Pulse Radiolysis of Ethyl
1553
Acetate
Conclusions A complete reaction mechanism for OH reactions with hydrogen cyanide in the pH range from 1.9 to 15 is presented (Scheme I). All major processes as proposed by Behar5 have been confirmed, including the rate data k6, kg, klo, 1213, and pK14. Equilibrium 5 has not been rechecked, as the species -O-CH=N. is only detectable by ESR.4 Original rate data have been derived for k3, k4, ks, k16, and pK7. The spectra and extinction coefficients of all transient species in this complete mechanism are now known. The only exception is the dianion -O-C=Nat pH >11.9, which seems to have a lifetime shorter than about 30 ns in all systems studied. The reaction mechanism discussed in this paper represents one of the rare cases where a complete quantitative understanding could be achieved.
Acknowledgment. This work was supported by the
Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung.
References and Notes (1)On leave from the University of Melbourne, Department of Physical Chemistry, Parkville, Victoria 3052,Australia. (2)H. Buchler and R. E. Buhler, Chem. Phys., in press. (3)D. Behar, P. L. T. Bevan, and G. Scholes, J. Phys. Chem., 76, 1537 (1972). (4)D. Behar and R. W. Fessenden, J. Phys. Chem., 76,3945(1972). (5) D. Behar, J. Phys. Chem., 78,2660 (1974). (6) I. Kraljic and C. N. Trumbore, J. Am. Chem. SOC., 87, 2547 (1965). (7) I. G.Draganic, 2. D. Draganic, and R. A. Holroyd, J. Phys. Chem., 75,608 (1971). (8) H. Ogura, T. Fujimura, S.Murozono, K. Hirano, and M. Kondo, J. Nucl. Sci. Technof., 9,339 (1972). (9)I. Draganic, 2. D. Draganic, Lj. Petkovic, and A. N. Kolic, J. Am. Cbem. Soc.,
95,7193 (1973). (IO)"Handbook of Chemistry and Physics", 54th ed, The Chembal Rubber Co., Cleveland, Ohio, 1973-1974. (11)G. V. Buxton, Trans. Faraday Soc., 66, 1656 (1970). (12)B. Hurni, U. Bruhlmann, and R. E. Buhler, lnt. J. Radiat. Phys. Chem., 7,499 (1975). (13) P. Pagsberg, H. Christensen, J. Rabani, G. Nilsson, J. Fenger, and S.0. Nielson, J. Phys. Chem., 73, 1029 (1969).
Pulse Radiolysis of Ethyl Acetate and Its Solutions1 G. Ramanan Chemistry Division, Bhabha Atomic Research Centre. Bombay 400085, India (Received November 26, 1975)
Pure ethyl acetate was subjected to electron pulse radiolysis in the liquid state and the absorption spectrum of the transient species produced was obtained between 280 and 650 nm. Two different species were produced, one with a short life of about 150 ns absorbing at longer wavelengths attributed to the solvated electron and a much longer lived radical absorbing at wavelengths less than 400 nm. The solute triplet yields were followed using anthracene and biphenyl at different concentrations. An upper limit for the yield of excited singlet anthracene was estimated from the study of fluorescence to be G = 0.1. Anthracene singlet yields in the presence of benzene a t different concentrations were measured and the contribution of ethyl acetate positive ions in forming the additional excited singlets are discussed. The free ion yield is G = 0.25. Yield of ethyl acetate positive ions scavengeable at high benzene concentrations is G = 0.63.
