C. D. Jonah, J. R. Mlller, and M. S. Matheson
1618
The Reaction of the Precursor of the Hydrated Electron with Electron Scavengerst Charles D. Jonah;
John R. Mlller, and Max
S. Matheson
Chemistry Division, Argonne National laboratory, Argonne, Illinois 60439 (Received March 4, 1977) Publication costs assisted by Argonne National laboratory
We show that there are several compounds whose ability to decrease the initial 50-ps yield of the hydrated electron is not correlated with the reactivity of that compound. Several mechanisms have been proposed. Explicit corrections have been included for time-dependent reaction rates. It is shown that other corrections require assumptions about the mechanism of the presolvation reaction. Introduction In the deposition of energy by ionizing radiation in polar media, the electrons ejected by the radiation can be solvated. A great many studies of the solvated electron in various media have been made and the properties and reactions of the solvated electron have been well catalogued.1)2 It has generally been assumed that an electron is ejected from the electron shell of a solvent molecule as a quasifree particle and is subsequently solvated. A great deal of information has been gained about the solvated electron but almost none about its precursor. This observation was made by Hamill 8 years ago3and is nearly as true now as it was then. One of the first pieces of direct evidence about the precursor came from the work of Hunt and co-w0rkers,4’~ after they had improved the time resolution of pulse radiolysis experiments to 30 ps. They found that the initial yield of the solvated electron was decreased in the presence of high concentration of scavengers and the fraction of the electrons remaining, f, could be described by the expression where [SI is the concentration of the scavenger and C37is a constant which depends on the solvent and the scavenger specie^.^,^ They assumed the initial yield of the electron could be determined by extrapolation of the data to zero time using the rate constants measured in the experiment.’ It was pointed out by Schwarz6 that corrections are required since rate constants for fast reactions are time dependent and will affect the extrapolation. Czapski and Peled7~8 pointed out that further corrections may be required because at high scavenger concentrations, some electrons may be solvated within a reaction radius of a scavenger. These initial “encounter pairs” are not accounted for by extrapolating a rate process to zero time. The work of Hamill et al. eliminated these difficulties in interpretation by finding a system where reaction of the solvated electron is minimal.g In their work, a strong decrease in the initial yield of the electron was found in ethanol when benzene was added. The rate constant of benzene is very small (less than lo7) and so no effect of time-dependent rate constants could be expected. In alcohols, the solvation time of the electron is slower than in water so one would expect an increase in the time allowable for precursor scavenging. Qualitatively similar results were seen by Thomas using both photogeneration and radiolytic generation of electrons.1° The major difficulty with the benzene-alcohol system is that a high concentration (2 M) of benzene is required. ‘Work performed under the auspices of the Division of Physical Research of the U.S.Energy Research and Development Administration. The Journal of Physical Chemistry, Vol. 81, No. 17, 1977
We have attempted to find reactants with similar reactivity with the solvated electron but different reactivities with its precursor. The work of Miller showsll that in glasses, precursor and the trapped electron may have greatly different reactivities, We started by examining several compounds which had been tried in glasses. Lam and Hunt had found that except for Haq+the reactivity of a scavenger with the precursor closely parallels the high concentration rate constant of the scavenger with the solvated electr0i.1.~We have found several compounds which were considerably more, or considerably less, reactive with the precursor than one would have expected from their reactivity with the solvated electron.
