2885
IONIC EVENTS IN THE RADIOLYSIS OF ETHANOL
Evidence for Very Early Ionic Events in the Radiolysis of Ethanol by Shamsher Khorana and William H. Hamill Department of Chemistry and the Radiation Laboratory,l University of Notre Dame, Notre Dame, Indiana (Received J a n u a r y 30,1570)
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Very early ionic events in polar systems, e.g., sec, precede dipole relaxation and they are expected to resemble in part those which occur in alkanes. Reactions or reaction efficiencies which are markedly enhanced in alkanes provide suitable tests. Benzene, as an additive, traps electrons efficiently in alkanes but reacts CeH6) Ei lo’ M-’ sec-I. In contrast H.+ reacts very fast slowly with solvated electrons, e,-, with k(e,with e,- and not measurably with the dry electron, e-. Preceding solvation, HzO+or C2HaOH+will oxidize, or be scavenged by, CL-, while H30,+ is a much weaker oxidant. Ethanol solutions have been chosen for study because G(e.-) E%1 and G(e-) should be large. Addition of 0.1 M alkyl halide RX was used to scavenge all e.- and some e- and the yield was measured as G(RH). Added CaClzincreased G(RH) and is attributed to hole scavenging followed by e- + e,-. Addition of acid decreased G(RH) to a limit of -50%, with G”(e,-) = 1.0, but further addition of benzene decreased G(RH) considerably. Addition of benzene to neutral 0.1 M RX decreased G(RH) with Go(e-) = 1.1.
+
Introduction There is a considerable amount of work which supports the hypothesis that the geminate dry charge pair, H20+and e-, can be trapped in aqueous systems with suitable solutes at 50.1 M . 2 This hypothesis is supported by quantitative correlations for aqueous systems of both recent and earlier ~ t u d i e s . It ~ ~is~also supported by the recent striking experiments of Hunt and coworkers5who have presented evidence from pulse radiolysis with 24 psec resolution that electrons react with high concentrations of appropriate solutes before becoming hydrated. Effective solutes were NaN03, CdC12,H202,and acetone, but Hap+was not effective. Ethanol as solvent offers significant advantages for studies of charge trapping. Since G(e,-) is only -1 there should be a large yield of scavengable dry electrons, possibly -3. Ion solvation energies and the rate constants k(e,S) are nearly the same as for water,*?’while the energetic primary positive ion is also degraded by protonation. It will be assumed hereafter that k(eaq- S) = k(e,S)ethsnol. The differences in chemistry are not relevant to charge trapping, while the improved solubility for organic compounds is critical for the high solute concentrations required. The primary objective of this work is to provide qualitative evidence for trapping the dry electron and the dry hole. Since hole trapping by anions increases the yield of solvated electron^,^ the two experimental procedures will be similar since AG(e,-) due to anions measures AG(ho1es). Benzene and phenol appear to be the most appropriate of the known traps for dry electrons because their rate constants for scavenging eaq- are -10’ M-l sec-l, and this very small value facilitates distinguishing between e- and e,-.4 The anions of both benzene and phenol abstract protons from alcohols4~* but it does not appear to be feasible to measure the ultimate products of the dienyl radicals.
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Instead, alkyl halides have been used as reference solutes in competition with H + or aromatic compounds. Alkyl halides react efficiently both with e- and e,- by dissociative electron attachment. I n ethanol as solvent the alkyl radical can be expected to yield alkane almost quantitatively and the change in yield of alkane measures the change in G(e1ectrons) due to trapping holes or electrons. The rate of reaction between the H atom and ethanol to form Hz under the conditions of this work greatly exceeds the rate of reaction between the H atom and 0.1 M propyl chloride to form HCl.‘j Consequently G(C3Hs)is expected to be a valid measure of dissociative electron attachment. The ratio of the corresponding rates of H atoms with ethanol and 0.1 M alkyl bromide is -206 and the contribution to G(C6HIo)is not important. The effect produced by addition of CaC12 will be attributed exclusively to hole trapping by C1-. The small difference between the first and second ionization potentials of Ca (6.1 and 11.9 eV) and the large single ion solvation energy of Ca2+(-18 eV) exclude electron (1) The Radiation Laboratory of the University of Notre Dame is operated under contract with the U. S. Atomic Energy Commission. This is AEC Document No. COO-38-717. (2) W. H. Hamill, J. Phys. Chem., 73, 1341 (1969). (3) P. L. T. Bevan and W. H. Hamill, Trans. Faraday Soc., in press. (4) T. Sawai and W. H. Hamill, J. Phys. Chem., 74, in press, ( 5 ) M. J. Bronskill, R. J. Wolff, and J. W. Hunt, J. Chem. Phys., in press. This and other related work was presented a t the “Symposium on Very Early Effects” at the International Meeting on Primary Radiation Effects in Chemistry and Biology, Buenos Aires, March 9-14, 1970. (6) M. Anbar and P. Neta, I n t . J. A p p l . Radiat. Isotop., 18, 493 ’ (1967). (7) L. M. Dorfman and M. S. Matheson, chapter on pulse radiolysis in “Progress in Reaction Kinetics,” Vol. 3, G. Porter, Ed., Pergamon Press, New York, N. Y.,1965, p 237. (8) T. Shida and W. H. Hamill, J. Amer. Chem. Soc., 88, 3689 (1966).