Introduction In recent years pulse radiolysis has been extensively used as a complementary technique to steady state radiolysis in studying the radiation chemistry of different chemical systems. The excited states both singlet and triplet and the free ions produced during radiolysis have been conveniently investigated using submicrosecond and nanosecond pulse radiolysis. The presence of solvated electrons in irradiated water and alcohols has been established by this technique.2 Both aliphatic and aromatic hydrocarbons having very low static dielectric constant are shown to contain free ions under irradiation by several investigator^.^-^ In this work, the radiation chemistry of ethyl acetate, whose dielectric constant is above those of the hydrocarbons and much below those of alcohols and water, has been studied using pulse radiolysis with 12MeV electron pulses of 100-ns duration. Very little work has been carried out on the radiolysis of esters in general and ethyl acetate in particular. The y radi-
olysis of ethyl acetate has been studied7a and the gaseous and liquid products have been analyzed. In the present study the main interest is not in the stable products of radiolysis but rather in the transient intermediates of ethyl acetate. The yields and behavior of ions and excited states in the intermediate dielectric constant region is interesting from the point of view of the general understanding of radiation chemical processes. Ethyl acetate in the presence and absence of suitable solutes has been pulse irradiated and the transient species have been analyzed by kinetic spectrophotometry.
Experimental Section Materials. Ethyl Acetate. Wilkinsons AnalaR ethyl acetate was kept over anhydrous potassium cqrbonate for about 24 h with occasional shaking. The ethyl acetate was filtered and fractionated over a 4-ft vacuum-jacketed column packed with stainless steel mesh, under a nitrogen atmosphere. The middle cut boiling at 77 "Cwas collected and stored under a nitrogen atmosphere. The Journal of Physical Chemistry, Vol. 80, No. 14. 1976
G. Ramanan
1554
Biphenyl and Anthiacene. BDH AnalaR grade biphenyl and anthracene were recrystallized from ethanol and used as such. Benzene. BDH AnalaR grade benzene was fractionally crystallized three times rejecting one third of the volume each time. The benzene was dried over sodium and then used. Nitrous Oxide. Anaesthetic quality nitrous oxide supplied by British Oxygen was used as received. Sample Preparation, Irradiation, and Optical Measurements. Ethyl acetate and its solutions were deoxygenated by bubbling argon through the liquids for about 1 h. A quartz irradiation cell having a path length of 2.5 cm was used and was connected to a flow system described el~ewhere.~ The cell containing the liquid was irradiated with a beam of 12-MeV electrons from a linear accelerator at the Christie Hospital and Holt Radium Institute, Manchester. The transient species were monitored by kinetic spectrophotometry, employing a xenon light source, a monochromator, and a photomultiplier. The amplifier signals were displayed on a cathode ray oscilloscope, Tektronix-445. The optical densities were calculated from a knowledge of the initial light level and the photographed decay curve on the oscilloscope. Absorption spectra were obtained by setting the monochromator to a particular wavelength and finding the optical density at the end of pulse. The electron pulses employed in the experiments were of 100-500 ns duration which delivered a dose of about 3-15 krads. The absorbed doses were measured from a secondary emission chamber which had been precalibrated with modified Fricke dosimeter.1° The fluorescence intensities were obtained by reading off the top flat portion of the emission during the pulse for the solution under study and correcting for the Cerenkov emission at that particular wavelength for the solvent alone.
Results Transient Absorptions. The end of pulse spectrum for a 100-ns pulse of 12-MeV electrons delivering 4 krads to pure ethyl acetate is given in Figure la. The spectrum shows an absorption maximum at about 310 nm and weak absorption bands in the 400-700-nm region. Kinetic analysis of the 310-nm absorption reveals that the growth period was over 200 ns and decays by second-order kinetics. The analysis of the decay plot gives a k/t value, at 310 nm, of 7.6 X lo7 cm s-l, where k represents the second-order decay constant and t the molar extinction coefficient of the absorbing species. The transient absorptions at 400 and 600 nm have approximate half-lives of about 150 ns. The 310-nm absorption was proportional to a dose absorbed between 0.2 and 3.6 krads and had a Gt value of 3.4 X lo3 units. Figure 1b gives the absorption spectrum a t the end of a 100-ns pulse in ethyl acetate saturated with nitrous oxide (-40 mM). The absorption maximum is around 300 nm with a Gt value of 4.3 x lo3 units. The absorption shows second-order decay with a k/t value of 2.6 X lo7 cm s-l. The longer wavelength absorption is no longer present in this case. Excited States and Their Yields. Singlets. Estimation of the yield of singlet excited ethyl acetate was made by measuring the emission intensities at 445 nm from excited singlet anthracene in pulse irradiated ethyl acetate containing anthracene. Table I shows the measured emission intensities in pure benzene, ethyl acetate, anthracene in benzene, and anthracene in ethyl acetate under the same geometry and absorbed dose conditions. Table I1 summarises the emission intensities a t various benzene concentrations along with the calculated G (IA). The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
TABLE I: Fluorescence Emissions at 445 nm with 0.2-ps Pulse, 0.71 krad
Emission intensity, au G(lA*)
Description of system Benzene Ethyl acetate Anthracene in benzene (20 mM) 5 mM anthracene in ethyl acetate 15 mM anthracene in ethyl acetate 25 mM anthracene in ethyl acetate a
40 36 620
1.62a
72.6
0.102
73.5
0.105
74.0
0.106
Reference 18.
