829
J. Phys. Chem. 1983, 87,829-832
Methyl Viologen Radical Reactlons with Several Oxidizing Agents Gerrlt Levey” and Thomas W. Ebbesen Radiation Laboratory. University of Notre Dame, Notre Dame, Indiana 46556 (Received: August 2, 1982; In Final Form: October 4, 1982)
The rates of oxidation of MV+. by peroxodisulfateand by hydrogen peroxide have been investigated. The methyl viologen free radical was produced by pulse radiolysis. The S202- + MV+-reaction was first order in both species with k = 7.4 X lo7 M-’ s-l. A y-radiation study revealed a chain decomposition of S 2 0 2 - involving MV+when methanol, ethanol, or 2-propanol were present. Loss of MV+. in pulse radiolysis studies was then no longer observed to be a simple first-order reaction. The H202 + MV+- reaction was very slow with k = 6.7 M-’ s-l in the presence of 1 M methanol. A much faster reaction in the absence of methanol was interpreted to be a reaction of MV+. with HOz (or 02-) where the latter was produced from pulse-generated.OHradicals reacting with H2O2. The rate constants for HOz and 02-reacting with MV+-were 2.1 X lo9 and 2.8 X lo9 M-l s-l, respectively. Hydrogen peroxide, in contrast to the chain decomposition of S20a2-,does not participate in a chain reaction involving MV+-and methanol. MV+. reactions with Cr207%, IO,, and Fe(CNJ6+have rate constants of 5.0 X lo9, 4.3 x lo9, and 8.7 X lo9 M-’ s-l , respectively. Introduction Much attention is being given to methyl viologen (paraquat; l,l’-dimethyl-4,4’-dipyridiniumdichloride), MV2+(C1-)2,chemistry. Work has been directed toward the possible use of methyl viologen in the photochemical conversion of solar energy.2 Its role in that application as well as in its herbicidal properties is related to the facile photochemical reduction to the radical cation, MV+.. Since herbicidal activity is dependent on both light and oxygen3 the reduction products of oxygen, i.e., 0, and Hz02,have been considered as possible active species responsible for plant death.4,5 It has been reported that MV+. reacts quantitatively with oxygen to reduce it to water on a sufficiently small time scale that the reaction can be utilized in the measurement of very low concentrations of oxygen.6~~The rate constants for the first two steps in the reduction, i.e., O2 to 02-(or H02) and then to HOz- or (H202)are reported by Farrington et a1.8 as 8.0 X lo8and 9.2 X los M-’ s-l, respectively. However, the next step, the reduction of H20z,was recently reported by Levey et to be very slow with a rate constant of 2.0 M-’ s-l. Some questions regarding interpretations in the latter study have prompted a more detailed pulse radiolysis study of the reaction of MV+. with H202. An interesting recent study by Winterbournlo in which xanthine oxidase catalyzes production of MV+. by electron transfer from xanthine to paraquat in the presence of oxygen and H202led to the suggestion that Hz02can compete with O2 for reduction by MI’+.. The experimental evidence is in contradiction to that predicted from the Farrington and Levey reports
described above and from the results of the present work. The report by Levey et aL9included comments on the unusual character of the reaction of MV+. with S202-.The reaction was observed to be too rapid for a stopped-flow kinetics study so pulse radiolysis production of MV+. was used. Due to limited access to pulse radiolysis equipment and some unexpected complications, the few experiments that were conducted yielded only an approximate rate constant value of 106 M-’ s-l. Furthermore, it was reported that the rate constant appeared to be independent of the presence or absence of methanol. Most of the work was done in the presence of methanol and “unexpected absorptions” created problems in interpretation of data. Methanol was regularly used in the studies since Patterson et al.5 had shown that the .OH radicals produced by the electron pulse react with MV2+(C1-)2to produce a relatively stable .0HMV2+adduct and that methanol scavenges the -OHradicals to prevent adduct formation. Furthermore the -CH20Hradicals produced from methanol react with MV2+(C1-)2to produce additional MV+-with a rate constant of 3 X lo8 M-l s-l. The present study has revealed, however, that methanol subsequently enters a chain decomposition sequence involving MV+. in the case of S202but not HzOz. Both S2082-11 and H20212undergo chain decomposition reactions in the presence of methanol when no MV2+(C1-)2is present. The present work was carried out in an attempt to elucidate the mechanisms responsible for the seven orders of magnitude difference for the reaction of MV+-with S20a2and H202as well as the eight orders of magnitude difference for the reaction of MV+. with O2or 02-and H20z.