Experimental Section The picosecond pulse radiolysis equipment has been previously described in detail.12 The Argonne microwave linear accelerator generates pulses of 20-22-MeV electrons 60 times per second. Each pulse consists of a main pulse containing greater than 90% of the charge with a width (fwhm) of 30 ps and a satellite pulse 770 ps from the main pulse. Approximately 30% of the electron beam is intercepted by a cell containing 1 atm of xenon. The Cerenkov light generated by the xenon cell is used as the analyzing light. The remainder (70%)of the electron beam is used for radiolysis of our sample. Using a focussed eV/g) was electron beam, approximately 1.5 krd deposited in the irradiated volume (approximately 0.25 cm3). Both the electron beam and the light beam are delayed. The delay of the light beam can be varied so that it reaches the irradiation cell before, during, or after the pulse of ionizing radiation. By varying the delay of the light beam the transmission of the sample as a function of time is determined. The light detection system used an RCA C7253A five-stage side-on photomultiplier. The photocurrent from the photomultiplier was integrated and stretched using a commercial charge sensitive FET preamplifier (Tennelec TC 162). This system has a risetime but the risetime is not similar to the ordinary electronic risetime. The effect of the finite pulse width which causes the risetime will mask any processes occurring during the pulse and will give a smaller absorption than is actually occurring. It will not decrease the apparent rate constant measured for an exponential process. Thus, risetime affects how soon a process may be measured but not the speed at which it appears to occur. Solutions were made using water purified by a deionizing water system which included charcoal filter and submicron final filter.13 Chemicals were used as received. Solutions were degassed using helium. To avoid complexes the perchlorate salts of reactive cations were used. Ferric and mercuric solutions were run in the presence of perchloric
Reaction of the Precursor of the Hydrated Electron
1819
TABLE I: Measured Values of c 37
Compd SeOd2-
c 37
(cow)
rc37
'dab
'C37
'cakl'dil
'ealcd
0.42 0.42 9.8 0.36 0.11 2.0 18.0 0.63 0.65 8.5 0.5 0.2c 1.5 5.0 oc10.48 0.52 9.1 0.7 2.75 4.0 1.6 Acetone 1.4 (1.4) 1.6 0.87 2.9 3.6 6.3 0.87 0.47 9.5 1.1 2.9 2.8 NO30.42 (0.45) 2.0 Te(OH)c' 0.075 0.09 3.2c 7.3 2.3 16.0 3.7 2.2 CU2+ 0.9 1.3 3.0 3.3 7.2 6.6 3-4c 7.3 1.7-2.1 0.09 16.0 HgC4 0.08 5.4 4.0 7.8 1.9 A&?+ 0.14 0.17 13.3 6.0 6.0 10.2 1.7 Cr3+ 0.30 7.8 7 .O 0.84 1.6 4.0 3.0 4.7 CH3N02 0.30 0.37 10.2 5.0 8.0 1.6 Cd2+ 0.38 (0.35) 0.55 9.0 4.0 6.OC 9.6 1.6 Fe3+ 1.3 > 3.5 300/f (telluric acid). This limit requires about twice the error limits we see on our data. A less conservative estimate would be in > 400/f.Either of these values of m is much larger than 6 to 10, the range of m proposed by Freeman.20 For the last step in the mechanism elo; S, it is clear that if the reaction radius for this reaction is greater than the radius for e,< with a scavenger no Czapski-Peled type correction is necessary. If the reaction radius is less such a correction would be necessary. However, if all of the reaction was due to el& reaction, the data of Table I are consistent with the elo; reaction radius being larger than the reaction radius of esoiwith the scavenger. If we paramatrize the reaction of efre; as
+
(5) and elo; as
and the Czapski-Peled effect as
(7) than the fraction of the electron that remains at time just greater than 0, f if the reaction takes place with elo; is f=
~ - [ S I / C ~ ~ ~ O ~
(8)
while if the reaction takes place with the free electron f = e - [ ~ o 1 / ~ g 7 ~ ~ ~ ~ e - =1 e~ g- 1[ /~ ~1 3 /7 ~~ ~3 7 ~ ~ ~ ~
(9)
We see then, if one wants to correlate the reactivity of a precursor of an electron with some property, the mechanism of the reaction will determine what sort of corrections should be made to measured data to obtain the number one should use to make correlations. Another mechanism has been proposed where all reactions of the solvated electron go through a mechanism of the formz1
CSIi.kf
-
S-
where edmp is the same stage of partial solvation. It would appear that this mechanism is ruled out by the data of chromate and selenate. Since all steps in the mechanism would be the same for chromate and selenate except the reaction step, one would expect that the ratio of 1/CS7of chromate to 1/C37of selenate would be the same as the ratio of their rate constants. However The Journal of Physical Chemistry, Vol. 81, No. 17, 1977
1622
P. Maruthamuthu and P. Neta
keeping the accelerator faithful. We also are grateful for the friendly and helpful discussions with John Hunt and Gidon Czapski. considerably different from one. It is conceivable that there could be a structure changing function of the selenate which would explain the data, however, in looking at Figure 3, we see that selenate is not unique. The data are well scattered on the graph and no slope of -1 would appear to be satisfactory. Since the data of Hunt seems to fit the line shown, it is possible we are looking at unusual species; however, the number of species differing greatly from the line argues for this not being the case.
