1558
Y. Ilan, J. Rabani, and A. Henglein
(11) E. J. Land, Proc. R. SOC.London, Ser. A, 305, 457(1968). (12) I. A. Taub, D. A. Hartner, M. C . Sauer, and L. M. Dorfman, J. Chem. Phys., 41, 979 (1964). (13) S. Arai, H . Ueda, R. F . Firestone,and L. M. Dorfman, J. Chem. Phys., 50, 1072 (1969). (14) I. A. Taub and L. M.'Dorfman,J. Am. Chem. SOC.,84, 4053 (1962). (15) M. Nakashima and E. Hayon, J. Phys. Chem., 75, 1910 (1971).
(16) S . Noda, K . Fueki, and Z.Kuri, J. Chem. Phys. 49, 3287 (1968). (17) J. L. Franklin et al., E d . , Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 26, 60, 131, (1969). (18) R. Cooper and J. K . Thomas, J. Chem. Phys., 48, 5097 (1968). (19) Balk, Hoijtink, and Schreurs, Recl. Trav. Chlm., Pays-Bas, 76, 813 (1957). (20) R.A. Holroyd, "FundamentalProcesses In Radiation Chemistry",P. Auslms, Ed., Interscience, New York, N.Y., 1968, p 433.
Pulse Radiolytic Investigations of Peroxy Radicals Produced from 2-Propanol and Methanol Yael Ilan,' Joseph Rabani,*2 and Arnim HengleW Berelch Strahlenchemle, Hahn-Meltner-lnstltut fur Kernforschung, Berlin GmbH, 1 Berlin 39, West Germany and The Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 9 1000, lsrael (Received May 7, 1975; Revised Manuscript Received October 28, 1975) Publication costs assisted by the Hahn-Meitner-lnstitut fur Kernforschung
--
The reactions of peroxy radicals produced from 2-propanol and from methanol were investigated. At pH 3 the peroxy radicals were found to decay by both second- and first-order processes: 02CHzOH HO2 CH20, k o 5 X lo2 s-l; 02COH(CH& HO2 (CH&CO, 12 = (700 f 150) s-l; 202CH20H products, 2K = (3 f 1 ) , X los M-l s-l; 202COH(CH& products, 2K = (1.1f 0.2) X lo7 M-l s-l. The reactivity of 02C(OH)(CH& toward OH- and HP0d2- ions, resulting in the ionization of the peroxy radicals, and the = (5.2 subsequent formation of 0 2 - , was studied. A rate constant ~((cH~)~c(oH)(o~)+oH--(cH~)~c(o-)(o~)+H~o) f 1) X IO9 M-l s-l was measured. The similar reaction with HP0d2- was found to have a rate constant of (1.07 f 0.15) X lo7 M-l s-l. All results are reported for an ionic strength of 0.1 M. -+
-
Introduction Several papers have been published recently concerning the formation and decay kinetics of various peroxy radic a l ~ .H ~ atoms,1° - ~ ~ eaq-,l0and COz- 5~14are known to reduce oxygen and form superoxide radicals. In general, when an organic solute, RH2, is present in an irradiated N2O-containing aqueous solutions, it reacts with the free radicals formed by irradiation according to H2O
eaq-, H, OH, H202, H2, H30+, OH-
--+
eaq-
HzO + N2O -+ OH + OH- + N2 OH + RH2 RH + H2O H + RH2 RH + H2
(2)
(3)
4
(4)
+0 2
4
RHO2
(5)
Note that 0 2 may compete with N2O for eaq-, and with RH2 for H, so that RHO2 radicals are expected to form along with some quantities of HO2 and 0 2 - . (HO2 has a pK at 4.9.4p5)Of the radicals RHO2 studied so far, two groups have been observed aliphatic radicals with no additional functional groups around the carbon atoms that add to the 0 2 (such as cyclopentyl peroxyl radicalsll (C5HgO2)), decay away by a bimolecular reaction of the type 2RH02
+
2R
+ H2Oz + 0 2
The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
RH02H
(6)
+R +0 2
On the other hand, a alcohol radicals, such as 02CH20H12and (CH3)2C(OH)(02) (which will be discussed in the present manuscript), are capable, under certain conditions, of producing HOz and 0 2 - radicals according to RHO2
+
R
+ H+ +
02-
(7)
or RH02
+
+
or
(1)
When 0 2 is also present, peroxy radicals, RH02, are produced according to RH
+
+
R
+ HO2
in competition with reaction 6. In a previous paper,12the formation of 0 2 - from OzCH2OH has been reported. It has been shown that the formation of 0 2 - according to stoichiometric eq 7 is quite complicated, and is enhanced by bases (OH-, H P O P ) . Evidence for a bimolecular process at the higher pulse intensities has been obtained, but no systematic study of this process has been carried out. The purpose of this manuscript is to investigate the bimolecular decay of (CH3)2C(OH)(02)and of 02CH20H, and the first-order noncatalytic formation of HO2. The effect of OH- and HP042- on 0 2 - formation in 2-propanol aqueous solutions is also measured, and the results support the idea that the ability to produce superoxide radicals is general to a peroxy radicals of alcohols. Previous work may now be better understood on the basis of our new findings.