(1) To whom correspondence should be addressed at College Box 2311, Berea, KY 40404. (2) See, for example, P. Keller and A. Moradpour, J. Mol. Catal., 12, 261-3 (1981); M. Gratzel, Acc. Chem. Res., 14,376-84 (1981); 0. Johansen, A. Launikonis, J. W. Loder, A. W. H. Maw, W. H. F. Same, J. D. Swift, and D. Wells, Aust. J . Chem., 34,2347-54 (1981); M. Kaneko, M. Ochiai, K. Kinosita, Jr., and A. Yamada, J. Polym. Sci., Polym. Chem. Ed., 20, 1011-9 (1982). (3) A. D. Dodge, Endeaoour, 30, 130-5 (1971). (4) J. A. Farrington, M. Ebert, E. J. Land, and K. Fletcher, Biochim. B i ~ p h y sActa, . 314, 372-81 (1973). (5) L. K. Patterson, R. D. Small, and J. C. Scaiano, Radiat. Res., 72, 218-25 (1977). (6) R. E. V. D. Leest, J. Electroanal. Chem., 43, 251-5 (1973). (7) P. B. Sweetaer, Anal. Chem., 39, 979-82 (1967). (8) J. A. Farrington, M. Ebert, and E. J. Land, J. Chem. SOC.,Faraday Trans. I , 74, 665-75 (1978). (9) G. Levey, A. L. Rieger, and J. 0. Edwards, J. Org. Chem., 46, 1255-60 (1981). (IO) C. C. Winterbourn, FEBS Lett., 128, 339-42 (1981).
Experimental Section H202,(CH3)2CHOH,HC104,and CH30H were certified analytical reagents from Fisher. K2S20sfrom Fisher was twice recrystallized from water. t-BuOH and NaOH were Baker analyzed reagent grade. MV2+(C1-)2,from Sigma Chemical Co., was recrystallized from absolute ethanol. The pulse radiolysis studies were done with a computercontrolled ARC0 LP-7 linear accelerator with 7-MeV pulses of 5- or 10-ns duration. The pulses generally produced about ~ - F M total radical concentration except in the dose-dependent studies of the MV+. + Hz02reaction where _ _ _ _ _ ~ ~ ~
(11) J. E. McIsaac, Jr., and J. 0. Edwards, J. Org. Chem., 34,2565-71 (1969). (12) C. E. Burchill and I. S. Ginns, Can. J.Chem., 48,2628-32 (1970).
0022-365418312087-0829$01.50/0 0 1983 American Chemical Society
830
The Journal of Physical Chemistry, Vol. 87, No. 5, 1983
Levey and Ebbesen
TABLE I : Effect of CH,OH and MVZ+(Ci-),on H,O, Decomposition by 7 Radiationa
1 1 0 1
0 0 2 2
5 10 10 10
propagating reaction 5 suggested by Burchill is eliminated entirely when MV2+(C1-)zis present. The .0HMV2+adduct does not appear to take part (as suggested by Levey et al.)9 in a chain decomposition of HzOz.
30 35 1.6 2.3
a Dose rate = 3380 rd/min.
radical concentrations ranged from about 1to 8 pM. The experimental arrangement was one employing a flow through 1.00-cm cell so that the solution being irradiated was constantly being replaced with fresh solution. It was verified that the behavior of MV+. at or near the 393-nm peak in the spectrum where the extinction coefficient is =40000 M-' cm-l and at 605 nm where the extinction coefficient is =11OOO M-' cm-' is identical. The y-radiation studies employed a Gammacell 200 6oCosource with a dose rate about 3 X 10'' eV g-' min-' or, when a lead shield was used, about 4 X 10l6eV g-' min-'. Absorbance measurements for following the effects of radiation were made by a Cary 219 spectrophotometer. Solutions were freshly prepared from triply distilled water and deoxygenated by bubbling with pure N2. The concentrations of the 3 or 30% H2O2stock solutions were checked each time prior to use by permanganimetry. Analyses for HzOzfor following the effects of y radiation were made by using the Tirv-H20z reaction described by Er1enme~er.l~Analyses for Sz02- in following the effects of y radiation were done spectrophotometrically in the ultraviolet. When MV2+(C1-)2 was present the persulfate was reduced by excess iodide ion and the absorbance of the triiodide so produced was measured at 350 nm. y Radiolysis of MV2+(C1-)2-H202Solutions
y-radiolysis studies were conducted to aid in elucidating the mechanism of HzOzdecomposition in the presence of MV2+(C1-)zor MV2+(C1-)zplus CH30H. It was of special interest to determine whether or not MV+. is involved in a chain decomposition of HzOzas was the case with S202described later. The data given in Table I for the CH30H-H202 solutions in the absence of MV2+(C1-)zare in general agreement with the work of Burchill et a1.12 The chain decompositions of Hz02in those cases are relatively short. Addition of MV+(C1-)2eliminates the chain entirely with the resulting G(-H202) values approximately equal to those expected if the ea; + HzOzreaction was the only reaction responsible for loss of HzOz. The relative concentrations of MV2+(C1-)zand HzOz were such that approximately equal numbers of electrons went to M V + and to H202. Hence the G value for the ea; + H20zreaction was 1.5. The mechanism suggested is as follows:
+
MV2+ ea;
--
HzOz + ea(
-
MV+.