Conclusions We have found that for most of the species studied, the mechanism proposed by Czapski and Peled together with corrections for time-dependent rate constants is insufficient to explain the decrease of the initial yield. In addition, we find that the correlation between CS7and rate constant does not seem to be general but only fit a restricted class of compounds. The mechanism that one invokes for the decrease of the initial yield of the electron determines what sort of corrections one must make to get a number which is indicative of the dry electron reactivity with a scavenger. To correlate dry electron reactivity with other parameters one must make an assumption on the mechanism of dry electron reactions. Acknowledgment. We acknowledge the assistance of R. M. Clarke in the running of these experiments. We are grateful to Donald Ficht, Lee Rawson, and Joe Becker for
References and Notes (1) “The Solvated Electron in Radiation Chemistry”, A. K. Pikaev, Translated by Israel Program for Scientific Translations, Ltd. IPST Cat. No.5848, Available from U.S. Department of Commerce, National Technical Information Service, Springfield, Va. 22151. (2) M. S. Matheson in “Physical Chemistry”, Vol. VII, Academic Press, 1975, New York, N.Y., Chapter 10. (3) W. H. Hamili, J. Phys. Chem., 73, 1341 (1969). (4) R. K. Woife, M. J. Bronskili, and J. W. Hunt, J . Chem. Phys., 53, 4211 (1970). (5) K. Y. Lam and J. W. Hunt, Int. J. Rsdiat. Phys. Chem., 7 , 317 (1975). (6) H. A. Schwarz, J . Chem. Phys., 55, 3647 (1971). (7) G. Czapski and E. Peled, J . Phys. Chem., 77, 893 (1973). (8) G. Czapski and E. Peled, “Radiation Research”, Proceedings of the Fifth International Congress of Radiation Research, 0. F. Nygaard, H. I. Adler, arid W. K. Sinclaii, Ed., Academic Press, New York, N.Y., 1975, p 356. (9) H. Ogura and W. H. Hamiil, J. Phys. Chem., 78, 504 (1974). (10) A. Bromberg and J. K. Thomas, J. Chem. Phys., 63, 2124 (1975). (11) J. R. Miller, J . Phys. Chem., 79, 1070 (1975). (12) C. D. Jonah, Rev. Sci. Instrum., 46, 62 (1975). (13) Continental Water Company, Chicago, IiI. (14) R. M. Noyes, R o c . React. Kinet., 1, 129 (1961). (15) Reference 1, p 158. (16) 1. Santar and J. Bednar in “Proceedings of the Second Tihany Symposium on Radiation Chemistry”, J. Dobo and P. Hedrig, Ed., Publishing House of the Hungarian Academy of Sciences, Budapest, 1967 n 17. (17) YI-K.’hm,.ref 8, p 219. (t8) D. H. Katayama, R. E. Huffman, and C. L. O’Bwan, J. Chem. Phys., 59, 4309 j1973). (19) R. L. Piatzman in “Radiation Research”, Proceedings of the Third International Congress of Radiation Research, G. Sllini, Ed., p 20. (20) G. L. Bolton, K. N. Jha, arid G. R. Freeman, Can. J . Chem., 54, 1497 (1976). (21) J. W. Hunt, K. Y. Lam, and W. J. Chase, ref 8, p 345. ’
Reactions of Phosphate Radicals with Organic Compounds‘ P. Maruthamuthu and P. Neta’ Radiation Laboratory, University of Noire Dame, Notre Dame, Indiana 46556 (Received March 8, 1977) Publication costs assisted by the U S . Energy Research and Development Administration
Phosphate radicals in the three acid-base forms, H2P64, HP04-, and were produced by the reaction of hydrated electrons with peroxodiphosphateions at pH values of 3-4,7-9, and 12, respectively. Rate constants for the reactions of these radicals with organic compounds were determined by following the rate of decay of the phosphate radical absorption in the presence of increasingconcentrations of the substrate. Rates of reaction of H2PO4 were found to be higher than those of HP04-and P042-by a factor of -4-10, while the latter radicals have similar rates. The rate constants for hydrogen abstraction from aliphatic compounds ranged from -10‘ M-’ s-l for acetic acid and 2-methyl-2-propanolto lo8 M-’ s-l for 2-propanoland formate. Addition to a double bond was not faster but reaction with aromatic compounds reached a rate constant >lo9 M-ls-’ for the most reactive compounds. The effects of substituents were found to be similar for H2P04and S64- and both radicals are suggested to react with the aromatic ring by one-electron transfer to the inorganic radical. In hydrogen abstraction reactions H2P04(and SO4-)were found to react with rate constants 10-100 times lower than those of OH and to have a selectivity about three times higher than that of OH. Radiolytic chain decomposition of peroxodiphosphate in the presence of some organic compounds was also studied.
-
-
Introduction Phosphate radicals can be produced most efficiently by the reduction of peroxodiphosphate, e.g., with hydrated electrons: P,o:-
-t eaq-
-+
PO:-
+ PO:-
(1)
Similar reactions take place with the protonated forms The Journal of Physical Chemistry, Vol. 81, No. 77, 1977
HP20s3-and H2P20;-. The rate constant for reaction of eaq- increases upon Protonation from 1.8x 10’M-’ S-l for p20s4- to 5.3 lo9 M-’ S-l for H2p20s2-.2 The Phosphate radical can also be produced by oxidation of phosphate ions with OH radicals but this reaction is very slow3and is impractical to use as a source of these radicals. Photolysis of phosphate4 or peroxodiphosphate can also yield the phosphate radical by electron detachment from the