T \ (b)
I
!
0 280
380
--
w --
I
580-
_. -
,
680
WAVELENGTH (nrn)-
Figure 1. End of pulse spectrum in the pulse radiolysis of (a)pure ethyl acetate (3.92 krads, 0.1 pus) and (b) nitrous oxide saturated (40 mM) ethyl acetate (3.6krads, 0.1 ps).
Triplet Yields. The absorption spectrum from a 0.2-ps electron pulse in ethyl acetate containing anthracene corresponds well with the anthracene triplet spectrum peaking a t about 420 nm. Using the knawn E value of 5.72 X lo4 M-l cm-I,ll the triplet yields were calculated. Biphenyl solutions were also examined in an analogous way. Figure 2 gives the absorption spectrum immediately after the pulse in a 25 mM biphenyl solution. Two broad peaks in the region of 360 and 400 nm are observed. The absorption at 400 nm decays rapidly leaving the much longer-lived absorption at 360 nm shown in Figure 3a, which is the spectrum after 0.8 ps of the pulse. The spectrum is identical with the biphenyl triplet spectrum.ll Using e360 nm of 3.54 X lo4 M-l cm-l for the triplet, the triplet yields were calculated and the results are given in Table 111. Figure 5a gives the plot of G(trip1et) vs. (anthracene) or biphenyl concentration. Ion Yields. The scavengeable free ion yields were calculated from the absorptions a t 700 and 410 nm for anthracene and biphenyl solutions, respectively, in pulse irradiated ethyl acetate solutions. Figure 3b gives the spectrum of the biphenylide ion obtained by subtracting the long-lived ab-
1555
Pulse Radiolysis of Ethyl Acetate TABLE 11: Emissions at 445 nm in the Pulse Radiolysis of Ethyl Acetate-Benzene-Anthracene Systema
a
Benzene concn, M
Benzene electron fraction t~
Emission intensity, au
G(IA*)
11.3 0 0.015 0.068 0.367 0.688 1.035 1.80 2.59 3.41 4.23 5.10 7.85 9.60
1 0 0.001 0.006 0.032 0.060 0.089 0.155 0.224 0.296 0.368 0.443 0.692 0.849
860 56 56 63 120 168 207 272 364 460 532 585 659 740
1.62 0.105 0.105 0.108 0.226 0.317 0.390 0.513 0.686 0.866 1.00 1.10 1.24 1.37
2i I!
I
I
300
350
400
WAVELENGTH (nm)
Flgure 2. End of pulse spectrum of 25 mM biphenyl in ethyl acetate (0.1
0.2-rs pulse, 0.84 krad, 25 mM A.
IS,
2.63 krads).
