T H E
J O U R N A L
O F
PHYSICAL CHEMISTRY Registered i n U.S. Patent Office 0 Copyright, 1978, by the American Chemical Society
VOLUME 82, NUMBER 24
NOVEMBER 30, 1978
Effect of pH on O ~ y g e n ( ~ PAtom ) Formation in y-Ray Irradiated Aqueous Solutions’ Weldon G. Brown* and Edwin J. Hart Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received February 3, 1978; Revised Manuscript Received June 22, 1978) Publication costs assisted by Argonne National Laboratory
The yield of O(”) atoms, G(03P),inferred from the yield of CzH4, G(C2H4),in y-ray irradiated aqueous cyclopentene solutions, has been measured in the pH range 1-13.5. In the range 1-11, G(C2H4) is independent of pH but it rapidly doubles between pH 11 and 12.5 and then falls abruptly toward 0 at higher pHs. The relative yield of O(3P)from photolyzed KBr03 solutions was also studied. In this case, pH independence was found in the range 7-11 followed by a pH dependent fall in yield at higher pHs. The pH dependent rise in G(03P)is attributed to dissociation of OH- by subexcitation electrons; the decrease in G(C2H4)is caused by reaction of OH- with O(3P). The fraction of O(3P)atoms producing C2H4from C5H8is reported for aqueous solutions and so G(03P)absolute may be calculated from G(CzH4). O(3P)reacts 2.6 times faster with CSHs than with Oz.
Introduction Previous studies2established that O(3P)atoms form in small yield in y-ray irradiated neutral aqueous solutions. This conclusion was inferred by the liberation of ethylene from cyclopentene (CP) in y-ray irradiated solutions. The reaction is C5H8+ O(3P) C2H4+ C3H40 (1) This reaction has been firmly documented in gas-phase studies where 25% of the 0 atoms proceed by this react i ~ n The . ~ yield G(C2H4),only 0.0017 in pure water, increases tenfold in irradiated 4 M KC104.2 In this case “direct” action by y rays decomposes the C104- by the reaction4 C104- C10,- + O(3P). That the 0 atom from pure water was identical with the one liberated by the above reaction was confirmed by the same relative scavenging effect of methanol.2 In the present paper we report G(C2H4)as a function of p H in the range 1-14. Near p H 12, G(C,H,) doubles from its constant value a t lower pHs and then approaches zero a t pHs above 13. We also establish the absolute yield of G(03P) by determining the fraction of 0 atoms proceeding via reaction 1. We assume that the G(03P)yields
-
-
0022-3654/78/2082-2539$0 1.OO/O
parallel G(C2H4)and interpret the pH dependent 03P formation by a mechanism involving water subexcitation electrons.
Experimental Section Except for the measurement of CP concentration, the preparation, irradiation, and analysis of solutions have been previously described.2 The CP concentration was determined by spectrophotometric measurements a t 185 nm in pure water. Saturated solutions of CP were prepared from the chromatographically purified compound by injection of 0.1 mL into a 100-mL syringe containing 80 mL of degassed triply distilled water. After addition of 5 cm3 of helium to provide a gas phase, the contents were shaken and the gas was expelled. Finally, the undissolved CP which had floated to the top was removed through the syringe capillary and the optical density of suitably diluted solutions was determined using a Cary Model 14 spectrophotometer. The spectrum of CP shown in Figure 1 was obtained on aqueous solutions prepared from a standard 0.1133 M CP in pure ethanol. In these measurements care was taken to replace O2 by K2 in the spectrophotometer cavities. Extinction coefficients are 0 1978 American Chemical Society
2540
The Journal
of Physical Chemistry, Vol. 82, No. 24, 1978
W. G. Brown and E. J. Hart
“: ‘ O O i
.OGll
t t
170
I
180
I
190
I
ZOO
210 X(nrn1
I
1
220
230
Flgure 1. Absorption spectra of water,lg hydroxide ion, and cycloHzO, (0)0.01 M OH-, (A) 0.1 M OH-, (0) 1.0 M OH-, peritene: (0) ( 0 )1 mM C5Hs.
