7839
J. Phys. Chem. 1992,96, 7839-7841 generous support of this research through Grant CHE-92009 17.
References and Notes (1) (a) McLauchlan, K . A. In Aduanced EPR: Applications in Biology and Biochemistry; Hoff, A. J.. Ed.; Elsevier: New York, 1990; pp 345-369. (b) Trifunac, A. D.; Lawler, R.G.; Bartels, D. M.; Thurnauer, M. C. Prog. React. Kinet. 1988, 14, 43. (2) Tominaga, K.;Yamauchi, S.; Hirota, N. J . Phys. Chem. 1990, 94, 4425. (3) (a) Closs,G. L.; Forbes, M. D. E.; Norris, J. R., Jr. J . Phys. Chem. 1987, 91, 3592. (b) Jenks, W. S.; Turro, N. J. Res. Chem. Intermed. 1990, 13, 237. (4) Terazima, M.; Hayakashi, S.;Azumi, T. J . Phys. Chem. 1991, 95, 4297. (5) Stehlik, D.; Bock, C. H.; Thurnauer, M. C. In ref 1, pp 371-403. (6) Budil, D. E.; Earle, K. A,; Lynch, W. B.; Freed, J. H. In ref 1, pp 307-340. (7) Lynch, W. B.; Earle, K.A.; Freed, J. H. Rev. Sci. Instrum. 1988,59, 1345. (E) (a) Forbes, M. D. E.; Myers, T. L.; Maynard, H. D.; Dukes, K.E. J. Am. Chem. Soc. 1992, 114, 353. (b) Forbes, M. D. E.; Peterson, J.; Breivogel, C. S. Rev. Sci. Instrum. 1991, 66, 2662.
(9) Millard, A. A.; Rathke, M. W. J . Org. Chem. 1978, 43, 1834. (10) Forbes, M. D. E.; Dukes, K. E.; Myers, T. L.; Maynard, H. D.; Breivogel, C. S.;Jaspan, H. B. J . Phys. Chem. 1991, 95, 10547. (11) Forbes, M. D. E. Rev. Sci. Instrum., submitted for publication. (12) Hyde, J. S.;Newton, M. E.; Strangeway, R. A.; Camenisch, T. G.; Froncisz, W. Rev. Sci. Instrum. 1991, 62, 2969. (13) McLauchlan, K. A.; Simpson, N. J. K.;Smith, P. D. Res. Chem. Inrermed. 1991, 16, 141 and references therein. (14) Levstein, P. R.; van Willigen, H. J. Chem. Phys. 1991, 95, 900. (15) Atkins, P. W.; Evans, G. T. Mol. Phys. 1974, 27, 1633. (16) Shkrob, I. A.; Wan, J. K. S . Res. Chem. Intermed. 1992, 17, 77. (17) Fischer, H. Chem. Phys. Left. 1983, 100, 255. (18) (a) Jaegermann, P.; Lendzian, F.; Rist, G.;Mobius, K. Chem. Phys. Lett. 1987, 140, 615. (b) Jent, F.; Paul, H.; Fischer, H. Chem. Phys. Lett. 1988, 146, 315. (19) (a) Spin Polarization and Magnetic Effects in Radical Reactions; Yu, N., Molin, Ed.; Elsevier: Amsterdam, 1984; p 18. (b) Batchelor, S. N.;
Heikkila, H.; Kay, C. W. M.; McLauchlan, K. A.; Shkrob, I. A. Chem. Phys. 1992, 162, 29. (20) Closs, G. L.; Forbes, M. D. E.; Piotrowiak, P. J . Am. Chem. Sot. 1992, 114, 3285. (21) Closs, G. L.; Forbes, M. D. E. J. Phys. Chem. 1992, 96, 1924. (22) Forbes, M. D. E. Unpublished results.