1559
Peroxy Radicals Produced from 2-Propanol and Methanol Experimental Section The linear accelerator at the Hahn-Meitner-Institute was used a t 8-12 MeV, 1-3 A current, and 10-2000 ns pulse duration. Tektronix 549 and 7613 memory scopes with plug-in units 1A5 and 7A22, respectively, were employed. The light source was 450-W Xe-Hg lamp. A 4 cm long cell with one light path was used for the irradiations. Carl Zeiss MCQIII prism monochromator, with no scattered light under our conditions, was also employed. Each solution was pulse irradiated once, and then replaced. Mixtures of N20-02 were used unless otherwise stated. The solutions were in equilibrium with the same gas mixture while filling the irradiation cell. Equilibration with the gas was carried out by vigorous bubbling of the gas mixtures (total pressure, 1 atm) through 1-1 flasks containing the solutions. The flasks were kept under gas mixture pressure until after use. The equilibration processes lasted 15-20 min, during which time 7-10 1. of gas were bubbled through the solution, above which the gas volume was about 200 cm3. The temperature was 22.5 f 1"C. Unless otherwise stated, experiments were carried out a t ionic strength 0.1 M, obtained by the addition of appropriate amounts of NaC104. Dosimetry was carried out by measuring the 248-nm extinction of 0 2 - produced in 0.1 M sodium formate solution at pH 7 (phosphate buffer loe2 M). The extinction coefficient t of 0 2 - is known as 2 X l o 3 M-l cm-l. Total radical yields in the test solutions were assumed to be the same as the 0 2 - yield in the dosimeter solution. Materials. Water, triply distilled after first being deionized with an ion exchanger, was used. Tetranitromethane (TNM, Fluka, puriss) was washed 15 times with water, and distilled a t room temperature by means of a stream of air through a solution of bicarbonate. It was collected in a trap of liquid Nz. Avoiding the last step of distillation did not affect the results. All other materials, products of Merck (pro analysis), were used as received.
LO psec
t
time
pulse
-
Flgure 1. The formation of 02- from (CH3)&(OH)(02): 0.033 M Na2HP04 M NaH2P04 added), at ionic strength 0.1 M. 67% N 2 0 at pH 7.3 and 33% O2 (by partial pressure above the solution). Total radical concentration 1.OO X M.The spectra were recorded at 248 nm.