0.I
01
240
1
I
I
10
mMC,H,
Flgure 2. Effect of cyclopentene concentration on G(C2H4)in y-ray irradiated neutral solutions.
7580, 6530, and 4190 M-l cm-l a t 185, 190, and 195 nm, respectively. Photolysis experiments were performed on solutions in a flat 1-cm deep silica cell of 13 mL volume exposed 2 min a t a distance of 7 cm to a GE G4T4/1 germicidal lamp (4 W) as a source of 253.7-nm radiation. The irradiated solutions were transferred to the Van Slyke apparatus for extraction of C2H4 and other volatile products. Gas chromatographic analysis was then carried out as in the previously described y irradiation experiments.
Results Reaction o f O(3P)A t o m s w i t h CP. The fraction of 0 atoms yielding ethylene in aqueous solutions is needed to place G(C2H4)on an absolute basis. The flash photolysis of 0.01 M Br03--02 solutions provided us with a method of measuring this ratio. The absolute number of 0 atoms generated by the reaction, Br03- + hu Br02- + O(3P) was measured after the flash by the transient concentration of ozone formed by 0, + O(3P) Os. A molar extinction coefficient of 3600 M-’ cm-’ a t 260 nm was used.5 In solutions where the 0, was replaced by CP, ethylene forms and from the relative yields of ozone and ethylene per flash in the two experiments, the fraction of 0 atoms producing ethylene may be calculated. This fraction, 0.24 in aqueous solutions, is identical with the fraction, 0.25, reported for the gas phasee3 This result further supports our conclusion that O(3P) atoms form in irradiated water. O(3P) atoms react 2.6 times faster with CP than with 02. By measuring the concentration of ethylene in flash-photolyzed 0.01 M Br03- solutions containing different ratios of C6H8/O2,we obtained the above result. An attempt was made to determine this relative reactivity from the suppression of the transient O3 band centered at 260 nm by CP in Br03--02 solutions. However, an anomalous absorption of light, probably by acrolein (the product accompanying CzH4in reaction l),interferes with the spectrophotometric measurement and this method had to be abandoned. Effect of C P Concentration. Over the available concentration range up to 0.009 M, CP displays the usual “indirect” action effect, see Figure 2. G(C2H4)rises from 0.0012 to 0.0017 as the CP concentration increases 25-fold to - 5 X M. This result rules out any from 2 X direct action producing CzH4. Earlier it was shown that eaq-,H , HOz, and OH radicals and O2 do not react with CP or its radicals to produce CzH4. Below it is also shown that 0-,H202,and O3 do not alter G(CzH4)to any appreciable extent. Hence we conclude that C2H4arises solely through 0 atom reaction with CP.
-
0°/
‘0
,
I
I
2
4
6
, , 8
IO
I,! 12
14
PH
-+
Figure 3. Effect of pH on G(C2H4)in y-ray irradiated solutions: (9) 0.7 mM CSH,; (0)4.5 mM C6H8;(a)4.5 mM C6H8 0.234 p M 02; (a) 4.5 mM C,H8 100 pM N,O.
+
+
4-
-
37.5
251 t
12.5
“6
8
10 PH
12
14
Flgure 4. Effect of pH on C2H4yields with photolysis at 253.7 nm in 0.0009 M cyclopentene-0.01 M KBrO, solutions.
Effect of p H . An unexpected increase and subsequent fall in G(C2H4)takes place in alkaline solutions. G(C2H4), constant in the range from pH 1to 11,rises sharply at pH 11 and subsequently falls abruptly beginning about pH 12.5 (see Figure 3). G(CzH,) is not altered in N20 solutions over the pH range 8-13.5. In strongly alkaline solutions with NzO present where OH has been converted to 0-, and ea?- to 0-, G(C2H4)follows the same pH dependence as in NzO-free solutions (see Figure 3). This demonstrates again that neither OH or 0- affect G(C2H4) since the amounts of these species are doubled in the presence of NzO.