Diffusion-Kinetic Calculations of the Effect of Nitrous Oxide on the Yields of Ionic Species in the Radiation Chemistry of Water' Simon M. Pimblott,* Robert H. Schuler, and Jay A. Laverne Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: July 6, 1992; In Final Form: August 13, 1992) Deterministic diffusion-kinetic calculations show that the yields of the cationic and the anionic species, H30+and eaq- + OH-, produced in the fast electron radiolysis of neutral water are significantly reduced by the addition of nitrous oxide as an electron scavenger. This reduction is demonstrated to be a direct consequence of the conversion of the hydrated electron to the hydroxide anion which undergoes more rapid intraspur reactions with H30+ than does ea;. These model calculations indicate that at 100 ns the OH- yield in N20-saturated water is 11% lower than the total anion yield in the absence of N20. These calculations may, in fact, slightly underestimate the effect of N 2 0 since the predicted decrease in conductance is 2-755 less than that determined experimentally.
-
Introduction Pulse radiolysis of N20-saturated solutions provides a convenient way of introducing a nonequilibrium population of hydroxide ions into water on the 10% time scale via reactions 1 and 2.,s3 While ea;
+N20
0'-+ HzO
kl = 9.1 X IO9 M-'s-'
+ 0''OH + OHN2
k, = 1.8 X IO6 M-Is-I
(1) (2)
the focus of most radiation chemical studies on the use of N 2 0 has been on the conversion of ea; to 'OH radical, recent studies have shown that the OH- produced in N20-saturated solutions can be used to examine directly the dynamics of the reactions involved in establishing the acid-base equilibria of aqueous solutions of weak acids such as phenols! Quantitative considerations require information on the radiation chemical yield of OH-. The present study examines the time dependence of the ionic intermediates as they are produced and subsequently react within the spurs of fast electron tracks. Nitrous oxide is an efficient scavenger of the hydrated electron with a half-period for reaction 1 of 2.9 ns in N,O-saturated (0.026 M) solutions.2 The subsequent reactions of the oxide radical anion are important in the intraspur processes since the half-period for reaction 2 is 7.0 The production of OH- is, therefore, controlled by processes which occur on the time scale of the evolution of the spurs of fast electron tracks. Preliminary experiments4using a-naphthol to scavenge OHindicate that the concentration of OH- on the 100-ns time scale corresponds to a yield of only ~ 2 . 8 5 This . ~ yield is substantially
less than that of -3.35 which is measured for ion pairs in N 2 0 free water by time-resolved conductivity method^.^,^ In fact, Schmidt and Ander8 as early as 1969 found that the ion pair yield on the microsecond time scale in N20-saturated water was 2.9, which is 14% lower than in argon-saturated water. Klever et al.7 found a similar result and suggested that spur processes were responsible for the observed difference. More recently, Anderson et al.9 compared the conductance of argon- and N,O-saturated solutions at 100 11s and also found a similar reduction at the earlier time. Other than these reports, the radiation chemical literature has not addressed the effect of ion or radical scavengers on the yield of anionic species in water radiolysis. The results of preliminary calculations reported here demonstrate that the yield of ions which escape the spurs of N,O-saturated solutions is 11% less than that in the absence of NzO. This difference is shown to be a result of more rapid intraspur reaction of radiation-produced H30+with OH- than with e, -. These calculations allow an extensive examination of the me&anistic details and of the contributions of the various intraspur reactions to the overall radiation chemistry. More detailed studies which include the effects of additional radical and ion scavengers on the ion pair yields are in progress.
-
Methodology In a series of paper~l*'~a deterministic diffusion-kinetic method for describing the intraspur reactions in the radiation chemistry of aqueous solutions has been developed. The nonhomogeneous kinetics of the various intermediates in the solution are modeled using a typical average spur. The initial radiation-induced in-
0022-365419212096-7839%03.00/0 0 1992 American Chemical Society
7840 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
Letters
f I .o
1
1
C
0
0 u ,
0 10-12
t
IO-^ time (sac)
lvll
I
IO-'
I
I ,
IO-^
Figure 1. Deterministic diffusion-kinetic modeling of the time dependences of the yields of the ionic species produced by the fast electron radiolysis of deaerated water (-) and of nitrous oxide-saturated solution (---).