TABLE 111: Anthracene and Biphenyl Triplet Yields in the Pulse Radiolysis of Ethyl Acetate Solutions
1.5 3.0 7.6 9.0 11.5 17.5 25.4
0.07 0.11 0.17 0.19 0.17 0.23 0.26
5.0 6.7 11.0 18.0 35.0 52.0 88.0 262.0
0.15 0.15 0.15 0.21 0.34 0.46 0.61 0.87
TABLE IV: Ion Yields in Pulse Irradiated Ethyl Acetate Solutions A, mM
G(A+) G(A-)
+
D, mM
G(D+)
7.6 11.5 17.5 25.4 24.0 24.0
0.46 0.50 0.51 0.53 0.53 0.47
5 6.7 11.0 18.0 35.0 52.0
0.16 0.20 0.27 0.30 0.30 0.25
WAVELENGTH inrnl-
Figure 3. (a)Transient absorption spectrum after 0.8 I S of biphenyl in ethyl acetate (0.1 IUS, 2.68 krads).(b) Spectrum of biphenyl cation ob-
sorption from the end of pulse spectrum. Table IV summarises the yields a t different concentrations of the solutes, making use of €700nm for the anthracene anion or cation as lo4 M-l cm-l and €410nm for the biphenylide ion as 5.8 X lo4 M-l cm-1.12,13
tained after subtraction of the long-lived absorption from end of pulse spectrum.
When nitrous oxide (NzO) is also present in the system the following additional reactions can be postulated:
Discussion
The following reactions can be postulated in the radiolysis of ethyl acetate (EtAc): EtAc EtAc*, EtAc+, e(1)
-+ + -
EtAc
EtAc+
e-
es-, EtAc-
(2)
EtAc*
(3)
e,-
EtAc* radical products (e.g., CH3C0, OC2H5, CHBCOO,etc.)
-
+
EtAc+ CH3COOCHCH3 H+ e,- or EtAcproducts (e.g., CH&OO-, CzH5, CHBCO,and CzHsO-)
(4) (5)
(6)
e,-
0-
+ EtAc
- + + NzO OH-
Nz + 0-
(7)
CH3COOCHCH3
(8)
The absorbing species in the 300-400-nm region can be attributed to radicals such as CHsCO, CH3CO0, and CH&OOCHCH3 though a definite assignment is not possible at this stage. The weak longer wavelength absorption found in pure ethyl acetate but eliminated completely in presence of NzO can be attributed to either the solvated electron or EtAc-. In the presence of NzO the yield of cations should increase as otherwise some would have reacted with electrons or negative ions and hence radicals arising from EtAc+ should also dominate over those arising from excited ethyl acetate. Thus one would expect to see a different absorption spectrum, The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
G. Ramanan
1556
yields, and decay kinetics. This, indeed, was observed in our experiments for the 310-nm species where the Gc value increased by about 30%, the klc value decreased by a factor of 3, and the absorption maximum slightly shifted to the lower wavelength region, when nitrous oxide was present in the system. It will be interesting to note that the a-ethanol radical, i.e., the CH3CHOH radical, has absorption maximum around 290 nm in ethanol14 and the CH2COCH3 radical in aqueous acetone has absorption maximum around 295 nm.15 The absorption maxima around 310 and 300 nm for the radicals obtained in ethyl acetate pulse radiolysis may indicate similar type of radicals such as CH3COOCHCH3 or CH2COOC2H5 in the system. The acetyl radical CH3CO in methyltetrahydrofuran (MTHF) matrix has been found16 to have absorption maxima at 340,500, and 540 nm and the very small absorptions in the 500-600-nm regions probably indicates that the CH3CO radical yield is very small. In the pulse radiolysis of pure ethyl acetate, the 310-nm transient absorption shows a grow-in time of about 200 ns. The explanation for this formation time can only be speculative with the present observations. The absorbing radical may be formed by the decomposition of ethyl acetate cation according to reaction 5. It is also probable that the radical i s 8 result of the ethyl acetate cation radical abstracting an H atom from the neutral molecule through an ion-neutral molecule collision complex haveing a lifetime of the order of 200 ns under the experimental conditions. In the prsence of anthracene (A) or biphenyl (D) in ethyl acetate the solute excited states can arise through energy transfer reactions from both excited singlet (lEtAc*) and the triplet (3EtAc*) ethyl acetate or charge transfer and subsequent neutralization reactions. These reactions are shown as follows: N
+ A (or D) 3EtAc* + A (or D) IEtAc*
1A* (or ID*)
-
--
-
-
IA* (or lD*)
3A* (or 3D*)
(9)
3A* (or 3D*) (intersystem crossing) (10)
+A Et&+ + D EtAc+
+ e,D+ + e,A+
A+
+ EtAc
(11)
D+
+ EtAc
(12)
lA* and 3A*
(13)
lD* and 3D*
(14)
