4058
E. HAYONAND M. MOREAU
Reaction Mechanism Leading to the Formation of Molecular Hydrogen in the Radiation Chemistry of Water
by E. Hayon and M. Moreau Service de Chimie Physique, C.E.A. Saclay (Seine et Oise), France (Received August 31, 1966)
The yields of “m~lecular’~ hydrogen produced in the y-radiolysis of air-free neutral solutions of potassium dichromate, potassium nitrite, and copper nitrate have been measured over a very wide range of concentrations, such that up to 99% of the hydrogen has been scavenged by these solutes. From a comparison of the scavenging efficiencies of the various’ solutes used, the results seem to indicate the presence of two precursors leading to the formation of “molecular” hydrogen. From a kinetic treatment of the data it is suggested that the precursors are electrons and H atoms, which react according to HzOHzO- -+ H, 20H- and H HzO- -+. H2 OH-, with a small contribution from H H + H,. On accounting for the different rate constants for reaction of the solutes with the two reducing species, it is possible to obtain better qualitative agreement between theoretical diffusion kinetics calculations and experiment.
+
+
The “molecular” yield of hydrogen produced on yirradiation of aqueous solutions, G H 2 = 0.45 & 0.01, is known to be reduced in the presence of a number of solutes (e.g., NO2-, NO3-, HzOz, Cu2+, acrylamide, Ce4+), the variation being dependent on the scavenger used and its concentration. It was suggested by Hayon and Weissl that the main precursor leading to the formation of “molecular” hydrogen is an electron, HzO-, which on recombination in the %purs” produces Hz, according to the over-all reaction HzO-
+ HzO- +Hz + 20H-
(1)
It was thus possible to explain the l o ~ e r i n gof~ G~ H ~~ in acid solutions compared to neutral solutions on the basis of a competition between HzOHf -+. H HzO and reaction 1, resulting in an expansion of the dimensions of the spurs. Dorfman and Taub4 have recently confirmed the existence of reaction 1. Mahlman6 has shown that on irradiation of water with y-rays, using nitrate ions as scavenger for the precursor of hydrogen, a plot of G(Hz)vs. the cube root of [NO3-] X activity coefficient showed two straight lines with different slopes, indicating two different modes of formation of “molecular” H2. The lower slope extrapolates to G(H2) = 0.1 at “infinite dilution.” This value seems to correspond to the yield of Hz ob-
+
The Journal of Physical Chemistry
+
+
+
+
tained on irradiation of frozen aqueous solutions of NaNOa5 or H2026at -1196”. The results of Anderson and Hart,7 using hydrogen peroxide as solute, were also showna to give two straight lines with different slopes when plotting G(H2) vs. [H202]1’8. Since Nosions and HzOt are known to react efficiently with HzO(k = 1O1O and 1.3 X 1O1O loll1 M-l sec.-l, respectively) and relatively slowly with H atoms (k = 2.4 X 106 M-1 sec.-112 and 4 X l o 7 M-1 sec.-l,la respecstlo
(1) E. Hayon and J. J. Weiss, Proc. 8nd Intern. Conf. Peaceful Uses At. Energy, Geneva, 29, 30 (1968). (2) E. Hayon, J . Phys. Chem., 65, 1602 (1961). (3) C. H. Cheek, V. J. Linnenbom, and J. W. Swinnerton, Radiation Res., 19, 636 (1963). (4) L. M. Dorfman and I. A. Taub, J. Am. Chem. Soe., 85, 2370 (1963). (6) H. A. Mahlman, J . Chem. Phys., 32, 601 (1960). (6) J. A. Ghormley and A. C. Stewart, J . Am. Chem. Soe., 78, 2934 (1966). (7) R. A. Anderson and E. J. Hart, J. Phys. Chem., 65, 804 (1961). (8) E. Hayon, Nature, 194, 737 (1962). (9) J. K. Thomas, S. Gordon, and E. J. Hart, J . Phys. Chem., 68, 1624 (1964). (10) J. H. Baxendale, E. M. Fielden, C. CapeUos, J. M. Franois, J. V. Davies, M. Ebert, C. W. Gilbert, J. P. Keene, E. J. Land, and A. J. Swallow, Nature, 201, 468 (1964). (11) S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, Discussions Faraday Soc., 36, 193 (1963).