TABLE I: Initial Absorption Spectra at pH 3 and 7.3= h, nm
102DX (pH 7.3Ib
102DX (pH 3Ib
Dx/D24s (pH 7.3)C,d
232 240 248 260 270 280 290 302 313 325 350
4.45 4.3 4.15 3.4 3.35 2.6 1.9 1.3 0.9 0.6 0.35
4.1 4.1 4.55 3.6 3.3 2.7
1.08 1.04
1.9
1.25 0.87
DX/D248 (PH 3 ) ' ~ ~ 1.02 1.03
1.00
1.00
0.83 0.82 0.64 0.50 0.33 0.22
0.81 0.77 0.63 0.44 0.30 0.21
0.15
0.09
Each value is an average of at least two determinations. Experiments at pH 3 were carried out after equilibration with 1:l (partial pressures) 02:NzO. Experiments at pH 7.3 were carried out in the presence of 3.3 X M NazHP04 and 1 X M NaH2P04, after equilibration with 1:2 (partial pressures) 0z:NzO. Total radical concentration 1.00 X M. Optical densities at wavelength A, extrapolated to the end of Results a n d Discussion the electron pulse. Optical densities at wavelength A, extrapThe 2-Propanol System. (a) Spectrum of ( C H ~ ) Z C ( O H ) O ~ .olated to the end of the electron pulse and corrected for the All the experiments were carried out in 0.5 M 2-propanol soabsorptions of HOz (9% of the total radical concentration) at lutions. In Figure 1we present a typical oscilloscope trace at pH 3 and of 0 2 - (5% of the total radical concentration) at pH 7.3. Corrected optical density at 248 nm (see also footnote c ) p H 7.3. Under the conditions of Figure 1, reactions 2,3, and was 3.8 X e Corrected optical density at 248 nm (see also 4 are very fast in comparison with the time resolution of the footnote c ) was 4.2 X apparatus, and reaction 5 has a half-life of about 0.4 p s , as will be discussed later. Thus, 2 p s after the electron pulse, 5% of The absorption of (CH3)2C(OH)(02) a t p H 7.3 was found the radicals are present as 0 2 - due to the competition of 0 2 to be the same as that observed at pH 3 (Table I). At pH 3, an for eaq- and H (according to the rate constants of the competing solutes), while 95% of the radicals are present as optical absorption is produced soon after the electron pulse, (CH3)zC(OH)(Oz)and OzCH2CHOHCH3. Of these, about 11% as is the case for pH 7.3. However, this is not followed by an are p radicals, and nearly 89% are a radicals (assuming that additional formation of absorbance. Instead, mixed secondH produced a radicals only, while 15%of the OH produced and first-order decay kinetics are observed. The initial (exradicals16). Thus, the optical absorption a t 2 p s after the trapolated) absorbance a t pH 3 is attributed mainly to a peroxy radicals, and some p radicals (-10% of the total radielectron pulse is mainly due to a and peroxy radicals. The increase in optical density observed in Figure 1was found to cals), and HOz (-9% of the total radicals). It can be seen that be strictly first order. The absorption increases are due to the extinction coefficients in Table I are somewhat higher (up to formation of 0 2 - according to reaction 7, catalyzed by lo%), at pH 3, but apart from this the spectra are identical and HP042-.12 The spectrum observed after 150 p s was taken and should be attributed to the same species. The results at pH found to be in very good agreement with the known spectrum 3 are more accurate, as the changes of D with time are much slower a t pH 3 as compared with our results at pH 7.3. of 02-.5 The initial spectrum (Table I) is due to the absorpTherefore, the extrapolation to time zero is more accurate a t tions of the a and p peroxy radicals (CH&C(OH)(O2) and 02CHzCHOHCH3. As we do not know the precise spectrum pH 3. (In fact a t p H 3 there is a negligible change of D with time on the microsecond time range, after 2 p s . ) Average exof the radicals, we are unable to correct for this, and calculate a more exact spectrum of the a radicals, which form the great tinction coefficients t @ & 3 ) 2 ~ ( ~ ~of) ( 01100 2) f 100 and majority of the peroxy radicals. It is reasonable to assume that t(CH3)2C(OH)(Oz) 290 of 500 f 50 M-l cm-l can be considered as the the radicals absorb in the same range, and have perhaps an best values. Note that both the a and the radicals have been absorption similar to the CbH902 radicals.ll assumed to have identical absorptions. a
The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
Y. Ilan, J. Rabani, and A. Henglein
1560
TABLE 11: Reaction Rate Constant of (CH&C(OH)(Oz) with HPOd2-
pulse intensity so that from 2 to 50 pM of (CH3)2C(OH)(O2) were produced did not affect the kinetics. This is in contrast with the results observed previously in the methanol system,lZ and, as will be discussed later, should be attributed to the relative slowness of reaction 6 in the 2-propanol system. The decrease in k8 a t high ionic strength is similar to the one observed in methanol.12 The reactivity of (CH3)2C(OH)(Oz) toward OH- has also been investigated.