Reactions 11and 12 can occur as the ionization potentials of anthracene and biphenyl are lower than that of ethyl acetate.17 Solute Singlet Yields.As seen from Table I, the emissions at 445 nm in pure benzene and ethyl acetate are approximately equal, thus showing it to be from Cerenkov radiation which is less than 5% of the emission of anthracene in benzene. Anthracene solution in benzene was used as a monitor for the singlet yield since G(lA*) in benzene is known to be about 1.62.18 The yields of anthracene singlet have been calculated using this value since the geometry and absorbed dose were kept identical. The G(lA*) in ethyl acetate was found to be very small (-0.1) and does not change between 5 and 20 mM of anthracene. ,This means that all the precursors to the excited solute singlet are scavenged even at 5 mM of anthracene. T o confirm this, benzene was added to the system since excited benzene transfers energy to anthracene to give anthracene singlet. Table I1 shows that even at thrice the concentration of anthracene benzene does not increase the singlet yield indicating this to be a plateau value. In addition to reactions 1and 3, where the directly excited ethyl acetate singlet The Journal of Physical Chemistry, Vol. 80,No. 14, 1976
and the one arising from the geminate recombination reaction are involved, the following reaction may also take place: EtAc+ k16_ F+
F+ + e,- or EtAc-
-
F*
(15)
Here F+ represents the different cations formed from the excited ethyl acetate cation. Thus the yield of G = 0.1 is an upper limit of the lowest (fluorescent) singlet ethyl acetate. At much higher concqntrations of benzene, benzene intercepts the geminate ion recombination reaction more efficiently giving rise to the benzene cation which on neutralization can give lB*. The ionization potentials of ethyl acetate and benzene (10.2 and 9.6 eV, respectively) favor this mechanism:
+ -
EtAc+
+B
B+
e-
lB*
+A
R+ EtAc
-
lB*
lA*
+B
(16)
(17) (18)
Thus if Go is the yield of lA* formed at low concentrations of anthracene (= O.l), EB the electron fraction of benzene in the system, and k15 the rate constant for formation of F+ which cannot transfer charge to benzene, and Go(EtAc+) is the yield of ethyl acetate cation formed initially then the observed yield of singlet anthracene can be given by
+
G(l.4'") = 1 . 6 2 + ~ ~(1- CB)GO (1- cB)Go(EtAct) x [hidB)Il[h15 f k16(B)] (19) In accordance with this mechanism the observed excited anthracene singlet does not show linearity with the benzene electron fraction as observed in Figure 4. From a reciprocal plot using this equation GO(EtAc+) was found t,o be 0.63 and the rate constant ratio k15/k16 to be 9.7 M-l. Solute Triplet Yields.Figure 5a gives the variation of the anthracene or biphenyl triplet yields as a function of the solute concentration in ethyl acetate. Both curves show similar behavior, with the triplet yield increasing with concentration. This suggests that the triplets arise from the singlet states produced by the neutralization of the anthracene or biphenyl cations. The efficiency of charge transfer reaction increases with increasing solute concentration as it has to compete with the geminate recombination. Similar behavior has been observed in cyclohexane.ll In order to investigate the formation of triplet biphenyl, i.e., whether it arises only through intersystem crossing of the singlet excited biphenyl, singlet and triplet biphenyl yields were measured in the presence of oxygen a t different concentrations. A linear relation between the yields of the singlet and triplet passing through origin is to be expected if the triplet biphenyl is formed only through intersystem crossing. Figure 5b proves this to be the case where the biphenyl singlet emission intensity at 310 nm is plotted against the triplet yield obtained from 360-nm absorption at a particular oxygen concentration. fiere the assumption made is that the triplet measurement is made before oxygen quenches the triplet biphenyl, which is much longer lived than the singlet. Ion Yields.As has been postulated earlier, both ethyl acetate cations and anions and solvated electrons are formed during the radiolysis. As shown in reactions 11 and 12, the cations of anthracene and biphenyl are formed when they are also present in the system, The solute anions also are formed according to their electron affinity compared with the solvent. The ionization potentials of ethyl acetate, anthracene, and biphenyl are 10.2, 7.7, and 8.6 eV, re~pectively,~ making re-
1557
Pulse Radiolysis of Ethyl Acetate
IB t-*,
I
-
02 06 OB ELECTRON FRACTION OF BENZENE
0
’
--Ad1 -
10
Figure 4. Variation of q ’ A ” ) as a function of benzene electron fraction
in benzene-anthracene in ethyl acetate.
