2274
P. S. Rao and E. Hayon
Ionization Constants and Spectral Characteristics of Some Semiquinone Radicals in Aqueous Solution P. S. Rao' and E. Hayon* Pioneering Research Laboratory, U . S.Army Natick Laboratories, Natick, Massachusetts 07 760 (Received Aprii 19, 1973) Publication costs assisted by Natick Laboratories
The ionization constants, absorption maxima, and extinction coefficients of the semiquinone radicals and radical anions of 12 quinones were determined. These were obtained in aqueous solutions in the presence of 1-3 M tert-butyl alcohol or isopropyl alcohol by electron eaq- attachment, using the technique of pulse radiolysis. Various benzoquinones, naphthaquinones, anthraquinones, diphenoquinone, epinephrine, and adrenalone were studied. The ionization constants range from 3.2 to 5.4. These values are correlated with the redox potentials of the corresponding quinones. The decay kinetics of some semiquinone radicals and radical anions were determined.
A large number of quinones are known2 to occur in nature and to play .an important part in oxidation-reduction reactions. The role of semiquinone radicals as active agents in biochemical electron transfer reactions has been i n d i ~ a t e d,3. ~Recently, the kinetics in aqueous solutions of electron transfer processes from free radicals (leading to the oxidation or reduction of these free radicals) to various quinones has been studied.4-10 I t became apparent that basic information on the ionization constants, absorption spectra, and extinction coefficients of most semiquinone radicals in water was lacking, in spite of a recent investigationll on this subject. Reported below is a pulse radiolysis study aimed a t obtaining this information. Twelve quinones were examined, including benzoquinones, naphthaquinones, anthraquinones, adrenalone, and diphenoquinine. These were selected on the basis of their redox potentials, ranging from E" 1 values of -0.266 to +0.534 V, and their relative solubilities in aqueous alcohol (1-3 M ) solutions.
tron transfer from (CH&COH radicals are low by -101570, due to the formation of /3 radicals on reaction of OH radicals with isopropyl alcohol.
Results and Discussion
+
.Q-+ H 2 0 .Q-+ H,O+
The pulse radiolysis set-up used has been described elsewhere.12 Single pulses of 2.3-MeV electrons of -30 nsec duration were used. Due to the intense photochemical decomposition of quinones in solution, particular care was taken to carry out the experiments with minimum exposure to light. Appropriate glass filters were used to reduce the effect of the monitoring light from a 450-W xenon lamp, in addition to placing a synchronized shutter which opened for -7-8 msec. The quinones used were supplied by Eastman and by Aldrich, and most of them were recrystallized and/or sublimed. The quinones were initially dissolved in 1.0-3.0 M tert-butyl alcohol or isopropyl alcohol, as indicated, and then appropriate amounts of triply distilled water were added. Solutions were buffered using perchloric acid, potassium hydroxide, and -1.0 m M phosphate or borate buffers. Dosimetry was carried out12 using KCNS solutions. The extinction coefficients were derived12 based on G(eaq-) = G(OH) = 2.8. Experimental conditions of concentration and dose were chosen for 100% formation of the semiquinone radicals. The extinction coefficients derived by elecThe Journal of Physical Chemistry, Vol. 77, No. 19. 1973
Q--H+
+ OH-
+
(2)
.Q--H+ H,O ( 3) Depending on the acid dissociation constant, the semiquinone radical anion can be protonated to give the corresponding radical, reactions 2 and 3. The OH radicals produced from the radiolysis of water were scavenged by tertbutyl alcohol or isopropyl alcohol OH 4- (CH,),COH
OH
Experimental Section
--
The semiquinone radicals were formed by the ieaction of hydrated electrons, eaq- with quinones (Q) eaqQ .Q(1)
+ (CHJ,CHOH
-
+ H,O (CHJ~COH + H,O
CH,(CH,),COH
---+
(4)
(5) The radical produced in reaction 4 is relatively inert and was found not to react with the quinones examined. The acetone ketyl radical, however, reacts with the quinones producing .Q--H+ radicals according to reaction 6
+
(CHJ,COH 4- Q -+ .Q--H+ CH,COCH, .Q--H+ + .Q- + Hf
(6)
(7) with values of he ranging from -1.0 to 6.0 X l o 9 M - l sec-1 (see ref 8). Under the experimental conditions used all the (CH&COH radicals produced semiquinone radicals quantitatively according to reaction 6. Typically, these experiments were carried out in 1-3 M t-BuOH or (CH3)2CHOH, 50-100 pM quinone, and argon (1 atm). Relatively low doses of 1-2 krads/pulse were used throughout. Transient absorption bands were observed with maxima in the 400-450-nm region for .Q- radicals, see Table I, except for 1,2-naphthaquinone, epinephrine, and adrenalone. Similarly, .Q--H+ radicals showed maxima in the 370-425-nm region. In most cases, the semiquinone radicals and radical anions have other absorption bands in the uv or far-uv region (see e . g . , ref 6 and 13), but these have not been examined in detail.
Semiquinone Radicals in Aqueous Solution
2275
TABLE I: Ionization Constants, Absorption Maxima, and Extinction Coefficients of Semiquinone Radicals . Q - and Radical Anions .Q--H+ in Aqueous Solutiona
1 2
3 4 5 6 7 8 9 10 11 12
AnthraquinoneC Anthraquinone-1-sulfonatee 2-Hydroxy-1,4-naphthaquinoned Menaquinone 1,4-Naphthaquinone Duroquinone 1 ,2-Naphthaquinoned 2,5-Dimethyl-p-benzoquinone
p-Benzoquinone Epinephrined Adrena.loned Diphenoquinoned
-0.266 -0.218 -0.139 4-0.002 +0.050
4-0.068 4-0.143 4-0.176 4-0.293
4-0.380 4-0.480 -l-0.534
7.8 x 103, 7 . 3 x 103 8.0 x 103,8.0x 103 6.3 x 103 1.2 x 104 1.3 x 104 7.1 x 103 4.0 x 104 6.8 X l o 3 7.2 x 103 3.3 x 103 1 . 7 x 104 2.6 x 103
395, 480
5.3 5.4 4.7 4.5 4.1 5.1 4.8 4.6 4.0f 3.7 3.6 3.2
400, 500 390 395
390 445 265 440 425
265 290 400
x x x x 7.3 x 4.0 x 1.6 x 3.6 x 4.5 x 1.1 1.2 5.9 9.7
104 104 103 103 103 103 104 103 103 1.3 X l o 3 1 . 0 x 104 4 . 7 x 103
375
385 370 370
370 425