The Journal
Atom Formation in y-Ray Irradiated Solutions
Photochemically generated O(3P) atoms display a different pH dependence. Missing is the increase in C2H4 yield in alkaline solutions although the fall in yield that occurs above pH 11 is similar to that found in radiolysis (see Figure 4). In this case O(3P) atoms were generated by the flash photolysis of Br03- solutionsa6Here O(3P)is a prominent product since the 010- ratio for the pair of reactions, Br03- hv Br02- O(3P) and Br03- + hv B r 0 2 0-, is 1.6.6 The quantum yield for the decomposition of Br0< is 0.19 in the spectral region 190-260 nm.’ The effect of X on q!J was not reported.
-
+
+
-
+
Discussion of Results We interpret our results in terms of two separate dissociation reactions, one involving excitation and dissociation of water, the other excitation and dissociation of OH-. The case for production of O(3P) from irradiated H 2 0 and from photolyzed BrO< is well documented2,6and will not be treated in the present discussion. Dissociation of Water. We attribute the pH independent C2H4yield of Figure 3 to O(3P)atom formation by dissociation of the excited water molecule
-
H~O*
+
o ( 3 ~ )H~
(2)
This reaction requires 5 eV8 and thus has a threshold of 246.8 nm, well above the primary absorption band of water. Compared t o the total yield of water dissociation products involving some five water molecules, this reaction is insignificant and since the pH independent G(C2H4)is 0.0016 and 0.24 of the 0 atoms produce C2H4 we calculate an absolute G(03P)of 0.0067. Thus, reaction 2 is a dissociation process of very low probability compared to the other excitation and ionization processes taking place during radiolysis. Dissociation of (OH-)*. In Figure 3, note the rapid rise in G(C2H4)in the first increment of increasing pH above 11,especially in the 0.7 mM C5H8solutions. This increase in yield indicates an efficient process quickly quenched at higher concentrations of OH-. Different is the pH dependence for 0 atoms generated by the photochemical dissociation of BrOy (see Figure 4). Here the increase in q!J(C2H4)above pH 11 is missing and only the common depressing effect of the OH- beyond this pH is observed. We ascribe the increase in G(C2H4)in alkaline CP solutions to dissociation of excited OH-. In our assumed mechanism the first step involves excitation of OH- by subexcitation electrons The reactions are OH-
+ es[
(OH-)*
-
(OH-)* + (ese-)
(3)
+
o ( 3 ~ ) H-
(4) Subexcitation electrons have been previously suggested as being responsible for “indirect” effects observed in the radiolysis of formic acid,l0J1nitrate ion,l2J3peptides,14-16 and uranyl ion.17 It is possible that reaction 3 is a particularly simple example of the action of es; on an important aqueous ion. T o assess the possible role of e8; in reaction 3, let us review the properties of es; as described by PlatzmanOg These free electrons, equal in number to the total number of positive ions generated by the radiation all have an energy E smaller than the smallest electronic energy Eo of the water molecule. This group of electrons include electrons ejected with initial energy E 5 Eo and electrons which initially had E > Eo but subsequently suffered one or more inelastic collisions. For liquid water -dt/dt 1013 eV/s for e&, whereas -dE/dt 10l6 eV/s for E 10 or 20 eV.18 This thousandfold disparity in rate of energy loss for e,;.provides a mechanism for activation of solutes
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--
of Physical Chemistry, Vol. 82,No. 24, 1978 2541
a
a
(OH-)bJ
Figure 5. Determination of the rate constant ratio, k ( 0 iOH-)/k(O 4- C,H8) from the data of Figure 4. Ro = CPH, yield in neutral solutions; R = C2H, yield at indicated OH- concentration.