(I
pH
[HP0d2-],M
b,b M
[HzPOd-], M
106k8 M-’ S-’
7.30~ 7.30~ 6.65d 6.65d 6.42d
3.3 x 10-3
0.1 0.1 0.7 0.7 0.9
1.1 x 10-3 3.3 x 10-3
10.8
1.0x
10-2
1.0 X loW2 2.0 x 10-1 2.0 x 10-1
10.5e 6.P 5.3
5.0 X
1.0 x 10-1 3.0 x 10-1
4.6
(CHdzC(OW(02)
Total radical concentration 1.0 X M. [2-propanol] = 0.5 M. Each value in the table is an average of at least two determinations. b Ionic strength. 1:2 Oz/N*O (partial pressure). 1:0.82 O2/N20 (partial pressure). e Including 5% correction for the contribution of OH- to the formation of 0 2 - under the conditions of the experiment. (I
4 XI 0’
I
I
/
I 2x10=1-
I
I
1
7x10’
4
1i I
;/
f
i
lo$ 7xlOf
1
1
4x10‘-
Zx1$c
1
-6
-5
-3
-4
-
pH 13.86
Figure 2. Effect on pH on the rate of formation of 02-from (CH3)2C(OH)(02). The ionic strength p was 0.1 M. Total radical concentration M; 1:l N20/02 (partial pressures). k, is the apparent rate constant for 02-buildup and is equals to the sum kg[HP042-] klo[OH-]: (0)1 X lov3 M HP042-; ( 0 )2 X M HP042-. The line represents klo = 5.2 X lo9 M-’ s-’. The value of Kw under our con-
+
ditions is 13.86.
(CH3)zC(OH)(Oz)+ HP04’-
-
(CHdzC(O-)(Oz)
+HzP04- (8)
followed by (CH~)ZC(O-)(OZ) (CHd2CO + 0 2 --+
(9)
Previous work in the methanol system has shown a similar reaction pattern.12 Under the conditions of Figure 1, by analogy with the methanol systemlz we assume that (CH&C(O-)(Oz) decomposes very quickly to 0 2 - , and reaction 8 is the rate-determining step in the formation of o~-. From the formation kinetics observed in figures such as Figure 1, at various [HP04*-] and pulse intensities, we measured ka. Some of the results are included in Table 11. Changing the The Journal of Physical Chemistry, Vol. 80, No, 14, 1976
+
(CH3)2C(O-)(Oz)
+ HzO
(IO)
The results are presented in Figure 2. We used k 8 = 1.07 x 107 M-l s-l. The experimental data yield klo = 5.2 X lo9 M-l s-l. The deviations from the straight line observed in the higher pH range can be accounted for if we assume that k5 N 4 x 109 M-I s-l. Thus, a t sufficiently high pH, reaction 5 is not well separated in time from (lo), and (10) cannot be considered as rate determining. The value k5 = 4 X lo9 M-l s-l agrees well with the values measured for m e t h a n o P and for cyclopenthylll (C5H9) radicals. (c) Effect of Changing Pulse Intensity. No effect of pulse intensity on the rate of buildup of optical absorption has been M HP042- or >5 observed in solutions containing >3 X X M OH-. (Pulse intensities which produced 2-50 pM (CH&C(OH)(Oz) have been tested.) The ratio Dm248/D,3248 = 1.65 f 0.05 (see Figure 1and related text) did not depend M on the pulse intensity in solutions containing >3 X HP04’- in the range studied. However, when no phosphate was present, at pH 3, no build up of absorbance could be observed after the formation of (CH3)2C(OH)(Oz).Instead, the optical density decayed away by mixed first- and second-order kinetics. Typical results are presented in Figure 3 at 290 nm. The curves have been calculated with the aid of a HewlettPackard 9810 A calculator. A program based on the method described by Schmidt17 was applied. Agreement with the data could be obtained taking kg = 1.1X lo7 M-l s-l and h7 = 5.5 X lo2 s-l. This value for k7 represents the spontaneous (noncatalytic) formation of HO2 from (CH3)2C(OH)(02). Curve fittings at 248 nm were also carried out, using identical solutions with similar pulse intensities. Best fits were obtained = 1050, = with the same values of h6 and k7, using 1200, ~$%o~ecu~arreaction = 250, and &?402 = 25 M-’ cm-l. Values of k l l = (2.5-3.3) X lo6 M-l s-l, which were used for the calculated curves, are higher than the value reported previously for the low ionic ~ t r e n g t h . ~ , ~ ‘“02’’
(b) Reaction of (CH3)2C(OH)(O2). In the presence of HP042- ions, (CH3)2C(OH)(02)reacts with it according to
+ OH-
+ “H02”
+
H202
+0 2
(11)
