1713
EFFECT OF TRANSITION METALIONS ON YIELDOF “RESIDUAL HYDROGEN”
The Effect of Transition Metal Ions on the Yield of “Residual Hydrogen77in Neutral Radiolysis Solutions
by M. Anbar and D. Meyerstein The Weizmann Institute of Science and the Soreq Research Establishment, Rehovoth, Ierael (Received September 10, 1969)
2-d-2-Propanol has been irradiated in dilute neutral aqueous solutions in the presence of acetone and the yield of H D determined. H D is formed by abstraction of the a-hydrogen of 2-propanol by the “residual hydrogen.” In acetone-free acid solutions H D is formed by abstraction of the a-hydrogen of 2-propanol by hydrogen atoms. C a f 2 and Ba+2ions have no effect on the yield of H D in either unbuffered or acidified solutions. On the other hand, in unbuffered solutions, certain transition metal ions diminish the yield of H D in the following order: Mf2, Cd+2, Cof2, Zn+2. The same ions were examined in acid solution, where hydrogen atoms are present, and were found to have no effect on the yield of HD. I n other words, these metal ions, which were found not to react with hydrogen atoms, do react with the “residual hydrogen” or with its precursor. It is suggested that the “residual hydrogen” or its precursor is actually an excited water molecular, HzO*, which reacts with these metal ions as a reductant in analogy to their reaction with hydrated electrons.
In the past few years attention hlas been drawn to a species formed in the radiolysis of neutral aqueous solutions, which abtstracts hydrogen from various organic solutes similarly to hydrogein atoms.1-8 This hydrogen-like species, the “residual hydrogen, ” was claimed to be absent in oxygen-saturated solution^,^ where the yield of OH radicals was also found to be diminished.’Onll T h e “residual hydrogen” was shown to be unaffected by electron scavengeri~,’-~1~1~ unlike the precursors of hydrogen atoms in acid solutions.12 It was suggested that excited water molecules are the precursors of the “residual h y d r ~ & ; e n . ” ~ JExcited t~ water molecules were postula,ted in two recent studies, to account for the increased radical yield in acid and in alkaline solutions.’31’4 In another investigation it was suggested that excited water molecules oxidize fluoride ions to fluorine atoms.’6 It occurred to us that there is no available experimental evidence that the “residual hydrogen” is actually a hydrogen atom rather than an excited water molecule. The present study was intended to examine this point.
Experimental Solutions (0.04 M ) of 2-d-2-propanol (>99% deute-
rium, Merck Sharp and Dohnie, Ltd.) containing 0.05 M acetone (Baker analyzed) in triple-distilled water, were irradiated under argon (200 mm.),by Co6O y-rays (dose rate 11,800 rads/min.). Argon was introduced to facilitate the mass-spectrometric measurement of the isotopic composition of hydrogen.16 Var(1) J. T. Allan and G. Scholes, Nature, 187, 218 (1960).
(2) J. Rabani, J . A m . Chem. SOC.,84, 868 (1962). (3) E. Hayon and A. 0. Allen, J . Phys. Chem., 65, 2181 (1961). (4) C. Lifshitz, Can. J . Chem., 40, 1903 (1962). (5) G. Scholes and M. Simio, Nature, 199,276 (1963). (6) J. Rabani and G. Stein, J. Chem. Phys., 37, 1865 (1962). (7) S. Nehari and J. Rabani, J . Phys. Chem., 67, 1609 (1963). (8) E. Hayon, Nature, 196,533 (1962). (9) G. Czapski and A. 0. Allen,
J. Phys. Chem., 66, 262
(1962).
(10) E. J. Hart and R. L.Platzman, “Merhanism in Radiobiology,” Vol. 1, M. Errera and A. Forssberg, Ed., Academic Press, New York, N. Y . , 1961, p. 188. (11) M. Anbar, R. Munoz, and P. Rona, J . Phys. Chem., 67, 2708 (1963). (12) M. S.Matheson, Ann. Rev. Phys. Chem., 13, 77 (1962). (13) F. S. Dainton and D. B. Peterson, Proc. Roy. Soc. (London), A267, 443 (1962). (14) F. 9. Dainton and W. S. Watt, Nature, 195, 1294 (1962). (15) M. Anbar and D. Meyerstein, Israel AEC Report IA-851 (1963).
