1738
G. SCHOLES AND XI. SIimc
Reactivity of the Hydrogen Atoms Produced in the Radiolysis of Aqueous Systems
by G. Scholes and M. Simic Laboratory of Radiation Chemistry, School of Chemistry, The Uniaersity, Newcastle u p o n Tune, 1 , England (Receined Yoeember 11 1963) ~
Neutral aqueous solutions of DCOONa containing YZO and one of several organic solutes which can be dehydrogenated have been irradiated with Co60 y-rays. The yields of H2 and of H D have been measured and used to obtain the relative reactivities of these solutes toward the primarily formed dehydrogenating species (hydrogen atoms). The relative reactivities of solutes, organic and inorganic, which are not dehydrogenated by hydrogen atonis have been studied using the system of 2-propanol and NzO.
Introduction
Experimental
The existence of two primary reducing species, formed by the action of ionizing radiations on neutral aqueous systems, was demonstrated by Allan and Scholes’ in studies of the ?-radiolysis of aqueous 2propanol-acetone solutions. One of these reacted with acetone so as not to yield hydrogen and was assumed to be an electron. The other species was capable of dehydrogenating 2-propanol and was postulated to be a hydrogen atom. The presence of these two species was also evident2 in irradiated aqueous solutions of different alcohols in the presence of COZ. These conclusions were fully confirmed by Rabani3 and by Rabani and Stein,4 who measured the hydrogen yields from various aquo-organic systems irradiated with 200-kv. X-rays, using several different electron scavengers, e . g . , bicarbonate, acetone, ferricyanide, and nitrite. The data of Hayon and Allen5 obtained from the yradiolysis of neutral deaerated chloroacetate solutions have also been interpreted on the basis of the formation of some hydrogen atoms. The identification of the dehydrogenating species as a hydrogen atom appears to be supported by kinetic evidence4 and by its reaction with OHThe independently-produced dehydrogenating species observed in the radiolysis of neutral aqueous system is designated as Ha. The radiation-produced electrons (negative polaronss) are represented by (HzO)-. In this paper the reactivity of Hawith various solutes has been investigated.
Coeo y-rays were used, the dose rate (2.4 X e.v./ml. min.-l) being determined by the Fricke dosimeter, taking G(Fe3+) = 15.5. Methods for the purification of KZO, the preparation of solutions containing NZO, and gas analysis for Hz and XZ by mass spectrometry have already been described.9 The relative sensitivities, in the mass spectrometer, of H2 and H D mere determined by J l r . P. Kelly. The gas yields are estimated to be accurate to &3%. Sodium deuterioformate, DCOONa, was obtained from nlerck, Sharp, and Dohme of Canada Ltd. and was stated to have minimum isotopic purity in the labeled position of 98 atom %. Sodium formate was recrystallized several times from mater. Wherever possible A.R. grade materials were used. “Spectrosol” 2-propanol was used without further purification. Methanol (A.R.), acetone (A.R.), t-butyl alcohol, allyl alcohol, ethylene glycol, and benzyl alco-
T h e Journal of Physical Chemistry
(1) J. T. Allan and G. Scholes, iVature, 187, 218 (1960).
(2) J. T. Allan, N. Getoff, H. P. Lehmann, K. E . Nixon, G. Scholes, and M. Simic, J . Inorg. Nucl. Chem., 19, 204 (1961). (3) J. Rabani, J . Am. Chern. Soc., 84,868 (1962). (4) J. Rabani and G. Stein, J . Chem. Phys., 37, 1867 (1962). (5) E. Hayon and A. 0. Allen, J . Phys. Chem., 65, 2181 (1961). (6) 3. T. Allan, M . C. Robinson, and G. Scholes, Proc. Chem. Soc., 381 (1962). ( 7 ) hI. S. Matheson and J. Rabani, Radiation Res., 19, 180 (1963). (8) J. Weiss, Nature, 186, 751 (1960). (9) G. Scholes and M.Simic, J . Phys. Chem., 68, 1731 (1964).
