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One-Electron Reduction Potentials for Radicals
One-Electron Transfer Equilibria and Redox Potentials of Radicals Studied by Pulse Radiolysis’ Dan Meisel’ Radiation Research Laboratories and Department of Chemistry, Mellon institute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 152 13
and
Gideon Czapski
DeparTment of Physical Chemistry, The Hebrew University, Jerusalem, / m e / (Received December 16, 1974) Publication costs assisted by Carnegie-Mellon University and the U.S.Atomic Energy Commission
The pulse radiolysis technique is utilized for measurements of the equilibrium constants for electron transfer between the durosemiquinone radical anion and oxygen, menadione, and indigodisulfonate. These equilibrium constants are in turn used for calculations of one-electron redox potentials for these systems. Each of these equilibrium constants was determined experimentally and independently and found to be selfconsistent. Only for the reactions of the semiquinone radical ions with oxygen could the electron transfer reaction be followed directly. For the reactions between the various quinone-semiquinone systems substantial indirect evidence is presented that these equilibria are achieved rapidly. In those cases equilibrium constants were determined from studies of the effect of quinone concentrations on the relative yields of the semiquinones. A method for distinguishing between kinetic competition and equilibrium is outlined and its usefulness is emphasized, The DQ1 DQ- (DQ = duroquinone) and IDS1 IDS- (IDS = indigodisulfonate) systems were employed as reference couples as the redox potentials for those systems are either available in the literature (IDSlIDS-) or may be calculated from available data (DqDQ-). Taking E7l, the redox potential for the first one-electron reduction step at pH 7 , of DQ1 DQ- as -0.235 V or of IDS1 IDS- as -0.247 V both yield E7l = -0.325 V for the 0 2 1 0 2 - system (1 atm of 02) and E7l = -0.20 V for the menadione system.
Introduction The experimental determination of redox potentials of short-lived radicals has been the aim of considerable effort ever since Michaelis2* presented his approach to the problem of two consecutive single steps of electron transfer redox reactions. The conventional potentiometric titration technique may be used for calculating such redox potentials provided that the buildup of the intermediates formed during the titration is sufficiently high to affect the titration curve. This technique has been used to calculate the one-electron redox potentials for several systems using the procedure suggested by Michaelis2* or Elema.2b For most of these systems (cf. ref 3 and references cited therein) the effect of the semiquinoidic form on the titration curve was observed only a t highly acidic or highly basic aqueous solutions. Half-wave potentials for many quinone( semiquinone systems were measured by polarography in aprotic solv e n t ~ * where -~ the semiquinone is rather stable. In such aprotic solvents 0 2 - is also very long lived and Peoverg was able to measure the half-wave potential of the 0 2 1 0 2 - system. It was shown that the energy of the first unoccupied orbital as well as solvation energy are major factors affecting the redox potentials of these systems.6J0 In spite of the wealth of information regarding these potentials in aprotic solvents, very few measurements have been performed in aqueous solutions at near neutral pH. This, undoubtedly, is the result of the short lifetimes of the radicals in aqueous solutions. Results from the polarographic pulse radiolysis techniquell are in certain instances indicative of the electrochemical behavior of short-lived radicals but the irreversible nature of the polarographic
waves in the short time range limits its usage. The work of Pate1 and W i l l ~ o n ’established ~ the existence of equilibrium between some semiquinone radical ions and 02 but no redox potentials were calculated since appropriate reference one-electron redox potentials were not available. In this study we have tried to establish a method for measuring single electron rdox potentials for several quinones and for the 0 2 1 0 2 - systems. Under all conditions used in this study all the semiquinone radicals were found to transfer an electron to oxygen.
