tion used. Electron scavenging parameters obtained for nitrous oxide in methanol and ethanol9 seem to indicate that the yield of nitrogen observed in these alcohols26-28is also greater than the yield of the scavenged electrons. I n the case of cyclohexane-nitrous oxide solutions, this excess nitrogen yield can be quantitatively accounted for if one assumes that a secondary reaction of the anion t o produce nitrogen competes with the neutralization p r o ~ e s s . ~Such an explanation also describes the excess nitrogen yield in benzene-nitrous oxide solutions.25 In the present study of diethyl ether-NzO solutions, a constant ratio G(Sz)/G(es-) of 1.46 is observed over the entire concentration range studied (Table 111) indicating that the mechanism used for hydrocarbons does not apply to diethyl ether. As yet there is not enough information available as to make further speculation about the reaction mechanism worthwhile.
Acknowledgment. The authors wish to express their appreciation t o Dr. R. H. Schuler for helpful discussions. (23) G. R . Freeman in “Actions Chirniques et Biologiques des Radiations,” Vol. 14, M.Haissinsky, Ed., Masson et Cie Editeurs, 1969, and references cited therein. (24) M. G. Robinson and G . R. Freeman J. Chem. Phys., 48, 983 (1968). (25) R. R. Henta and W. V. Sherman, J . Phys. Chem., 73, 2676 (1969). (26) W. V. Sherman, $bid.,71, 4245 (1967). (27) J. C. Russell and G. R. Freeman, ibid., 72, 816 (1968). (28) K. N. Jha and G. R. Freeman, J. Chem. Phys., 48, 5480 (1968)
O