+ OH*OH + CH3OH HOH + *CHZOH CHZOH + MV2+ MV+. + H2CO + H+ .CHZOH + H2Oz HzCO + HzO + *OH MV+- + H2O2 -OHMV2+adduct + OH-
-
.OH
-+
(1) (2)
(3) (4)
(5) (6)
Since the second-order rate constant for reactions 4 and 5 are 3 X and 4 x 104,12respectively, the chain(13) H. Erlenmeyer, Helu. Chim. Acta, 47, 792 (1964).
Pulse Radiolysis of MV2+(C1-)2-H202Solutions The initial radiolysis experiments involving MV2+(C1-)2-Hz0z solutions suggested a very large effect by CH30H on the rate constant for the MV+. + HzOzreaction in contrast to the little or no effect observed earliergin the stopped-flow kinetics study of that reaction. Upon examination of the H202concentration dependence and dose dependence it was soon realized that the reaction in the absence of methanol was one between MV+- and a dose produced species rather than with HzOz. Since the dose produced species must have been the .OH radical or a product of its reaction with a solute, its concentration was necessarily comparable to that of MV+.. The reaction of MV+. with .OH was ruled out in view of the large rate constants for the reaction of -OH with MV2+(C1-)2(k = 2 X and with HzOz (k = 3 X 107).14 With high HzOz concentrations the .OH + H202 HOH + HOz reaction predominates so the dose produced species reacting with MV+. is assumed to be HOz or Oz- depending on the pH. Also, at high Hz02concentrations, Hz02competes very favorably (k = 1.6 X 1O'O M-' s-l)15with MV2+(k = 7.7 X 1O1O M-' s-', an average of values reported by Patterson et al.5 and Farrington et al.4) for electrons and in this way additional .OH radicals are produced. Although the MV+. yield is greatly reduced it is still possible to follow the pseudo-first-order loss (by reaction with HOz or Oz-) because of the very large extinction coefficient of about 4.0 X lo4 M-' cm-' at 393 nm. Calculation of the HOz or 02concentration for each experiment requires consideration of the dose, the spur scavenging of eaq-and .OH by high Hz02concentrations, the competition between H20zand MV2+(C1-)2for electrons, the competition between MV2+(C1-)2and HzOz for .OH produced directly in the pulse as well as from the ea!- + HzOzreaction, and the H + HzOz HzO + -OH reaction. Dose measurements were regularly done with a thiocyanate dosimetry. Spur scavenging of electrons was taken into account by use of calculations suggested by Schuler et a1.16 Values for G(ea;) ranged from 3.6 to 4.0 for the concentration range of HzOzused. The G(0H) formed directly in the pulse (in contrast to that from the eaq-+ Hz02and H + HzOzreactions) was calculated to be in the range of 2.8-3.0 depending on the &02concentration. This includes spur scavenging of .OH (by H202)by using calculations which follow the suggestions of Schuler et al." The contribution to G(0H) from the ea; + H20z OH- + .OH reaction employed the rate constants k(eaq-+H20z)= 1.6 X 1O1O l4 and k(ea;+MV2+) = 7.7 X 10'0495along with the concentrations of H20z and MV2+(C1-)2.The contribution to G(0H) from the H H20z HzO + .OH required information regarding the competition for H between M V 2 + ( W 2and HzOz. The rate constant for the H + MV2+(C1-)2reaction was found to be 1.6 X lo8 M-' s-' for solutions that were 0.1 or 1 M in HC104. The literature value for the H + HzOzreaction is 6.1 X lo7 M-' s-l.18 In
-
-
-
+
-
(14) J. K. Thomas, Trans. Faraday Soc., 61, 702-7 (1965). (15) R. R. Hentz, Farahataziz, and E. M. Hansen, J. Chem. Phys., 56, 4485-8 (1972). (16) T. I. Balkas, J. H. Fendler, and R. H. Schuler, J . Phys. Chem., 74,4497-4505 (1970). (17) R. H. Schuler, A. L. Hartzell, and B. Behar, J. Phys. Chem., 85, 192-9 (1981). (18) M . Anbar and P. Neta, Znt. J.Appl. Radiat. Zsotop., 18,493-523 (1967).