The J o u T of ~ Physical ~ ~ Chemi8try, Vol. 7.4, No. 16, 1570
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SHAMSHER KHORANA AXD WILLIAM H. HAMILL
trapping by Casz+. It will be assumed tentatively that H,+ traps only e,-, for which there is some basisJ2 and HC1 will be used at constant concentration of C1-. It will be assumed tentatively that the relative probabilities for trapping e- by alkyl halide and benzene is given by K~ [RX]/KI[CeHa], where the K’S are cross sections. For competition between R X and H + for e,- the relative probabilities become K~ [RX]/ KI[H+]but the K’S will be read, in context, as rate constants. An additional rate process for “first-order” recombination of e,- or solvation of e-, as appropriate, will be represented by KS. The expression A is quite
Table I: The Effects of C6H6, H +, and C1- on the Yield of CsHs from 0.1 M l-CsH7CI in Ethanol Yield CaHs, [Solute], arbitrary M units
Solute
None C6H6
0.05 0.10 0.20
0.30 0.50 0.70
0.90 1.00
similar to expressions used previously for trapping dry electron^.^^^ It should be observed that the usual plot of l/AG vs. 1/[S] will give an intercept l/Go but the slope will be (KZ[RX] 4- K ~ ) / G ’ Kand ~ [RX] > 0 in all systems.
Experimental Section n-Propyl chloride and c-pentyl bromide, Eastman White Label grade, and phenol, Mallinckrodt AR grade, were used as received. Benzene, Fisher’s certified grade, was recrystallized 3 times. All solutions contained 0.1 M n-propyl chloride or c-pentyl bromide in 95% ethanol of USP grade. The only reaction products of interest were propane or c-pentane which were analyzed using a Carbowax-1500 packed gel column a t 70’. All solutions were purged with nitrogen and 6oCoy irradiated t o 5 X 1019eV/g at ambient temperature. Results Both benzene and phenol depress G(C3Hs)and at 0.7, 0.9, and 1.0 M of either the effects are substantially equal. Benzene was used for all other experiments. It call he seen from Table I that -0.9 M benzene depressed the yield of propane (expressed in arbitrary units) by -SO%, even though a considerable part of this product must have been formed by es-. This component is not susceptible t o appreciable interference by benzene in competition with propyl chloride since k(eaqCaH7C1) Ei 109 M-l sec-’ while k(eaqCsHc) = 1.4. X Equation A can be fitted to the 5 most concentrated solutions with Go S 7 0 (in arbitrary units); Le., -70% of the precursors of propane are scavengable by benzene. The hydrogen ion reacts -lo2 faster with eaq- than does propyl chloride and acid should therefore suppress strongly the component of propane with e,- as precursor. The results in Table I show the expected decrease followed, however, by a marked increase. The latter might be a consequence of hole trapping by C1- and this possibility was tested by adding CaC12. As Table I shows, the yield of propane was increased
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The Journal of Physical Chemistry, Vol. 74, No. 16,1970
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None CaClz
0.1
0.2 0.4 0.6
0.8 1.0
100 82.3 73 70 69 59 52 49 46 100 107 114 134 139 130 145
Solute
Kone HC1
Yield CaHs, [Solute], arbitrary IM units
100 0.2 0.4 0.6
0.8 1.o
c1H + , C1-
HC1, C6Ha
1.0
0.05, 1 . 0 0.1, 1 . 0 0 . 2 , 1.0 0.6, 1.0 0.8, 1 . 0 1.0,l.O 1.0, 1 . 0
69 57 69 72 75
144 114 96 81 76 76 75 30
considerably. The effect of H + was then examined in solutions containing both HC1 and CaCI2, keeping [CI-] constant (Table I). The yield of propane decreased and leveled off a t 52% of the value in neutral solution. That is, only 48% of the precursor of propane was scavengable. These results are fairly well described by eq A with Go = 70, in arbitrary units. Since ~3 = 0 in this system the ratio Kl(es- 4- H+)/ C3H7CI) = 42 can be evaluated. The Kz(escorresponding ratio in water is -40. The limiting yield a t high acid concentrations was further reduced to 20% of the initial value by addition of 1 M benzene, the AG(C3Hs) being slightly less than that for 1 M benzene in neutral solution. The results for cyclopentyl bromide, which proved to be more satisfactory than isopropyl chloride for analytical reasons, are in good qualitative agreement with the earlier data and quantitatively somewhat more precise. These data will be presented in the format of competition kinetics using eq A. The observed 100eV yield of c-pentane in neutral 0.1 M c-C6H9Br was 1.32 but AG = 1.40 - G(c-CaHlo)fits eq A better for acidic solutions and has been used in Figure 1. The intercept gives Go = 1.02 or 73% of the precursor of C5Hlo is scavengable by H+, about the same as the result using propyl chloride. From the slope, 0.48, one finds Kl(esH+)/Kz(e,C5H9Br) = 2.1 provided Ka = 0. For aqueous butyl bromide the corresponding ratio of rate constants is 2.0.0 The dependence of the yield of c-pentane from 0.1 M c-pentyl bromide on the concentration of benzene appears in Figure 2 for a best fit AG(CbH10) = 1.45 G(C.&,). From the intercept we find Go = 1.10 for the precursor. From the slope (KZ[C~H&]4- K3)/K1
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IONIC EVENTS IN THE RADIOLYSIS OF ETHANOL
I
I
I
ob
*
I
d
I
4
l/[flM'
Figure 1. The dependence of G(c-C;Hlo) from c-C,jHBBrin ethanol as a function of the concentration of HC1 at constant [Cl-1.