(Note that k l l is an apparent reaction rate constant, related to the overall decay rate of HOQ 0 2 - “HOz”.) Measurements a t ionic strength 0.1 M at pH 3, in HC02H-HCOZNa0 2 solutions, where HO2 and 0 2 - are the only decaying product^,^ gave h l l = 3.0 X lo6 M-l s-l. The effect of ionic strength on K 1 1 at pH 3 is expected, since at this p H the reaction of HOz with 0 2 - still contributes to K11. At higher ionic strengths more 0 2 - is expected. As the reaction of HO2 with 02-is relatively fast,4,5an increase in hll can be expected, and is indeed observed. Methanol Solutions. When 0.1 M methanol was substituted for 2-propanol, results which are in qualitative agreement with those presented in Figure 3 were obtained. Using pulse intensities corresponding to 10-50 pM radicals, best fits between the calculated curves and the experimental data were obtained at 290 nm (pH 3) with k6 = 3 X 108 M-l s-l. A first-order decay of 02CH20H, h = 5 X lo3s-l, had to be included in the
+
Peroxy Radicals Produced from 2-Propanol and Methanol
1561
c
mation is controlled by the rate of reaction 7. The optical absorption finally reached a constant value, which was stable for at least 100 ms, The final optical absorption was only 85% of that measured when total radical scavenging by TNM took ' 3 place. From these results we conclude that a pseudo-first2 order process was competing with reaction 7, presumably, a 5 reaction of peroxy radicals with impurities. After correcting P 2 for this effect we obtained k7 = (700 f 150) s-l. When the .t f T N M concentration was doubled, the fast process became more important, as expected. ai The kinectics of the slower build up of absorbance remained unaffected. This shows that the possible reaction of RHO2 peroxy radicals with T N M was not important. 0 1 2 3 L 5 Similar experiments in methanol showed a higher sensit[rnsecl tivity of this system to the effects of impurities. However, k6 Figure 3. The effect of changing pulse intensity in 2-propanol solutions 5 X lo2 s-l was measured, a value similar to that found in (partial pressure above the at pH 3; measured at 290 nm, 1:l 02/N20 the 2-propanol system. The first-order rate constant of 5000 solution). Initially formed HOn was calculated to compose 9 % of the initial total radical concentration. The ionic strength was 0.1 M. W e s-l, which was used for the curve fittings in the absence of assumed /qeas-+,,+)= 1 X 1O1O M-' s-l for this ionic strength. E& = T N M (see previous section), represents mainly the reaction 500, .$klecular product) = 250, and 4'' = 10 M-' cm-l were used of 02CH20H with impurities. The reliability of the T N M for the calculations of the curves. The pberoxy radicals were disresystem for the measurements of k6 was demonstrated when garded (equivalent to assuming that the 0radicals and the (Y radicals have identical absorptions and kinetics). ~ ~ ~ H O ~ - + ~ =* H3.0 O ~X~ *lo6 ) we used solutions containing phosphate buffer (10 pM) at pH M-' s-l was used for all computations: (A,0,0, 0 )experimental 6.6 instead of HClO4. The enhancement of reaction 7 by OHresults. (a)Scale reads real optical densities if multi lid by loT2.Curve was computed taking ~ ( R o * + R o=~ )1.O X 1O7 M- Ps- and k p o 2 - ~ 0 2 ) resulted in the formation of NF-, the rate of which was still controlled by (7). Although the impurity effect could not be = 6.0 X lo2 s-l. Initial total radical concentration is 10.7pM. Initial [H202]assumed 1.2 pM. (b) Scale reads real optical densities if mulneglected, we were able to measure by this method tiplied by Curve was computed taking /((RO~+RO~)= 1.3 X lo7 M-' ~(O~CH~OH+OH =- )1.5 X 1O1O M-l s-l which is in agreement s-l and k(ROz4H02) = 5.5 X lo2 s-l. Total initial radical concentration with the value published previously.12 was 21.5 pM. Initial [H202]assumed 2.5 pM. (c)Scale reads real optical The optical changes during the decay of the a peroxy radCurve was computed taking densities if multiplied by 2 X = 5.5 X lo2 s-l. Total ~ ( R O ~ + R O= ~ )1.2 X lo7 M-l s-l and icals and especially the accompanying changes in conductivity initial radical concentration was 38.5p M . Initial H 2 0 2 assumed 4.5 pM. of the solutions have previously been attributed to the for(d) Scale reads real optical densities if multiplied by 4 X lo-'. Curve mation of intermediate hydrotetroxides. However, a number was computed taking k(R02+ROz) = 1.1 X lo7 M-l and ~ ( R O ~ - H O * ) = of reasons against the existence of hydrotetroxides can now 5.0 X lo2 s-l. Total initial radical concentration was 60 pM. Initial be given, the most prominent one being the postulate that [ H 2 0 2 ] assumed to be 7 yM. tetroxides have extinction coefficients twice as large as the corresponding peroxy radicals (this postulate had to be made in the earlier investigations). The mechanism of eq ?