FORMATION OF MOLECULAR HYDROGEN
4059
tively), it was prolposeds that the major portion of molecular hydrogen which can be readily reduced by the addition of these scavengers has as precursor HzO-, while the rlemaining portion (G(H2) N O.l), less susceptible to €LO2 and NO3- ions, has H atoms as the precursor
H
+ tf20IH + H
4 Hz --j.
4-OHHa
(2) (3)
To check this meehanism further, the variation of G(H2) with the concentration of a number of selected solutes was examined: Cu(NO&, KN02, and Na2CrZO,. The solutes were chosen on the basis of increasing reactivity towards H atoms,12 with h(H. S) = 2.5 X 108, 6.1 X 108, and 5.6 X lo9 M-l sec.-l, ~, and Na2Cr20,, respecwhere S = C U ( N O ~ )KNOZ, tively. The corresponding rate constants for reaction with HZO- were 4.5 X lolo, 4.0 X 109,9J0and 3.3 X 1O1O M-l
+
Experimental Sectian Copper nitrate and sodium dichromate supplied by Hopkin and Williams and potassium nitrite by Baker and Adamson were used without further purification. All other reagents were of analytical grade. A 200-curie e°Co y-source was used with a dose rate of 6.5 X 1019e.v./l. min. based on the Fricke dosimeter taking G(Fe3+) =; 15.5. Total doses given ranged from 4 X 1021 e.v.,/l, at low solute concentrations to 3 X e.v./l. for the highest solute concentrations used. The water employed was purified as described e1~ewhere.l~The method used for degassing, filling the 10-ml. irradiation tubes, extracting the gaseous products, and measuring them by gas chromatography has already been given.12 The solutions containing C U ( N O ~and ) ~ NasCr207were irradiated in the presence of M KBr to protect the molecular hydrogen from OH radical attack. Bromide was not added to solutions of nitrite since it is a good scavenger for OH ra,dicals. Direct interaction of the radiation with the solutes used in this work becomes important at the high solute concentrations used. The doses absorbed in these solutions was therefore corrected2i16using the equation
where DS is the corrected dose, D F ~is~ the + dose as measured by the Fricke dosimeter, ES and ED are the electron density of‘ the irradiated solutions and dosimeter, respectively, and T is the correction for the photoelectric effect. The yields of Hz and O2 were corrected
as indicated above and are given on the basis of total energy absorbed by the solution. The correction factor used below represents the ratio Ds/Dm+. Each G value measured is the result of five or six irradiations carried out at different times to give linear yield-dose curves, and are good to better than &3% at the low solute concentrations and .t5% at the high solute concentrations.