termediates, H30+, eaq-, 'OH,and H', are assumed to have spatially nonhomogeneous concentration profiles which are Gaussian.13 The spur is divided into concentric spherical shells, and the reactants are allowed to diffuse between shells as prescribed by the appropriate microscopic diffusion laws and to react according to chemical rate equations. The technique has been described in detail previously." The time evolution of each of the intermediates is then examined. The predictions have been shown to reproduce the concentration dependence of the radical and molecular product yields observed in scavenger experiments.' The model used to describe the radiation chemistry of water employs a scheme of 10 reactions, which are given in Table I of ref 1 1. For solutions containing N 2 0 it is necessary to include additionally reactions 1-5. The rate coefficients for reactions
0'-+ eaq-
k, = 2.2 x 10'O M-' s-'
0'- + *OH
0'-+ H 3 0 +
b
20H-
(3)
HOT
(4)
* *OH
(5)
k, = 2.0 X 1Olo M-'s-' k S = 6.1
X
IO" M-'
s-I
3-5 have been taken from the compilation of Buxton et al.'4J5 A complete description of the anion chemistry requires inclusion
of reactions 6 and 7 to take into account the small transient yield of HOT produced in reaction 4.15
HO2- + H20
+
k, = 5.9 X I O J M-' s-'
H202
+ OH-
k, = 6 X IO'O M-I s-'
H02- H 3 0 + ' H202 (7) In the calculations reported here, the kinetic parameters obtained in earlier studies"J2 were used without modification. The equivalent conductances of the hydrated proton and hydroxide anion are well-known and are respectively 350 and 197 S an2mol-l at 298 K.I6 The temperature dependence of the equivalent conductance of ea; has been measured by Barker et a1.6 and is 180 S cm2 mol-' at 298 K, Le., only slightly less than that for the hydroxide anion. The equivalent conductances of 0'-and HOT were assumed to be similar to the value of chloride ion, 76 S cm2 mol-I.l6 Results and Discussion The predictions of these calculations for the time dependences of the yields of radiation-induced ionic species produced in the fast electron radiolysis of neutral deaerated water are given as the solid curves in Figure 1. These calculations are based on an assumed initial ion yield of 4.78for H 3 0 +and The yields of the hydrated proton, hydroxide ion, and hydrated electron at 1 ps are predicted to be 3.52,0.88, and 2.64,respectively. These values are -5% higher than the experimental estimates of the
1
0,0-12
lo-lo
10-9
,0.7
lo-8
,()-e
time (sec) Figure 2. Modeling of the transient conductivity of deaerated water (-) and of nitrous oxide-saturated solution (---) radiolysis.
following fast electron
measured yields for escape from the radiation track." The actual yield of OH-produced by intraspur reactions is 1.54,which is considerably greater than the net yield of 0.88 given above. This difference reflects the annihilation of OH-by reaction with H30+ within the spur (reaction 8). These calculations show that the k = 1.4 X IO" M-'s-I
OH- + H 3 0 + H2O production of OH-is adequately described by the reactions of ea< within the spurs and that the initial yield of OH-produced directly from water must be very low. The predicted time-dependent conductance for argon-saturated water is given by the solid curve in Figure 2.'* Because the observed conductance is dominated by the proton and because the equivalent conductances of the hydrated electron and of hydroxide ion are similar, the observed decrease in the conductance with time essentially reflects intraspur charge annihilation reactions 8 and 9. While the concentration of OH- in the spur is less than eaq-+ H 3 0 +
k
-
2.3
X 1010 M-I 6-I
H'
(9)
that of e, the rate constant for reaction 8 is considerably higher than for reaction 9 so that each of these reactions contributes almost equally to the reduction in conductivity noted in Figure 2. Our calculations show that at microsecond times the yields for the reactions 8 and 9 are respectively 0.66 and 0.60. The time-dependent yields of the radiation-induced ionic transients in a neutral saturated nitrous oxide solution are illustrated by the dashed curves in Figure 1. Since nitrous oxide reacts with the hydrated electron on the nanosecond time scale, scavenging competes effectively with the intraspur reactions of ea