Volume 68, Number 7
J u l y , 1964
M. ANBARAND D. MEYERSTEIN
1714
ious inorganic salts (all of A.R. grade) were added to the irradiated solutions. No further reagent was added to adjust the pH. The pH of the solutions was determined using a Metrohm Kompensator Type E 148 C, with an accuracy of 0.05 pH unit. The sealed irradiation vessels contained a glass-coated magnetic stirrer to facilitate the complete degassing of the solutions. The stirrer was removed from the solution and stored in a side arm during the irradiation. The side arm ended with a break-off tip for subsequent gas analysis. The irradiation vessels were cleaned by boiling nitric acid, rinsed with triple-distilled water, and dried in a vacuum oven a t l l O o . After irradiation (total dose 2.0 Mrads) the gases were introduced into a mass spectrometer (CEC Model 21-401) and the masses, 2 , 3, and 4 were determined, as well as masses 28 and 32-40 check on air contamination. The _ _masses _ - - 2,- -3,- and 4 were measured in the sequence 2,3,4,3,2,3,4,3,2. (2 X X))/(2- 3) was calculated The ratio R = (3 from the mean values obtained from the sequence of measurements. The R values obtained showed a standard deviation of ~ 4 7 ’ ~ .
+
The effect of various cations on the yield of the abstracted HD as expressed in terms of R is summarized in Table I. It can be seen that R increases from 0.58 in neutral solution in the presence of acetone to 2.0 in acid solution in its absence, owing to the formation of hydrogen atoms. A G((cresidual hydrogen”) of 0.49 is calculated from R = 0.58, and from the total hydrogen yield in neutral solutions = 0.95,5*19 taking into account that 28% of the hydrogen abstracted from 2-d2-propanol is Hz.l8 Table I: The Formation of HD from the Radiolysis of 2-d-2-Propanol in Water: 2-d-2-Propanol = 4 X M; Acetone = 5 X M ; Total Dose 1.2 X lozoe.v./g.
+
[Additive] X 102 M
pH
...
6.0 3.8 0.9“
0.01 10
1.1“
2.00 1.80
0.ga 4.7 4.3 5.2 4.9 3.5 4.0
2.09 0.57 0.48 0.39 0.41 0.47 0.45
1.0
1.0 2.0 10 10
+ 10
+ 10
When aliphatic alcohols are irradiated in dilute aqueous solutions, hydrogen is evolved in a yield equal to the sum of the molecular hydrogen, the radical hydrogen, and the “residual hydrogen” yields, G(H2) = GBn GE G H ~ .Using ~ deuterium-labeled ethanol, it was shown that the hydrogen originating from hydrogen atoms is formed by hydrogen abstraction from the alcohol.17 It has been further demonstrated that 72% of the hydrogen abstracted from 2-d-2-propanol in neutral solutions, originates from position 2 (the a-hydrogen) .Is We have, therefore, taken the yield of HD from solutions of 2-d-2-propanol as a measure of the yield of the [‘residual hydrogen” in neutral solution. The ratio ( R ) of HD to Hz formed by radiolysis, after correcting for isotopic equilibration, was taken as the measured variable in this work. In order to avoid the eventual formation of hydrogen atoms from hydrated electrons, an efficient electron scavenger has to be added to the solution. Acetone (0.05 M ) was added, as this reagent reacts with electrons a t a very high rate. The specific rate of reaction of electrons with acetone is 0.3 of that with HsO+,6 in other words, a t 0.05 M acetone, virtually no hydrogen atoms may be formed from electrons down to pH 3.0. On the other hand, it was shown that acetone reacts with the “residual hydrogen” a t a relatively low rate, thus it does hardly interfere with the hydrogen abstraction from 2-propanol. 1 ~ 7 ~ ~ The Journal of Physical Chemiatrpl
2.02 2.05
1.0
1.0 1.0
’
0.60
0.9” 0.9”
10
+
0.58
+ 10 1 . 0 + 10 1.0
Results and Discussion
+
HD 2Da R = Hz - Ds
10 10 10
3.2 4.3
0.21
4.1 4.0
0.20 0.60
10
6.0
0.57
0.36
Xckel, cobalt, and calcium ions in acid solutions have no effect on R. R was slightly diminished in the presence of cadmium ions in acid solution, probably because they compete with HaO+ for electrons.20 In unbuffered solutions a t pH 3 zinc, cobalt, cadmium, nickel, and manganous ions a t M concentration diminish R. This effect is even more pronounced a t 10-1 M concentration. On the cther hand, 10-1 M calcium and barium ions have no effect on R in either unbuffered or in acidified solutions. (16) M. Anbar and D. Meyerstein, Israel AEC Semiannual Report, IA-900 (1963). (17) C. Lifshitz and G. Stein, J . Chem. Soc., 3706 (1962). (18) M. Anbar and D. Meyerstein, Israel AEC Report, IA-902 (1963). (19) J. T. Allan, M.G. Robinson, and G. Soholes, Proc. Chem. SOC.; 381 (1962). (20) J. H. Baxendale and R. S. Dixon, ibid., 148 (1963).