+lo is able reaction scheme. This decay, as will be shown later, is not due to describe not only the observed optical effects but also the to reaction 7 . In these calculations we had to assume an E of conductivity changes described in the earlier work. 175 M-l cm-l for the product of reaction 6. Solutions Containing Tetranitromethane. When tetranitromethane (TNM) is also present, in addition to the 0 2 , N20, and alcohol, it competes for eaq-, H, and RH radicals.18Jg Acknowledgment. This work was supported in part by the T N M does not react with peroxy radicals as will be shown later US. Energy Research and Development Adminstration under Therefore, T N M can be but reacts efficiently with 02-.18J9 Contract No. E(11-1-3221). used to discriminate between peroxy radicals and superoxide radicals. The reactions of eaq-, H, RH, and 0 2 - with T N M produce nitroform, NF-, which strongly absorbs a t 350 nm References and Notes ( 6 15 000 M-l cm-l) 1,
I
I
'
-
TNM
+X
+
NO2
+ NF- + X+
(1) In part fulfillment of the requirements toward a Ph.D. Degree at the Hebrew
(12)
X represents a reducing radical and X+ represents the product formed from X upon its reaction with TNM. Solutions containing 0.1 M 2-propanol, saturated 0.5 atm of N20, and 0.5 atm of 0 2 were pulse irradiated at pH 4 in the presence of 40 pM TNM. Pulse produced 0.7 WMradicals. An optical absorbance corresponding to the reactions of T N M with 30% of the radicals was built up within 100-200 ps. This was due to the reactions of T N M with eaq-, RH, and 0 2 - (in equilibrium with HO2. Some 0 2 - is quickly formed by the reactions of eaq- and H with 0 2 ) . This relatively fast process was followed by an additional formation of absorbance which obeyed a first-order rate law, k = 1100 s-l. We attribute the slower build-up process to the additional formation of 0 2 according to reaction 7 . Under our experimental conditions, the rate of NF- for-
University.
(2) Department of Physical Chemistry, The Hebrew University, Jerusalem 91000,Israel. (3)Sektor Strahlenchemie, Hahn-Meitner-lnstitut, 1 Berlin 39,Berlin, Germany. (4) J. Rabani and S. 0. Nielsen, J. fhys. Chem., 73, 3736 (1969). (5)D. Behar, G. Czapski, L. M. Dorfman, J. Rabani, and H.A. Schwarz, J. fhys. Chem., 74, 3209 (1970). (6) K. Stockhausen, A. Fojtik, and A. Henglein, Ber. Bunsenges. fhys. Chem., 74, 34 (1970). (7)T. Eriksen, A. Henglein, and K. Stockhausen, Trans. Faraday Soc., 69,337 (1973). (8)E. Hayon and M. Simic, J. Am. Chem. SOC., 95,6681 (1973). (9)K. Stockhausen and A. Henglein, Ber. Bunsenges. fhys. Chem., 75, 833 (1971). (IO)G.Czapski, Ann. Rev. fhys. Chem., 171 (1971). (11) J. Rabani, M. Pick, and M. Simic, J. fhys. Chem., 78, 1049 (1974). (12)J. Rabani, D. Klug-Roth, and A. Henglein, J. fhys. Chem., 78, 2089 (1974). (13)M. Simic and E. Hayon, J. fhys. Chem., 75, 1677 (1971). (14)A. Fojtik, G.Czapski, and A. Henglein, J. fhys. Chem., 74, 3204 (1970). (15)(a) M. Anbar, M. Bambenek, and A. B. Ross, "Selected Specific Rates of Reactions of Transients from Water in Aqueous Solutions", National Bureau of Standards, U.S. Department of Commerce, Washington, D.C., 1973. (b) L. M. Dorfman and G. E. Adams. "Reactivity of the Hydroxyl Radical in The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
Sara Abramovitch and Joseph Rabani
1562 Aqueous Solutions", National Bureau of Standards, US. Department of Commerce, Washington, D.C., 1973. (16) K. D. Asmus, M. Mijckel, and A. Henglein, J. Phys. Chem., 77, 1218 (1973). (17) K. H. Schmidt, ANL Report No. 7199, Argonne National Laboratory, Ar-
gonne, Ill., 1966. (18) K. D. Asmus, A. Henglein, M. Ebert, and J. P. Keene, 6er. Bunsenges. Phys. Chem., 88, 657 (1964). (19) J. Rabani, W. A. Mulac, and M. S. Matheson, J. Phys. Chem., 89, 53 (1965).