The Journal of Physical Chemistry, Vol. 87, No. 5, 1983 831
MV+- with Oxidizing Radicals
TABLE 11: Rate Constants for the MV'. (or 02-) Reaction
10-9k,
compd
pH
MVZ+(ClO,-),
7 7 3.5 3.5
MVZ+(Cl-), MVZ+(CIO,-), MVZ+(CI-), a
M - ' s - ' G(H0,)" 2.9 f 1 6.12 2.6 6.19 2.2 t 1 6.51 2.1 2 1 5.98
-+
HO,
G(e) + G ( 0 H )
+
G(H) 7.32 7.30 7.43 7.37
An average of the values used for HO, concentration
calculations.
view of the high H20zconcentrations used it was assumed that all of the H atoms reacted with H20zto produce .OH. Finally, the G(HO2)evaluation utilized the total G(0H) and the competition for -OH between MV2+(C1-)2and HZO2.An experimentally determined value for the ratio of rate constants of 6.7 was used, i.e., k(.0H+MV2+)/ k(-OH+H20z). Pulse calibration data and G(H02)values were used to calculate HOz concentrations. Since the pK, for HOz is 4.8, the pseudo-first-order loss of MV+. was followed for solutions with pH 3.5 and 7.0 to determine rate constants for reactions of MV+. with HOz and 02-, respectively. There was sope concern that the C1- OH HOC1- reaction might also play a role in the kinetics observed. Swallow et al.l9 report a rate constant of 4.3 X lo9 M-' s-l and an equilibrium constant of 0.7 for the reaction. Chloride of MV2+(C1-)zwas replaced by perchlorate by use of an anion exchange column and the study repeated. In all the experiments the H202concentrations were in the range of 40-160 mM in order to arrive at pseudofirst-order conditions for the MV+. decay. For each concentration the dose was varied from 300 to 1200 rd and three or more kinetic traces were analyzed for each doseconcentration condition. The averaged rate constants were treated by linear regression calculations to arrive at the second-order rate constants shown in Table 11. I t will be noted that the G(H02)values are in all cases a large fraction of the total G(e) + G(OH) + G(H) average of about 7.3 and is, therefore, not expected to be greatly sensitive to uncertainties in the two sets of relative rate constants used in its evaluation. The rate constants for the perchlorate and chloride salts are very similar. This suggests that the .OH C1- reaction and possible subsequent reactions are not significant under our conditions. Although the MV+. Oz- rate constant, k = 2.8 X lo9 M-' s-l, is larger than that for MV+. + H02, 2.1 X lo9 M-ls-l, the difference is less than might be expected in view of the fact that the first is a cation-anion reaction whereas the second is a reaction between a cation and a neutral species. The rate constants are larger than the value of 9.2 X los M - ' d observed by Farrington et al.8for the MV+. 0, reaction. Our determinations were made under different conditions and is a more direct method than theirs. Their method depended on following the kinetics after first following the MV+. O2 MV2++ 02-reaction which has a comparable rate constant, i.e., 8.0 X lo8 M-l s-l.