I-
o
I
0
I
2
I
I
4
I/
I
I
6
I
I
8
I
10
LYl
Figure 2. T h e dependence of G(c-C;Hlo) from c-CsHsBr in ethanol as a function of the concentration of CaHe, with [cl-] = 1 M .
= 0.12. The corresponding result for propyl chloride (both halides 0.1 M ) and benzene is 0.36.
Discussion There appear to be two principal precursors of alkane from alkyl halide in the solutions examined, benzene being nearly specific for one, acid for the other. It has been shown that benzene traps mobile electrons in methanol a t 77"K, the product observed being CeH7. It is not produced by scavenging H atoms but is produced from C6H6-by proton abstraction from the solvent.8 It was shown that acid had no effect on the yield of C6H7 and very little on the yield of CeH60H from phenoL4 The facts reported here follow the same pattern, and they cannot be accommodated by the conventional description of ionic processes in polar media. Alkyl chlorides and bromides in 0.1 M cyclohexane solution appear to generate alkyl radicals exclusively by electron capture, as shown by the recent work of Warman, et al.9 This applies equally for reactions of benzyl acetate with electrons in ethanol.1° If substantially all R H from R X arises from dissociative electron attachment, then there appear to be two kinds
2887
of electrons. One of these is characterized by reacting efficiently with H,+ and RX and is the solvated electron, e,-, The residual yield of RH which remains a t highacid concentrations can be attributed to another population of electrons which still reacts with R X and which can be scavenged by benzene. They behave the same as electrons in alkane liquids or glassy solids and are considered to be dry electrons, e-. I n the absence of benzene, 0.1 M c-pentyl bromide produced 1.45 c-pentane per 100 eV. The extrapolated yield of c-pentane precursors susceptible to scavenging by benzene was Go = 1.10. Since the rate constant for scavenging e,- by the halide is -lo3 larger than that for benzene it is concluded that benzene scavenges e-. Since e- is presumably the precursor of e,- it might be expected that in the limit of high benzene concentration G(c-C6Hlo)would approach zero, or Go = 1.45, rather than 1.10. Since the extrapolated value was appreciably smaller it may be that the trial function (A) is inadequate, although it has been satisfactory for a considerable number of aqueous system^.^^^ Alternatively, there may be a small yield of solvated electrons produced as such from an excited state below the ionization potential of ethanol, e.g., Rydberg states. The interpretation of results for systems containing benzene would be complicated if alkyl radicals were scavenged by benzene, rather than abstrpting H from ethanol. This appears to be very unlikely. The thermal decomposition of biacetyl peroxide in toluene solution produces C2H.5, CH,, and COZ with (2CzH6 CH4)/C02 = 0.98, the alkanes having only CHs for precursor." The rate constant for H abstraction by CH, is about the same for toluene and ethanol.12 Since [C2H60H]/[C&&] 5 17 in the present work, addition of radicals to benzene is not to be expected. Neither was there analytical evidence of such products. The lifetime 7 of e- in a liquid alkane is limited by the rate of recombination, K ~ . In a polar medium it depends also on the rate of solvation and T = ( K ~ ~ , ) - - 1 , A rather rough estimate of the relative rates is provided by G,-/Gs- = K , / K , , about 3 in ethanol and 0.5 in water. That is, T is about as great in water as in an alkane, in qualitative agreement with the higher concentrations required to scavenge e- in water. To trap 50% of the dry electrons in water requires -0.4 M phenoL4 If the thermalized dry electron reacts upon encounter with a molecule of phenol, then on the average e- sweeps out a volume occupied by -0.4 X 55 or -22 HzO, or its mean path is -70 A in length. At
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(9) J. M.Warman, K. D. Asrnus, and R. H. Schuler, J . Phys. Chem., 7 3 , 931 (1969). (10) J. A. Ward and W. H. Hamill, J . Amer. Chem. SOC.,87, 1853 (1965). (11) A. Rembaum and M. Szwarc, ibid., 77, 3486 (1955). (12) A. F.Trotman-Dickenson and E. W. R. Steacie, J . Chem. Phys. 19, 329 (1951). The Journal of Physical ChemCdry, Vol. 74, No. 16,1970
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 see, but the microscopic Mozumder to be 4 X 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.
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(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).