Results The yields of hydrogen obtained on y-irradiation of air-free aqueous solutions of sodium dichromate, copper nitrate, and potassium nitrite were determined over a wide range of concentrations such that up to 99% of the yield of H2 was scavenged by the solutes used. In all cases the decrease in G(H2)is linear with at low solute concentrations. Extrapolation of this linear portion of the curve to infinite dilution = 0.44. At higher solute concentrations gives the yields of Hz are no longer linear with and a smooth curve can be drawn through the experimental points. Figure 1 presents the results according to the method used by Schwarz.16 Here the fractional lowering of the molecular Hz yield G(H2)/GH, is plotted against the logarithm of the solute concentration and the resulting series of curves brought into coincidence by multiplication of the concentration by a normalization factor f. This factor is chosen to give the “best” coincidence of the curves in the low concentration regions, and was obtained relative to the nitrite system. Comparison of the f values for the solutes used here shows that they are related to the reactivity of these solutes toward HzO-, as obtained by pulse radiolysis. The full curve drawn in Figure 1 is theoretical and was calculatedlBfor the nitrite system on the basis of the one-radical diffusion model. The yields of H2 in NaN03 solutionss as well as those in H 2 0 P 7 and acrylamide18solutions have been included in Figure 1, and all have been plotted as a function of the solute concentration. Oxygen is formed on irradiation of Na2Cr207and Cu(NO& (none is observed in NaNOz solutions), particularly at higher solute concentrations. These (12) E.Hayon and M. Moreau, J . c h h . phys., 62, 391 (1965). (13) J. K, Thomas, J. Phys. Chem., 67, 2593 (1963). (14) E.Hayon, Trans. Faraday SOC.,60, 1059 (1964). (15) H. A. Mahlman and G. K. Schweitzer, J . Inorg. Nucl. Chem., 5, 213 (1958). (16) H. A. Schwarz, J . Am. Chem. SOC.,77, 4960 (1955). (17) J. A. Ghormley and C. J. Hochanadel, Radiation Res., 3, 227 (1955). (18) F. S. Dainton, Radiation Res. Suppl., 1, 25 (1959).
Volume 69,Number 1.9 December 1.966
4060
E. HAYON AND M. MOREAU
I-
0.7
0.6
t
:i 0.2
0.1 O
10-5
u 10
L
10-1
-4
x f. Figure 1. G(H2)/&, as a function of the logarithm of 6he solute concentration multiplied by normalization factorf: KN02 (a, B)f = 1; NaN03 (+,ref. 5), f = 2.4; H202(@, ref. 7, X, ref. 17), f = 2.5; acrylamide (e, ref. 18), f = 4.0; Na&rtOT (o),f = 8.2; Cu(NOd2 (A), f = 10.
yields were not too reproducible, and are probably a result of the “direct effect” of radiation on the solutes. The results obtained are shown in Figure 2. In all cases these yields increase with increase in the solute concentration but with a positive G(02) intercept. Thus the intercept for Cu(NO3)Z is approximately double that for NaN03,1Q No explanation is offered for these intercepts. Molecular Hz Yields in Alkaline Solutions. The variation of the yields of “molecular” hydrogen with [H+] having been shown,ZJ it seemed of interest to measure the yields of GR*in alkaline solutions. Potassium nitrite was chosen as solute since it is stable in alkaline solutions and reacts efficiently with OH radicals or 0- ions, thus protecting the hydrogen that is formed. The variation of G(H2) with [KNOz]l/s is shown in Figure 3. On extrapolation to infinite dilution, one obtains G values of 0.44, 0.43, and 0.37 at pH 6, 13.7, and 14.0, respectively. Discussion From a plot of G(H2) os. [S]”’, the slopes of the linear portions of the curves at the lower solute concentrations have been found to be greatest for those solutes which react more efficiently with HzO-, and the order is C U ( N O ~2 ) ~C r ~ 0 , ~>- NOz- in accord with the rate constants Ic(Hz0S). This is in agreement with the postulate1s8 that the precursors of the molecular hydrogen scavenged on this portion
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The Journal of Phgsicul Chemistry
of the curve are electrons, HzO-. At higher solute concentrations, however, the decrease of G(Hz) with concentration is no longer proportional to the reactivity of the solutes toward HzO-. This effect can be seen in Figure 1, where the relative lowering of the molecular yield G(H2)/G, is plotted against the log* rithm of the solute concentration, after normalizing the results obtained from Crz0,2-, CU(NO~)~, Hz02;J7 and acrylamide18systems with respect to the nitrite system on the basis of the reactivity of these solutes toward HzO-. Thus at high solute concentrations the decrease of G(H2)/G~,follows the order CrzOT2-> NO2- > Cu(NO& > NO3- > HzOzand acrylamide. This order is different from the one shown to exist at the lower solute concentrations: Cu(N03)2 2 C ~ Z O , ~> - acrylamide > HZOZ> NO3- > NOz-. With the exception of the NaNO3 results, the order at high solute concentrations is in agreement with the reactivity of these solutes toward H a t o m ~ . ~ ~ J ~ Diffusion kinetic computationsl*~20J1 can predict the variation of the yields of “molecular” hydrogen as a function of solute concentration, taking a certain value for the rate constant of the reaction of the solute (19) H. A. Mahlman, J . Phys. Chem., 67, 1466 (1963). (20) A. Kupperman and G. G. Belford, J . Chem. Phys., 36, 1427 (1962) ; A. Kuppermann, “Action Chimique et Biologique des Radiations,’’ Part 5 , M. Haissinsky, Ed., Masson, Paris, 1961, p. 154. (21) K. Shinohara, T. Shida, and N. Saito, J . Chem. Phys., 35, 1899 (1961).