-
+
+
+
+
+
-
Pulse Radiolysis of MV2+(C1-),-HzO2-CH30H Solutions Solutions 1.0 M in methanol and 2.0 mM in MV2+(C1Q were pulsed with increasing concentrations of H202for a series ranging from 25 mM to 215 mM. Pseudo-first-order rate constants for MV+. loss plotted vs. concentration resulted in a linear plot with a slope of 6.7 M-ls-I. In light of the y-radiation mechanism suggested earlier it appears (19) G. G. Jayson, B. J. Parsons, and A. J. Swallow, J. Chem. SOC., Faraday Trans. 1 , 69, 1597-1607 (1973).
that the primary reaction must be that of MV+.with H202 This experimental value establishes the reaction to be eight to nine orders of magnitude slower than the MV+-reactions with H02 (or 0,) and OF The difference between the rate constant reported here by pulse radiolysis (6.7 M-' s-l) and that observed previously by stopped-flow kinetics (2.0 M-l s-l) may be due, in part, to uncertainties in the pulse radiolysis method application to such very slow reactions. The latter study was much more limited in scope than the stopped-flow study so the 2.0 M-' s-l value is considered the more reliable.
of MV2+(C1-)2-S20s2-Solutions Solutions containing MV2+(C1-)z,S202-, and several alcohols were irradiated with y radiation. Several pulse radiolysis experiments had previously indicated a great difference in the rate of loss of MV+. absorption for systems containing MV2+(C1-)2-SzOs2- or MV2+(C1-)2-S20s2--t-kuOH and those containing MV2+(C1-)z-Sz02-plus 1.0 M methanol, ethanol, or 2-propanol. Table I11 lists data collected in the y-radiation study. The data for S202- plus methanol or ethanol reactions indicate the existence of a chain reaction decomposition of Sz02-. 2-Propanol was reported to have an even longer chain length for decomposition of Sz02-.20These data in the absence of MV2+(C1-)2are in general agreement with chain length observed for the thermal reaction of about 50,600, and 1800 for methanol, ethanol, and 2-propanol, respectively, by Edwards et The dramatic increase in G(-S20,2-) upon adding MV2+(C1-)2to these systems indicates that MV+. enters the reaction in chain-propagating roles. The mechanism suggested for the chain decomposition reaction of S202-is as follows: MV2+ ea; MV+(1) y Radiolysis
+
--
SzOa- + eaq-
-
SO-:
+ SO4--
+ CH8OH CH20H + HOH CHZOH + MV2+ MV+. + H2CO + H+ MV+- + S20a- MV2++ + SO4-. .OH
SO4-- + CH30H
HS04- + CHzOH
(7) (3) (4) (8) (9)
The same mechanism applies when ethanol or 2-propanol replace methanol. In the absence of MV2+(C1-)2the chain reaction requires that the alcohol radical reacts with S20a-, Le., CHzOH + Sz02- HS04- + SO4-- + HzCO. This reaction appears to be much slower than the reactions of MV2+ with .CHZ0H and CH,CHOH which have rate constants 3 X los5and 2.7 X lo9 M-l s-l, respectively. The latter value was determined in this study for MV2+(C1-)2 concentrations ranging from 0.2 to 1.0 mM and 1 M ethanol. Pulse Radiolysis of MV2+(C1-)2-S20~Solutions In the earlier brief investigationgof the pulse radiolysis of MV2+(C1-)2-S202--CH30Hsolutions the decay of the absorbance of MV+-was reported to exhibit unusual behavior. Although the Sz02- concentration (0.2-1.0 mM) was clearly in great excess over the MV+. concentration (about 3 pM) in the presence of alcohols (except t-BuOH) the decay of MV+. absorption could not be analyzed by any simple kinetic order. This might be expected if a chain reaction occurs as was observed for the y-radiation study described above. In the presence of 50 mM t-BuOH or in (20) G. Levey and E. Hart, J. Phys. Chem., 79, 1642-6 (1975). (21) A. R. Gallop0 and J. 0. Edwards, J. Org. Chem., 36, 4089-96 (1971).
832 The Journal of Physical Chemktry, Vol. 87, No. 5, 7983
Levey and Ebbesen
TABLE 111: Effect of Alcohols and MV2+(C1-),on S,0,2- Decomposition by 7 Radiationa [CH,OH], M
[C,H,OH], M
[t-BuOH], M
[ M v z + ( a - ) 2I, mM
1.0 1.0 1.0 1.0 1.0 1.0
a
[S,O,z-] = 2.00 mM in all cases.
dose rate, rdlmin
0 1.0 0 1.0 0 1.0 0 1.0
7 15 7 15 7 15
71 5 3380 3380 3380 3380
G(-S,O,'-) 98 >3800
605 b 6.1
30 6.4 1.9
Too large to measure with radiation source.
