REACTIONS IN
THE
IRRADIATION OF WATERVAPOR
such an exercise. We have found, however, that the experimental relaxation time does decrease significantly with rising temperature (see Table 11). This
2775
fact is consistent with our assignment of the measured D F E to equilibrium 1 since the concentration of U02OH+ increases with t e m p e r a t ~ r e . ~ , ~
Reactions of Electrons and Hydrogen Atoms with Oxygen and Methyl Bromide in +radiated
Water Vapor
by G. R. A. Johnson and M. Simic Laboratory of Radiation Chemistry, School of Chemistry, The University, .l’ewcastle u p o n T y n e , England Accepted and Transmitted by The Faraday Society
~
(December 7 , 1966)
~~~
Oxygen and methyl bromide react efficiently with the electrons and hydrogen atoms produced by irradiation of water vapor-propane mixtures. I n the D 2 0 C3H8 0 2 system, the dependence of the H D and H2 yields on the ratio Oz/C3H8is consistent with a simple competition of 0, and C8H8for the D and H atoms produced radiolytically from D20 and = 105 f 10 (11 = CaHs, respectively. The rate-constant ratio [~I]IC~D+~*+J~)/~(D+C~H~) D 2 0 , 140”). The same value, within experimental error, was obtained for the corresponding ratio for H atoms. The D atom yield, extrapolated to zero concentrations of O2 and C3H8 (G(D),O = 5.5 0.5)) is consistent with efficient electron capture by 0 2 , which prevents the formation of D atoms by hydronium ion neutralization. I n the system DzO C3H8 -I- CH3Br, the dependence of the H D and CH3D yields on the ratio CH3Br/ C3H8has been studied. It is concluded that the reaction, D CH3Br + CH3D Br, competes with the dehydrogenation of C3HRby D atoms and that G(D),..! = 4.6 f 0.4 and JC(D+CH~B~)/~C(D+C~H~) = 2.9. It is suggested that electron capture by CH3Br gives CH3Br- and that neutralization involves reaction of this negative ion with D 3 0 + to give CH3D as a product. The formation of CH3D by this process is eliminated by SFs and by 02,which are more efficient electron scavengers than CH3Br.
+
+
Introduction I t has previously been shown that in the radiolysis of mater vapor, the formation of hydrogen atoms can be attributed to a t least two different One of these involves hydronium ion-electron neutralization; the other does not involve hydronium ions. X20 and SF6,which are known to be capable of capturing electrons to form negative ions, completely supress the formation of hydrogen atoms from hydronium ions. I t ha5 been concluded that neutralization of hydronium ions by the negative ions from these compounds does
+
+
+
not involve simple electron transfer, but that atomic rearrangement also O C C U ~ We S . ~ have now investigated the neutralization of hydronium ions by the negative ions formed when oxygen and methyl bromide are used as electron scavengers. These compounds have previously been shown to be effective as electron scavengers in the radiolysis of methanol vapor.4 We (1) J. H. Bxxendale and G. P. Gilbert, Sciencr, 147, 1571 (1965). (2) G. R. A. Johnson and M. Simic, Nature, 210, 1356 (1966). (3) G. R. A . Johnson and hi. Simic, J . Phys. Chem., 71, 1118 (1967).
Volume 7 1 , Xumber 9
August 1967
G. R. A. JOHNSON AND 11.SIMIC
2776
have also investigated the reactions of the radiationproduced hydrogen atoms with these scavengers. The reaction of hydrogen atoms with methyl bromide has not previously been studied in detail, and relatively little is known about this reactio~i.~ Experimental Section Gaseous mixtures were irradiated in Pyrex vessels (400 ml) with 6oCo y rays, and the noricondensable gaseous products mere collected and analyzed mass spectrometrically (A.E.I., JIS3). The experimental arrangements have been d e ~ c r i b e d . ~ Oxygen (British Oxygen Gas, medical grade) and methyl bromide (Ahtheson) were condensed at 77°K and distilled several times before use. DzO (isotopic purity 99.Sa/,) was distilled from alkaline I:
0
+
+
+
+
+ OD D20 -+ D20+ + eDz0+ + D20 +D30+ + OD R H + + DzO +HDzO+ + R HDzO + DzO D30+ + HOD D30++ e- +D + D20 D20 w+ D
(1) (2)
(3) (4)
(3 (6)
(Reaction 1 represents the formation of D atoms by all processes other than hydronium ion neutralization. RH+ represents the stable positive ions formed as a result of energy a,bsorption by C3H.s.) It is assumed that every D atom dehydrogenates propane
+ C3Hs
4H D
+
Figure 2. 6oCoy radiolysis of D2O CH3Br C3H8 at -140'. Dependence of yield on the ratio [CH3Br]/(C3H8]: 0,G(HD); 0 , G(CH3D). Filled points are with SFe (-0.1 mole %); lines calculated as described in Discussion.
G(D),O = 0) and the yield from DzO is depressed to G(D),O = G(D)residual= 4.9 f 0.4. The depression by XzO or SF, corresponds to the yield from hydronium ion electron neutralization, G(D), = G(D)hydronium = 2.7 =k O.S. Radiolysis of C3Hs, in the absence of water vapor, gives H2 by three different processes:' (i) the reaction of H atoms produced by neutralization of positive ions by electrons; (ii) the reaction of H atoms produced by processes other than positive ion neutralization; and (iii) directly as molecular Hz, by processes other than positive ion neutralization. It is probable that processes ii and iii originate from electronically excited C3HS molecules. In the presence of water vapor (or of certain electron scavengers) it has been found that the first process is eliminated and the residual yield, G(H2),0= G(H),O G H ~= 6.3, where G(H),O and G H ~ are the yields from processes ii and iii, respectively. Oxygen as Scavenge?. The effect of O2 on G(HD)w can be explained if it is assumed that, at all concentrations (Le., at least down to 0.1 mole 70), O2 scavenges all of the electrons and thereby prevents the formation of D atoms from D3Of. Electron capture by 0 2 , with electrons of low energy (