Experimental Section The computer-controlled pulse radiolysis facilities of the Radiation Research Laboratories of the Carnegie-Mellon Universityz9 were used in this study. The solutions flowed through either a 2-cm or a 0.5-cm optical-path Spectrosil cell, the latter being used for solutions containing the highly absorbing indigodisulfonate dye. It was verified by changing both the flow rate and the rate of pulsing of the Van de Graaff that no observable accumulation of radiation products or depletion in the original solutes occurred. By inserting light filters in the light path it was verified that no appreciable photochemical reactions occurred as the result of irradiation by the pulsed 450-W Osram Xe lamp. For time scales longer than 0.4 msec the‘ unpulsed mode of the lamp was used. No effect of the lamp pulsing on yields and kinetics was observed in the entire time range. The 2.8-MeV electron pulses of 0.5-1.5-wsec pulse width from the Van de Graaff gave total concentration of radicals of 0.5-2 wM. The total yield of the radicals never exceeded The Journal of Physical Chemistry, Vol. 79, No. 15, 1975
D. Meisel and 0 . Czapski
1504
10% of the lowest concentration of the solutes used. However, initial concentrations of solutes were corrected for depletion due to their reaction with the radicals whenever this effect exceeded 3%. No difficulties were encountered in measuring yields of radicals with Gc = 3.5 x IO3 mo1/100 eV M-' cm-', using the computer averaging option. The secondary emission dose monitor was calibrated against N20 saturated M KSCN solution assuming G(SCN2-) = 6.0 molecules/100 eV and c4s0(SCN2-) = 7600 M-' cm-'. Kinetic analysis was carried out on a 9830A Hewlett-Packard calculator using a least-squares best fit program. When 2-propanol was used as OH and H scavenger Gred. = 6.2 was assumed, allowing for lo?? yield of the relatively unreactive cH2CHOHCH3. When HCO2- replaced 2-propanol Gred. = 6.6 was assumed. The use of the alcohol greatly accelerated the solution of the quinones but might complicate the analysis of the results due to formation of alcoholic peroxy radicals. Therefore, 2-propanol was used in all deaerated solutions while HC02- was used whenever oxygen was present. All substances used were of the highest commercially available purity. Freshly prepared solutions were flushed with the desired gas for at least 15 min. Extra pure nitrogen was used for deaeration. Four different oxygen-nitrogen mixtures were used: 0.96%, 9.6%, dry air, and 100% oxygen. The concentration of 0 2 in saturated solutions under 1 atm M and Henry's law was asof 0 2 was taken as 1.25 X sumed. Water was distilled and the vapor passed with oxygen through a silica oven. Solutions were buffered to pH 7 using 5 mM phosphate unless otherwise stated. Extinction coefficients were corrected for loss of absorption by the parent substrate. The 6oCoy source (Gammacell 220) has a dose rate of 6.7 x 1017eV g-' min-l and samples were irradiated for 1 min in a specially designed cell which enables deaerated solutions to be measured in the Cary 13 spectrophotometer several minutes after irradiation. The in situ radiolysis ESR technique was described in great detail.30
Results and Discussion
A . Characterization of t h e Radicals. Two semiquinone radical ions and one quinoidic dye were tested for their possible electron transfer to oxygen. These were the semiquinones of duroquinone (DQ-), menadione (MQ-), and indigodisulfonate (IDS-). On irradiating deaerated solutions containing M of OH and H scavengers (HC02M of the quinone, or 2-propanol designated RH) and the spectrum observed is that of the corresponding semiquinone radical ion which is formed by the sequence of reactions 1-5. Reactions 1-4 are completed a t the end of HzO
--- eaq-, eaq-
H, OH, H202, HZ
+Q
QH s Q(H)OH + RH
R
+Q
-
+
-.Q-
+ H+
R
Q-
(1)
(2)
(3)
+ H20(H2)
(4)
+ R+
(5)
the pulse under our experimental conditions. The rate of reaction 5 was measured p r e v i ~ u s l y ' ~ -for ' ~ both C02- and (CH&cOH radicals. This reaction is over in less than 10 psec under our experimental conditions. The spectra of DQ- and MQ- radicals were observed in the present study (Figures 18 and lb); and are very similar to those obtained The Journal of Physical Chemistry, Vol. 79, No. 15, 1975
hnm
4,0001 ,
'300
/
350
,
400 hrn
~ofJ ~both J ' the formation of such species and the reactions they undergo have been studied. In some solvents, the optical absorption spectrum of the solvated electron is very greatly altered upon ion pair formation. Thus, the absorption maximum for (Na+,e,-) in T H F is 890 nm4 compared with a maximum a t 2120 nm for e,- in THF,'zJ3 corresponding to a change of 0.8 eV in the transition energy. Rate constants for the attachment of the
electron to various substrates are substantially diminished4J1upon ion pair formation with sodium cation. The reactions involved in this pairing e,- t Na'
es-
(Nat,es-) eNa-
(1)
are by no means unique to sodium cation. To obtain a broader knowledge of alkali metal-electron pairing, information has been obtained about the pairing of the solvated electron with lithium cation in THF. The results have been obtained by pulse radiolysis of T H F solutions of various dissociative lithium salts. We report here the optical absorption spectrum of the species (Li+,e,-) as well as rate constants for the attachment of this species to anthracene and to biphenyl.
Experimental Section The source of the electron pulse, as in our earlier studies,14 was a Varian V-7715A electron linear accelerator, deThe Journal of Physical Chemistry, Wol. 79, No. 15, 1975