present in concentrations of 0.001-1 M. If this minor component x has a smallest electronic energy E , < Eo practically all of the water subexcitation electrons having energy in the interval E, < 6 < Eo may excite x. We suggest that this is the mechanism of activation of reaction 3. An alternative mode of dissociation of (OH-)* is (OH-)* OH + eaq-. That the OH- has the properties required for activation by es; may be observed in Figure 1where the absorption spectra of water, C5H8, and OH- are compared. These electronic spectra of Figure 1 clearly illustrate the energy intervals in which es; possesses the energy sufficient to excite electronic transitions in C5H8and OH-. In a typical experiment a t pH 12 the hydroxide ion absorption curve is displaced from the water curve by about 1 eV toward lower energies. From the above discussion we see that es; within the interval -5.6 < E < -6.7 may generate (OH-)* via reaction 3 from the minor component OH-. As expected, increasing of OH- concentration increases G(03P). A second reaction involving the OH- appears to destroy O(3P)atoms. The spectrum of the C5H8is included from comparison with that relative to liquid water and OH-. At this stage no quantitative assessment of the role of es; interactions with C5H8to produce CzH4 can be made. We conclude, however, that this interaction is minor a t the concentrations used since G(C2H4)= 0.0017 f 0.0002 over the 40-fold concentration range from 0.1 to 4.0 mM C5H8(see Figure 2). Reactions of the (OH-). The diminishing yield of C2H4 a t pH >12.5 for the radiolysis of CP (Figure 3) for the photolysis of BrO; solutions (Figure 4) suggests a pH dependent reaction of O(3P) with the (OH-). We propose the reaction
-
O(3P) + (OH-),,
-+
(HOZ-)aq
(5)
This reaction has a large favorable free energy change. A competition plot of the data of Figure 4 yields the linear plot of Figure 5 , and a relative rate constant ratio k 5 / k l of 0.037. Although we have interpreted O(3P)atom formation in alkaline solutions as an OH- excitation by es;, this is by no means the only mechanism conceivable. Reasonable too is a “direct” action mechanism on (OH-)aq, Triplet water energy transfer to (OH-)aq is also a possible mechanism. Experiments are now in progress to provide further evidence for the mode of activation of the hydroxide ion. Acknowledgment. We thank Robert M. Clarke and Patricia D. Walsh for their technical assistance throughout these studies. We also gratefully acknowledge the advice
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The Journal of Physical Chemistry, Vol. 82, No. 24, 1978
M A . Chan and A.
and cooperation of Dani Meisel and Max S. Matheson in connection with the photochemical studies and their interpretation.
C. Wahl
(IO) E. J. Hart, J . Am. Chem. Soc., 81,6065 (1959). (1 1) E. J. Hart and R. L. Platzrnan, "Mechanisms in Radiobiology", Vol. 1, M. Errera and A. Forssburg, Ed., Academic Press, New York, 1961, Chapter 2, pp 105 and 189. (12) M. Faraggi, D. Zehavi, and M. Anbar, Trans. Faraday Soc., 67,701 (1971). (13) M. L. Hyder. J . Phys. Chem., 69, 1858 (1985). (14) M. A. J. Rodgers and W. M. Garrison, J. Phys. C k m . , 72,758 (1968). (15) W. M. Garrison, M. E. Jayko, M. A. J. Rodgers, H. A. Sokol, and W. Bennett-Carniea, Adv. Chem. Ser., 81, 384 (1968). (16) M. A. J. Rodgers, H. A. Sokol, and W. M. Garrison, Bbchem. Siophys. Res. Commun., 40,622 (190). (17) C.Gopinathan, G. Stevens, and E. J. Hart, J. Phys. Chem., 76,3698 (1972). (18) H. Frohlich and R. L. Platzman, Phys. Rev., 92, 1152 (1953). (19)J. L. Weeks, G. M. A. C. Meaburn, and S. Gordon, Radiat. Res., 19, 559 (1963).
References and Notes (1) Work performed under the auspices of the Division of Basic Energy Sciences of the U.S. Department of Energy.
(2) W. G. Brown and E. J. Hart, Radiat. Res.. 51,249 (1972). (3) R. J. CvetanoviE, D. F. Ring, and L. C. Doyle, J. Phys. Chem., 75, 3056 (1971). (4) D. Katakis and A. 0. Allen, J . Phys. Chem., 68, 3107 (1964). (5) H. Taube, Trans. Faraday Soc., 53, 656 (1956). (6) 0.Amichai and A. Treinin, Chem. Phys. Lett., 3, 611 (1969). (7) L. Farkas and F. Klein, J. Chem. Phys., 16, 886 (1948). (8) R. S. Dixon, Radiat. Res. Rev., 2, 237 (1970). (9) R. L. Platzman, Radiat. Res., 2, 1 (1955).