1739
REACTIVITY OF' HYDROGEN ATOMSIN RADIOLYSIS OF AQUEOUS SYSTEMS
-
hol were fractionally distilled prior to use. Solutions of sodium glyoxalate were prepared by neutralization of glyoxylic acid with sodium bicarbonate. All solutions were made up in triply-distilled water, the pH of which was usually in the range p H 5.3-5.7. Results and Discussion 1 . Ezperimmts with Solutions Containing DCOONa. In the ?-radiolysis of deaerated solutions of sodium deuterioformate containing another organic solute (RH) which can be dehydrogenated by hydrogen atoms, the ratio G(HD)/G(H,) will be governed by the competing reactions
H
yields of the gaseous products from these systems; in Table I, the G-values corresponding to a total dose of 1.5 X lo1*e.v./ml. are given. It can be seen (Table I) that the gas yields are also concentration dependent, decreasing with increasing solute concentration. Such an effect can be explained if formate reacts with (H2O)- so as not to yield hydrogen, lo,ll possibly according to
HCOO-.
+ (H2O)- +HCO + 20H-
and if this latter reaction is in competition with the process12v13 (Hz0)-
+ DCOO- +H D + COOH + KH Hz + 12.
(1)
(3)
--+
H
+ OH-
(4)
The nonlinearity of the yield-dose plots is then pre-
(2)
--+
If the experimental conditions are such that Ha is the only dehydrogenating species entering the above competition, measurement of the yields of H D and of Hz should lead directly to the rate ratio k ( ~ +aDCOO-)/ k ( H a + RH). To find the most favorable conditions for an investigation along these lines experilments were first carried out with solutions containing only HCOOSa or DCOONa. Neutral deaerated solutions of these compounds were irradiated with Co60 y-rays and the yields of Hz and of H D determined as a function of radiation dose. It mas found that the yields of Hz from HCOONa solutions and of H D from DCOOSa solutions were nonlinear with dose. 'The formation of Hz from irradiated DCOONa solutions was, within experimental error, linear with dose. Figure 1 shows some typical yield-dose plots from DCOOSa solutions. No attempt has been made to determine the initial Table I : Effect of' Solute Concentration on the Yields of Hz and HD in the -,-Radiolysis of Aqueous Solutions of DCOONa and of HCOONa, pH -7 Solutions of DCOONa Yield, @------
[DCOONa], M
w--
10-3
10-2
H2
HD
0 56 0 51
1 25 1.05
Solutions of HCOONa [HCOONa],
a
,n
Q(Hd"
10-3 10 - 2 3 x 10-9 10-1 5 x la-'
2 02
After a total dose of 1.5 X 10'8 e.v./ml.
1 50 1 40 1 25 1 05
0
I
I
I
1
2
3
Dose, e.v./ml. X
10-18.
..
Figure 1. Formation of H, and HD on y-irradiation of aqueous deaerated solutions of DCOOXa. [DCOO?;a] M, H,, 0; HD, 0. [DCOONa] M, H,, 0 ; HD, (10) D. Smithies and E. J. Hart, J . Am. Chem. SOC.,8 2 , 4775 (1960). (11) G. Csapski, J . Rabani, and G. Stein, Trans. Faraday SOC.,58, 2160 (1962). (12) G. Czapski and H. A. Schwars, J . Phys. Chem., 66, 471 (1962). (13) J. Jortner, AT. Ottolenghi, J. Rabani, and G. Stein, J . Chem. Phys., 37, 2488 (1962).
Volume 68, .\'umber
7
J u l y , 1064
G. SCHOLES AKD $1. SIMIC
1740
sumably due, as in the radiolysis of neutral deaerated solutions of various other organic solutes,' to a reaction of (H20)- with the products of irradiation (oxalate, HzOz) thus decreasing the extent of reaction 4. That oxalate can scavenge (HzO)--as well as Ha-is evident from the observation that irradiation of a solution of M DCOOSa containing 10-I M KZCz0, gave the values, G(HD) = 0.28 and G(Hs) = 0.48. To study the competitive reactions 1 and 2 involving only Ha, it is necessary to suppress reaction 4 by a suitable electron scavenger. It is known14 that bicarbonate is an electron scavenger, its effectiveness being associated with the COZ c ~ n t e n t . ~Irradiation ,~~ of a solution of DCOONa (low3M ) and SaHC03 (lo-' M ) gave the values, G(Hz) = 0.49 and G(HD) = 0.57 indicating effective suppression of reaction 4. Rabani and Stein3,*have, in fact, used bicarbonate in several systems in order to measure the reactivity of Hatowards some solutes. In the present studies using DCOOXa solutions, however, XZO was selected as the electron scavenger. This substance has a relatively high reactivity toward (HzO)- leading to the formation of nitrogen,16a reaction which may be represented by the over-all equation
XzO
+ (Hz0)- --+
Nz
+ OH + OH-
(5)
without specifying the nature of any possible intermediate. On the other hand NzO has a low reactivity toward hydrogen atoms.l7 The radiation chemistry of E20 solutions containing various organic and inorganic solutes has also recently been investigated in some detail.g In all of the following experiments the concentration of S20 was 1.6 X 10-2 M . Solutions were irradiated with doses up to 3 x 1018 e.v./ml. and the yields of Hz and H D determined from the linear yield-dose plots.