the absence of any alcohol the rate constant for the reaction between MV+. and S20S2-was 7.4 X lo7 M-* s-l. The slower, nonfirst-order decay of MV+. in the presence of alcohols must be due to regeneration of MV+. during the chain decomposition of Sz02-. The spectrum for a C2H50H-MV2+(C1-)z-S202- system exhibited a MV+. spectrum after 50 119, i.e., after >lo0 lifetimes (in the absence of any alcohol). Clearly, therefore, the slower decay and the "unexpected absorptions" observed by Levey et al? in the earlier work were due to the chain regeneration of MV+. when methanol was present. Additional MV+ 0 Reactions As expected the MV+. radical reacts rapidly with strong oxidizing agents. Rate constants for the reaction of MV+. with Cr2072-, IO4-, and Fe(CN):- were observed to be 5.0 X lo9, 4.3 X lo9, and 8.7 X lo9 M-l s-l , r espectively, with uncertainties of f10% or less. The solutions were 1mM in MV2+(C1-)2and the solute concentrations were varied up to 0.2 mM for Cr2072-and IO4-, and to 0.05 mM for Fe(CN)63-. Summary and Conclusions 1. The reaction between MV+. and H202is a slow reaction as observed earlier by stopped-flow and now by pulse radiolysis methods. In the latter case methanol must be present to avoid the reaction of MV+. with H 0 2where the latter is produced by the .OH + H202reaction. The reason for the eight orders of magnitude difference for the reaction of MV+. with O2 or 02-and with H202 is not obvious. In the reaction with O2or 02-an electron transfer to O2 or 0, is assumed. However, the reaction with H202 is not a simple electron transfer since this would produce .OH and then a chain decomposition of H202would be predicted when methanol is present. This does not occur. 2. MV+. inhibits the normal radiation-induced chain decomposition of H202 when 1 M CH,OH is present. However, MV+. greatly enhances the radiation-induced chain decomposition of S202- in the presence of 1 M CHBOHor 1 M C2H50H. 3. The reaction between MV+. and H 0 2 or 02-,depending on pH, was observed by the pulse radiolysis of solutions containing ratios of MV2+(W2or MV2+(C10,-)2 concentration t o H202up to 80. Rate constants for the reactions were observed to be 2.1 X lo9 and 2.8 X lo9 M-l at pH 3.5 and 7.0, respectively. Chloride ions (from
MV2+(Cl-)2)apparently are not involved in post pulse reactions since essentially the same rate constants were observed with MV2+(C104-)2 and MV2+(C1-)z. 4. Although the study yielded definitive information regarding rate constants for the reaction of MV+- with S202-and with H202,no explanation was obvious for the seven orders of magnitude difference in the rate constants. A reviewer of this manuscript, however, has suggested an explanation based on difference in bond dissociation energies for the 0-0 bond in S202-and H202 He suggested plus MV+. reaction might that the faster rate of the S20S2be explained by the formation of resonance stabilized SO4-. in the slow step as opposed to .OH with no such stabilization. A resonance stabilization energy for SO4--of 7.5 kcal/mol can be calculated to account for at least 5.5 of the 7 powers of ten difference in the rate constant. The Coulombic effect for the S202-plus MV+. reaction can account for the additional 1.5 powers of ten. The resonance stabilization energy of 7.5 kcal/mol is calculated by taking one-half of the difference in 0-0 bond dissociation energies for H20z (48-50 kcal/mol) and S202- (33.5 kcal/mol). Implicit in this approach, as is the usual case for these kinds of comparisons, is that the nature of the 0-0 bond in H202is about the same as it is in S202-. In any event, the factors that contribute to the difference in bond dissociation energies of H202and S202-can be expected to be operative in the formation of .OHand SO4-.. 5. The strongly oxidizing inorganic anions, Cr2072-,IO,, and Fe(CN):-, react with MV+. with rate constants 5.4 X lo9, 4.3 X lo9, and 8.7 X lo9 M-ls-l , r espectively.
Acknowledgment. The authors recognize with gratitude the helpful suggestions from P. Neta, R. Schuler, and J. Grodkowski of the Notre Dame Radiation Laboratory regarding experimental work and data interpretation and also the technical assistance in the use of the linear accelerator by T. Deal. Discussions with John 0. Edwards of Brown University are also appreciated. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2370 from the Notre Dame Radiation Laboratory. Registry No. MV2t(Cl-)2, 1910-42-5; M V . , 25239-55-8; H2O2, 7722-84-1; CH,OH, 67-56-1; S20a2-,15092-81-6;Cr2072-,13907-47-6; IO3-, 15454-31-6; Fe(CN):-, 13408-62-3; ethanol, 64-17-5; 2propanol, 67-63-0.