Pulse Radiolytic Investigations of Peroxy Radicals in Aqueous Solutions of Acetate and Glycine Sara (Dinur) Abramovltch and Joseph Rabani" Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 9 1000, Israel (Received January 7, 1976)
Aqueous solutions of glycine and acetate have been pulse irradiated in the presence of 0 2 . Radicals produced from these compounds by H abstraction reacted with 0 2 to form peroxy radicals. The reaction rate constants ~(RH+O= ~ ) (2.1 f 0.3) X 109'and -1 X lo9 M-l s-l were measured for RH = acetate radicals and glycine radicals, at pH 8.2 and 7.9, respectively. The peroxyacetate radicals decay by a second-order reaction, k = (6.5 & 1) X los M-l s-l at both pH 5.7 and 8.2. The peroxyglycine radicals produce 0 2 - by a first-order process, k = (1.5 f 0.3) X lo5 s-l at pH 7.9. Evidence for the formation of 0 2 - from peroxy glycine radicals is presented.
Introduction The suggestion made by McCord and Fridovichl that superoxide radicals may cause damage to living cells is relevant to irradiated biological system. As water consists the major part of living cells, reactions of the radiation primary species which are written on the right-hand side of eq 1 must be considered. H2O
---
eaq-, H, OH, H202, H2, H30+, OH-
(1)
Of these species, eaq- is known to react efficiently with 0 2 in the presence of air, according to eaq-
+ 02
+
02-
k2
= 2 X 1O1O
M-l s-l
(2)
The hydrated electron, eaq-, may also react with a variety of biomolecules to form reduction products and intermediates which may later transfer electrons to oxygen. A t present, it is not known how eaq- is shared between the various scavengers. OH radicals are expected to react with organic molecules, usually according to OH
+ RH2
+
RH
+ H20
(3)
RH2 and RH, respectively, represent organic molecules and radicals formed from them upon H abstraction. Many radicals of the type RH are known to react with 0 2 and produce either peroxy radicals, RH02, or 0 2 - . RH+ can be obtained as R H+.
+
RH
+
0 2
---*
RHO2 RH+
+02-
(4)
Thus, ~yclopentane,~ methan01,~and 2-propanol5 have been reported to belong to the RHO2 forming group, while formate6 forms the COz- radical ion which transfers an electron to 0 2 . Of the RHO2 forming compounds, the alcohol peroxy radicals The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
were found4v5to decompose and form 0 2 - , a process which is accelerated by bases base
RHO2 (alcohol) +R
+ H+ + 0 2 -
(5)
In order to obtain information concerning the possible role of 0 2 - in the damage of irradiated cells, one has to know whether 0 2 - forms at considerable yields. For that purpose it is necessary to investigate various peroxy organic and bioradicals, with respect to their ability to produce 0 2 - . We have previously ~ t u d i e d very ~ - ~ simple molecules such as C5Hg02, 02CH20H, and (CH&(COH)02. Now we report on more complicated molecules, which are also of greater biological significance.
Experimental Section The experimental technique has been described previously.6 5 meV, 200 mA electrons were used. The light source was 150-W Xe, 200-W He-Hg, or 60-W D2 lamps. The irradiation cell was 4 cm long with 1-3 light passes, so that the optical path was 4 or 12.3 cm. Bausch and Lomb grating monochromator with 32 nm/mm resolution was used with slits ranging from 1 to 2 mm. The monochromator was calibrated every time before use, with the aid of the mercury lines of Hg or X e H g lamps. IP 28 photomultiplier and Tektronix 556 or 549 oscilloscopes were applied. Scattered light was less than lo%, for which corrections have been carried out, assuming no absorption of scattered light by the transients. The pH was determined by an El-Hamma pH meter. The temperature was 20 f 1 OC unless otherwise stated. Dosimetry was carried out using formate solutions, which contained 0 2 or NzO/Oz mixtures. The formate dosimeter solutions were always at the same pH, and had the same [ 0 2 ] and [NzO] as the test solutions of acetate and glycine. The