EFFECT O F TRASSITION RfETAL
I O K S ON
YIELD
O F “RESIDUAL
Xckel and cadmium ions are the most effective of the ions examined, whereas zinc ions have the smallest effect on the yield of HD. It has been shown that the effective yield of “residual hydrogen” is suppressed by certain cations of the transition metals. On the other hand, these cations do not interact with hydrogen atoms. This is in accord with previous findings that certain cations including zinc, cadmium, cobalt, and nickel which react with hydrated electrons, do not react with hydrogen atoms.20 It may be thus concluded that the “residual hydrogen” is chemically different than the hydrogen atom. Alternatively, the given metal ions may interact with the precursors of the hydrogen atoms which constitute the “residual hydrogen.” we have shown that nitrate In a parallel ions do not compete with 2-propanol for hydrogen atoms in acid solution. On the other hand, nitrate ions were found to react with ‘‘residual hydrogen” with a rate comparable to that of f!-propanol.21 The latter finding is corroborated by the results of Nehari and Rabani.? It was further shownz1that iodide and bromide ions diminish the yield of HD from dilute 2-d-2-propanol solutions. As these reagents do not react with electrons at any appreciable rate, we may exclude the possibility that the “residual hydrogen” is a “molecular product” formed from eaq- H + in the spurs. A critical review of previous studies points out differences between the chemical behavior of hydrogen atoms and the ‘(residual hydrogen,” which have been overlooked. It has been found that the rate of reaction between hydrogen atoms and &of is extremely slow, namely, 2 x lo2 sec.-1.22,23 On the other hand, the rate of reaction of the “residual hydrogen” with H30+ may be estimated from the effect of acidity on the formation of N2 under XZOl3to be faster than lo4 M-I sec.-l and if, is most probably of the order of 106 M-’ ~ e c . - ~ . Further, ‘~ the specific rate of interaction between hydrogen atoms and hydrogen paoxide was found to be of the order 106 M-1 sec.-1.9 When the interaction of hydrogen peroxide with hydrogen was followed in neutral solutions, namlely with “residua1 hydrogen,” a specific rate constant of the order of lo8 M-‘ see.-‘ was derived.24 A third system in which the relative rates of reaction of hydrogen atoms and “residual hydrogen” with a series of substrates was studied, is their interaction with alcohols. The relative rate of interaction of “residual hydrogen” with methanol, ethanol, and 2-propanol may be calculated from the rates of their competition with OH- ions.lg Taking GEa = 0.39,6 one obtains the relative reactivities of “residual hydro-
+
1715
HYDROGER’”
gen” with methanol :ethanol :2-propanol = 1:5.1 :11.4. The relative rates of the same alcohols with hydrogen atoms, measured in acid solutions, is 1: 10.4 :30.6.25 It has been satisfactorily proven that the “residual hydrogen” differs in its chemical behavior from the normal hydrogen atom. The only plausible species which may abstract hydrogen from organic solutes and form molecular hydrogen are excited water molecules HaO*.