406 1
FORMATION OF MOLECULAR HYDROGEN
I 0.3
0.1
I
yields are normalized relative to the nitrite system for k(HzOS) = 4.5 X lo9 M-l sec.-I, and this happens to be very nearly the value for k(H Cr20T2-) = 5.6 X lo9 &I-lsec.-l. Since the values of k(H S) for all the other solutes used are lower than 4.5 X 109 M-1 sec.-1, one would expect to find a displacement to the right for the Hz yields at high solute concentration. Such a displacement would be greater the lower the k(H S). This is what is observed (Figure l), and qualitatively the displacement can be seen to be proport,ional to the scavenging efficiencies of the solutes for H atoms. The disagreement with the NaNOa data is disturbing, particularly since no obvious explanation can be given at present. Nevertheless, it is our belief that the results obtained in the five other systems justifies the interpretation given to them. Other mechanisms have recently been proposed to explain the formation of molecular H2. It was suggested22that the H30 radical is the main precursor of hydrogen. It is not apparent from this brief communication what is the evidence for introducing such a new species. Baxendale and DixonZ3have proposed that most of the molecular Hz is formed from H atom recombination as a result of reaction 4 occurring in the spurs
+
1 1
i
0
1
0
2
3
4
[SI, M .
Figure 2. G(O2)yields as a junction of solute concentration: NazCrzO, (0);Cu(NO& (a).
0.1
0.2
0.3
0.4
[KNOz]Y*.
Figure 3. Yield of mol~eoularHZin alkaline solutions: 0,neutral p H ; a,pH 1.3; A,pH 13.5; f, pH 13.7; @, p H 14:.10.
with the precursor of Hz. No agreement has, however, been 0bserved7~1~~~0~ between the theoretical curve and experimental data over the whole concentration range investigated. This was the case whether one dealt with cube-root, or logarithmic plots, and whether one used theoretical curves derived on the basis of the one-radical16 or two-radical model.20~21It was found necessary to change one or more of the parameters’ over some part of itlhe curve to obtain agreement with the experimental results. This discrepancy between theory and experiment can now be explained as due to the fact that the solutes examined up to then happened to have been efficient reactants for HzObut rather poor ones for H atoms. On accounting for the presence of two precursors each reacting with the solutes with significantly different rate constants, a better qualitative agreement can be obtained between theory and experiment, as can be seen in the case of potassium dichromate (Figure 1). Here the
+
+
HtO0
+
+ H+
----t
H
+ HzO
(4)
but that the decrease of GHzby solutes is due to their reaction with HzO-, the precursor of the H atom. If this mechanism prevailed, the decrease of GHs by solutes should be independent of [Hf]. In actual that the decrease of fact, it has been shown7J7~24 CH% is pH dependent. SchwarzZ4has suggested that some of the precursors of molecular Hz are H atoms formed via reaction 4 in the spurs, since the local concentration of H+is at least equal to that of HaO-. For the reasons stated above, if reaction 4 occurs in the spurs, one would expect certain solutes present at high concentrations to be capable of competing with H+ for HzO-. The results shown in Figure 1 indicate that such a competition does not take place. It is concluded that H atoms as well as HzO- are the precursors leading to the formation of “molecular” hydrogen
+ HzO- +HS + 20HH + HzO- + + OH-
HzO-
(1) (2)
(22) T. J. Sworski, J. Am. Chenz. Sac., 86, 6034 (1964). (23) J. H. Baxendale and R. S. Dixon, 2. physik. Chem. (Frankfurt), 43, 11 (1964). (24) H. A. Sohwars, Radiation Res. Suppl., 4, 89 (1964).