Rate of Electron Exchange between Iron, Ruthenium, and Osmium Complexes Containing 1,l Q-Phenanthroline,2,P'-Bipyridyl, or Their Derivatives from Nuclear Magnetic Resonance Studied2 Man-Sheung Chan and Arthur 6. Wahl" Department of Chemistry, Washington University, St. Louis, Missouri 63 130 (Received May 26, 1978) Publication costs assisted by the National Science Foundation
The rates of a number of electron-exchange reactions between M L F and ML?+ complex ions have been measured by the NMR line broadening method in the temperature range -5 to 35 "C, the symbols M and L representing, respectively, Fe, Ru, or Os and 1,lO-phenant,hroline (phen), 2,2'-bipyridyl (bpy), or their derivatives. Most measurements were made with acetonitrile as the solvent, but some were made with formic acid, and a few were made with water; ionic strengths were usually close to 0.1 M. The exchange reactions followed a second-order rate law, first order in each reactant concentration. The rate constants determined were all of the same large magnitude ( -IO6 M s-'), and their temperature dependences were small N 2 kcal/mol). Exchange rates increased with anion concentration, and were 2 to 3 times larger when Clod- was the anion than when PF6was the anion. The dependence of exchange rates on anion concentration and other data indicate that exchange occurred mainly between reactants that were associated to some extent with anions to form ion pairs, triplets, etc. The rate of electron exchange was 15 times smaller when cyclohexyl groups were substituted for hydrogen in the 4,7 positions of the I?e(phen)2+t3+reactants, and the rate with cyclohexyl substitution was 20 times smaller than when phenyl groups were substituted in the 4,7 positions. This latter observation indicates that phenyl groups conduct electrons better than do the cyclohexyl groups, since the groups are of similar size and should have similar steric effects. Substitution of methyl groups on the ligands increased the electron exchange rate by a factor of about 2, and several possible reasons for this effect are discussed. Also, changing metal centers from Fe to Ru to Os increased the exchange rate at 25 "C about a factor of 2 for each change. No difference in rates at 25 "C was observed for electron exchange between racemic R~(bpy)?+,~' mixtures and between the d - R ~ ( b p y ) , ~ +isomers. ,~+ The activation parameters derived from the data are similar to theoretical values calculated from the Marcus theory of electron transfer on the assumption that reactants are spherical, are 14 A in diameter, have 2+ and 3+ charges, and react in a continuous, unsaturated dielectric medium with zero ionic strength. However, the similarity in values is most likely fortuitous, since, as discussed, the reactants are probably associated with anions and thus have charges less than 2+ and 3+, and since the reactants are not spheres, but have sufficient space between the ligands to accomodat,e anions and/or solvent molecules. The method used to derive exchange rate constants from NMR line shapes that were broadened due to spin-spin coupling as well as exchange is discussed, and NMR data are given for the complex ions investigated.
(e
Introduction T h e rates of electron exchange between ML?" and MLi3+,M representing Fe, Ru, or Os and L representing 1,lO-phenanthroline, 2.2'-bipyridyl, or their derivatives, are of considerable interest as examples of: electron transfer via the outer-sphere mechanism. The ligands are not labile, the complex ions are large so energies of activation due to Coulombic repulsion and outer-sphere rearrangement should be small, and, since electron transfer involves only the electrons in the tQg orbitals and metal-nitrogen bond distances3,* and stretching frequencies5 in the 0022-3654/78/2082-2542$0 1.OO/O
reactants are essentially the same, the inner-sphere rearrangement energy should also be srnalL6 In addition, there are strong charge-transfer interactions between the tzgorbitals and the ligand P orbitals in these complex ions,7 an indication of metal-ligand electron delocalization, which should facilitate electron transfer via ligand conduction and may account for the much faster electron transfer observed for complexes with conjugated ligands than for complexes with saturated ligands (e.g., k Ilo7 M-? s-l for Ru(bpy)32+,3+compared to k < 8 X lo2 M-' s-l for Ru(en)32i*3+) .8,9
0 1978 American Chemical Society