Addition of other organic solutes (RH) to the DCOOSa-NzO system led to the Hz and H D yields given to Table 111. In these experiments the ratio of the concentrations of DCOONa to R H was varied, depending upon the relative reactivity of Ha toward the two solutes. On the basis of the simple competition reactions 1 and 2 it follows that G ~ h[RH] _z -- G(Hz) - G H , GI G(HD) kl [DCOO-]
(1)
where G(Hz) and G(HD) are the measured yields of Hz and HD, and G H , is ~ the molecular yield of hydrogen under the conditions of the experiment. I t has been shown previously9 that the molecular hydrogen yield in solution containing 1.6 X lo-* M K20 has a G-value of 0.36. However, the yields of HP from DCOO--K,O solutions (Table 11) are slightly higher than this. Likewise, G(Hz) from solutions of DCOO(Table I) are higher than would be anticipated since in the absence of NzO, G H =~ 0.45. ~ These higher yields of Hz are due to isotopic contamination of the DCOONa with HCOOXa. To calculate the rate ratios IC(H + R H ) / k ( +~DCOO -1 from the data of Table I11 correction must first be made for this contamination, Thus, in addition to reactions 1 and 2 there will also be some reaction of Ha with HCOO-, uiz.
H
+ HCOO-
+ Hz
+ COO-
(6)
The situation is then expressed by the equation
+
G(Hz) - G H , ~ kz[RH] ke[HCOO-] G(HD) k,[DCOO-]
kz [RHI kl[DCOO-]
+
ks[HCOO-] kl[DCOO-l
so that Table 11: Initial Yields of the Gaseous Products in the ?-Radiolysis of Aqueous Solutions of DCOOXa in the Presence of NzO (1.6 X 10-2 AI'); pH -7
__-_ Ha
Solutions
10-3 M DCOONa 10-l M DCOONa
0.42 0.42
Yield, HD
e------
0 47 0 51
Na
3.62 3 73
Table I1 gives the gaseous-product yields from jrradiated DC00?;a-K20 solutions. The observed yields of nitrogen, also included in Table 11, are much higher than would be expected from scavenging according to reaction 5 and are probably due to interaction of the formate radical with SZO, leading to further quantities of nitrogen.g The Journal of Physical Chemistry
"[ kl
G(Hz) - 0.36 - ke[HCOO-] kl[DCOO-]] [DCOO-1 [RH] G(HD)
(11)
In eq. I1 the term k6[HC00-]/kl(DC00-] will be constant for the various systems examined, and its magnitude has been arrived a t as follows: the ratio k6/k1 is obtained directly from the gaseous-product yields on irradiation of solutions containing DCOOKaNzO-HCOONa, since here the isotopic impurity effect (14) G. Scholes, M.Simic, and J. Weiss, Nature, 188, 1019 (1960). (15) A. Appleby. unpublished results. (16) F. S. Dainton and D. B. Peterson, "lature, 186, 878 (1960); Proc. Roy. Soc. (London), A267, 443 (1962). (17) G. Caapski and J. Jortner, Nature, 188, 50 (1960).