Volume 69, Number 18 December 1966
KJELL-IVAR DAHLQVIST AND STUREF O R S ~ N
4062
H+H+H2 (3) The rate constants of reactions 1-3 are known: 2kl = 1 X lOlo 144-I sec.-l (ref. 11); k2 = 2.5 X 10Io M-I sec.-l (ref. 25), and 2ks = 1.2 X 1O1O &?--I sec.-l (ref. 13). Since the yield of H atoms is about one-fifth that of electrons in the radiolysis of neutral taking the above rate constants the contribution from reaction 3 compared to reaction 2 becomes rather small. Thus the main reactions forming “molecu1ar” hydrogen are (1) and ( 2 ) . The H atoms may originate from the dissociation of excit,ed water molecules, as
has been p r o p o ~ e d ~to~ explain ~ ~ ’ the formation of H atoms in the bulk of the solutions.
H2O* *H
+ OH
(5) The results obtained above would tend to favor such a mechanism, though one cannot dismiss entirely a small contribution from reaction 4. (25) M. S. Matheson and J. Rabani, J. Phys. Chem., 69, 1324 (1965). (26) E. Hayon, ibid., 68, 1242 (1964). (27) J. T. AIIan and G. Scholes, Nature, 187, 218 (1960).
The Barrier to Internal Rotation in 2-Furanaldehyde
by Kjell-Ivar Dahlqvist and Sture Fors6n Research Group for NMR, Division of Physical Chemistry, The Royal Institute of Technology, Stockholm 70, Sweden (Received August 31, 1966)
The rotation barrier in 2-furanaldehyde has been studied by nuclear magnetic resonance The rate of interconversion of the two rot& (n.m.r.) at temperatures down to -115’. tional isomers, which are present in unequal amounts, has been calculated from the line shapes of the n.m.r. signal from both the aldehyde group and the ring proton in the 3position. A small systematic difference in interconversion rates deduced from the line shape of the signals from the aldehyde proton and the H-3 proton is noted. The results are analyzed in terms of the theory of absolute reaction rates.
1. Introduction The factors determining the barriers restricting rotation around carbon-carbon single bonds between sp2type hybridized carbon atoms
\ / c-c / \ are not well known. It seems likely, however, that the study of barriers to internal rotation in systems of this type should give increased insight into the significance of .rr-electron delocalization in conjugated molecules. However, comparatively few rotational barriers in systems of this nature have been reported. The Journal of Physical Chemistry
The technique of ultrasonic relaxation has been used by de Groot and Lamb to study the unsymmetrical rotational barriers in acrolein and a few related unsaturated aldehydes.172 Their results are summarized in Table I. Nuclear magnetic resonance spectroscopy has recently been applied by Anet and Ahmad to estimate the free energy of activation for the symmetrical rotational barriers in benzaldehyde and two parasubstituted benzaldehyde derivatives3 (cf. Table I). The rotational barriers in butadiene and styrene have (1) M. S. de Groot and J. Lamb, Proc. Bog. Sac. (London), AZ42, 36 (1957). (2) J. Lamb, 2. Elektrochem., 64, 135 (1960). (3) F. A. L. Anet and M. Ahmad, J. Am. Chem. SOC.,86, 119 (1964).