RHz
+ HzO* +Hz + RH + OH
(1)
+ H20*+Hz + R + H 2 0
(la)
or perhaps RH2
Alternatively, i t may be suggested that the reactants examined in the present study react with the precursor of the hydrogen atoms which constitute the “residual hydrogen.” The most likely precursor of the “residual hydrogen” are hydrated electrons26 which were shown to be the precursors of hydrogen atoms in acid medium (eaqH+)as well as of the “molecular hydrogen” by the eaq- eaq- reaction.28*27Most of the reagents examined show fast rate constants with eaq- and it might have been suggested a t first sight that their effect is due to their reaction with the eaq- in the spurs. The rather high concentrations of scavengers necessary for affecting the yield of “residual hydrogen” do support the suggestion that the reactions under study are taking place in the spurs. However, certain reagents, e.g., the iodide and bromide ions, which are nonreactive both toward hydrogen atoms and toward eaq-, still do strongly affect the yield of the “residual hydrogen,”21 while others which are good eaq- scavengers, like acetone, do not affect it at all, in the same range of concentration. Moreover, if the reactants under study would scavenge eaq- in the spurs, then they should affect the yield of “molecular hydrogen” far more than that of the “residual hydrogen”; this would result in an increase of R instead of the observed decrease. This effect on R has actually been observed21 in the case of Fe(CN)? and C O ( X H ~ ) ~both + ~ , excellent eaq- scavengers. Consequently one is left with H20* as a plausible precursor of the “residual hydrogen.”
+
+
~~
~
~~
(21) M. Anbar and D. Meyerstein, to be published. (22) H. L. Friedman and A. H. Zeltman, J . Chem. Phys., 28, 878
(1958). (23) G. Ceapski, J. Jortner, and G. Stein, J . Phys. Chem., 63, 1769 (1959). (24) A. R. Anderson and E. J. Hart, ibid., 65, 804 (1961). (25) J. Rrtbani, ibid., 66, 361 (1962).
(26) C. Lifshite, Can. J . Chem., 41, 2175 (1963). (27) L. M. Dorfman and I. A. Taub, J . A m . Chem. Soc., 8 5 , 2370 (1963).
Volume 68, Number 7 J u l y , 1964
1716
n1. ANBARAND D. MEYERSTEIN
The scavenging of HzO* by transition metal ions may take place by the following reactions HzO*
+ ,I1(H2O)ncz Hzf
+
+ OH + M(Hz0).-10H+
(2)
+ HaOf +Hz+ + OH + HzO
This mechanism is not plausible because Ca+2 and B a t 2ions have no effect on R. Alternative reactions are H20*
+ M(Hz0),+2+eaq- + M(HzO),fS
(3)
and HzO*
+ M(HzO).+’
+
H+
+ OH + M(HzO)n+
(4)
The last reaction is analogous to the interaction of hydrated electrons with the same metal ions.20 Moreover, there is a parallelism between the effectiveness of scavenging of electrons and that of scavenging of HzO*. For this reason, reaction 4 should be preferred to reaction 3 as a probable mechanism. The only exception in this comparison is M n f 2 ions which exhibited no effect in Baxendale’s study.20 These ions were, however, shown to be reduced by electrons, but RIiif was found to produce Hz from water through a hydride transfer.28 Oxygen was found to be an efficient scavenger for the ‘lresidual hydrogen” (about 100 times as reactive as formic acid).5 The reaction between O2 and HzO* may proceed according to
The Journal of Physical Chemistry
+
0 2
+OH
+ HOz
(5)
or in analogy to reaction 4 HzO*
in analogy tola
H20*
HzO*
+ O2 +H + + OH + Oz-
(54
The last reaction has been suggested by Hayon.8 To conclude, it has been shown that a species different from H, OH, and eaq- is formed in the radiolysis of water. This species is a precursor of hydrogen atoms, or alternatively it abstracts hydrogen atoms while forming Hz. Further work is required to decide between these two possibilities. It is suggested that this species is an excited water molecule. The molecular excitation of water under radiolysis has been recently demonstrated by spectroscopic methods.29 Excited water molecules have been suggested many times as intermediates in the formation of H and OH radical^,^,^^*^^ as intermediates in the formation of molecular H202,32 and as the species liable for the increased radical yields in acid and alkaline solution^.^^^'^ It is here suggested that this species has a lifetime long enough for interaction with solutes in relatively dilute solutions.
Acknowledgment. The authors wish to thank Mrs. V. Fischoff and Mrs. 0. Asher for their devoted help in the mass spectrometric analysis. (28) M. Anbar and D. Meyerstein, Proc. Chem. SOC.,23 (1964). (29) D. N. Sitharamarao and J. F. Duncan, J. Phys. Chem., 67, 2126 (1963). (30) M. Burton, J. L. Magee, and A. H. Samuel, J. Chem. Phys., 2 0 , 760 (1951). (31) A. H. Samuel and J. L. Magee, ibid.,21, 1080 (1953). (32) M. Anbar, S. Guttmann, and G. Stein, ibid., 34, 703 (1961).