1741
REACTIVITY OF HYDROGEN ATOMSIN RADIOLYSIS OF AQUEOUS SYSTEMS
Table I11 : Initial Yields of H2 and H D in the -,-Radiolysis of Aqueous Solutiona M ; p H -7 of DCOONa-N20 Containing Other Solutes (RH), [NzO] = 1.6 X
-RH
10-2 10-2
10-2 10-2 10-2 10-2 10-2 10-2 10-
HCOONa 2-Propanol Sodium glyoxalate 2-Deoxy-~-ribose Ethanol Ethylene glycol Sodium chloroacetate Methanol Acetone Sodium acetate &Butyl alcohol Ammonium sulfate
5 x 10-3 5 x 16)-3 3 x 10-3 2 x 10-2 2 x 10-8 10-2 2 x 10-3 2 x 10-8 10-8
lID-1 1iD-I
10-1 10 -1 10-1
10-8
is essentially "swamped out" and eq. I can be used. The experimental results (Table 111) lead to the value ka/lcl = 6.6 f 0.7. 'The extent of islotopic impurity in the DCOONa can then be arrived at from the yields of H, in the radiolysis of solutions of (DCOONa NzO) given in Table 11; the calculated values of [HCOO-]/ [DCOO-] are 0.0178 and 0.0194 from1 the irradiation of solutions containing 10"' M DCOONa and M DCOOKa, respectively. Using the average value of 0.0186 the constant term in eq. I1 has been taken as 0.12. The relative rate ratios kz/kl for the different solutes are given in the last column of Table 111, the quoted errors corresponding to an experimental error of *3yo in the yields of Hz and HD. It should be pointed out that there is some evidenceg to suggest that the reaction of Ha with formate does not only lead to dehyldrogenation but, also to a hydrogen atom adduct; similarly, other ijolutes (e.g., acetone,18 chloroacetate6Jg) are known to react with hy-
+
Table IV: Yields of Hz and H D on the -,-Radiolysis of Deaerated Aqueous Solutions of DCOONa (10-2 M ) Containing Other Solutes ( R H ) , p H -7
RH
HCOONa Methanol 2-Propanol
[RH], M
10-l
--Yield, H2
1.36 1.04 0.91
0-
k H + RH)
HD
k(H+ DCOO-)
0.14 0.78 0.22
6.4&00.5 6 . 4 f 0 . 7 x 10-3 2 . 0 rt 0 . 2
drogen atoms in more than one way. Hence the Gvalues of HZ and H D are only a measure of the extents of the dehydrogenatioin reactions 1 and 2 and not one
%(H+ RH)
Yield, 0---
Ha
HD
0.89 0.81 0.74 0.62 0.73 0.84 0.57 0.67 0.66 0.59 0.54 0.43
0.08 0.19 0 .I 1 0.12 0.16 0.21 0.39 0.36 0.20 0.34 0.32 0.27
k(H+ DCOO-)
6 . 6 i0 . 7 2.25 f 0.2 1.6 f0.1 1.0 f0.1 6.6 i1.0 X 4.3 f0.5 x 8.4 f 1.2 X 7.4 f0 . 8 x 2.8 & 0 . 3 x 1 . 2 f0 . 1 x 4 . 4 i0 . 7 X C=C< and >C=Nbonds in pyridine and adenine), the reactivity is somewhat less. Cupric sulfate and some of the solutes in Table VI lower the yields of nitrogen in irradiated NzO solutions due to competition between these substances and NzO for (HzO)-. As already discussedg such experiments are useful for the determination of the relative reactivities of various solutes toward (HA))-. Thus, the
effect of cupric ion concentration on the yields of nitrogen in 2-propanol-NzO solutions given in Table V is due to the process CU'+
+ (Hz0)- +CU+ + HzO
(9)
competing with reaction 5, and the data follow simple competition kinetics leading to a value ~ L ( H , o ) - + ~ ~ ~ + 1 / ~ ~ ( H , o ) - + N , o I= 4.7 f 0.4. This compares quite well with the corresponding ratio of 3.85 f 0.3 obtained from the radiolysis of SZO-CUSO~solutions. Some of these 2-propanol-K20-X systems are therefore of interest in that the reactivities of both Ha and (HzO)with X can be studied.
Acknowledgment. Thanks are due to Professor J. J. Weiss for his support of this work and to Mr. P. Kelly for mass spectrometry.
Volume 68, Number 7
J u l y , 1964