0
0.05
0.1
G(3Dl
-
0.15
Figure 5. (a) Variation of solute triplet yields with concentration: (0) anthracene (3.9 krads); (X) biphenyl (0.9 krads). (b) Relation between biphenyl singlet emission at 310 nrn with q3D) in ethyl acetate-biphenyl in presence of different oxygen concentrations. actions 11 and 12 possible. The observed ion yields can be attributed to the free ion yields which are scavengeable since the solute concentrations are kept below 50 mM, when the geminate recombination reaction is not interfered with. It can be seen that the ion yield obtained from the biphenyl ion absorption is only half that obtained from the anthracene case. Since the absorption spectra of both cations and anions are observed to be similar13J9it is likely that both anthracene cations and anions are observed in the anthracene case while only the cation is observed in the biphenyl case. This is possible only if the electron affinity of biphenyl is much less than that of ethyl acetate and that of anthracene is much more than that of ethyl acetate. In order to throw light on this point, anthracene and biphenyl solutions in 2-propanol were pulse irradiated (0.1 ws, 1.6 krads) and the absorptions at 700 and 575 nm for the an-
thracene anion and biphenyl anion were examined in the presence and absence of ethyl acetate. 2-Propanol was chosen as the solvent because of its efficiency for scavenging cations and the absorptions at these wavelengths can safely be assigned to the anions only. It was found that ethyl acetate did not affect the yield or the decay kinetics of the anthracene anion, but in the case of the biphenyl both the yields and the decay kinetics of the biphenylide anion absorption were profoundly affected by the presence of small amounts of ethyl acetate. Thus, in 7 mM anthracene, in the absence of ethyl acetate, in 2-propanol the Gc value was 9.4 X lo3 at 700 nm. When ethyl acetate was added at concentrations of 0.065, 0.415, and 0.875 M, the Gc values were 9.6,9.6, and 9.1 X lo3, respectively. Further, the 700-nm absorption decays by first order with a rate constant of (3.4 f 0.6) X IO5 s-l in all the cases. On the other hand, in a 9.7 mM biphenyl solution in 2-propanol the G t values were 8.6 X 103 and 4.2 X lo3 at 575 nm in the absence and presence (0.032 M) of ethyl acetate, respectively. Further the absorption decays by second-order kinetics in the absence of ethyl acetate with a k / c value of 1.8 X 108 cm s-l a t 575 nm whereas in the presence of ethyl acetate the absorption shows a first-order decay with a rate constant of 2.3 X lo6 s-l at 0.082 M ethyl acetate. Thus there would be little chance of observing the biphenyl anion in pure ethyl acetate. It can be concluded that the free ion yields in the radiolysis of ethyl acetate is G = 0.25. This yield for free ions is consistent with the static dielectric constant (viz. 6) of ethyl acetate. Schmidt and Allen4i5have found free ion yields (in terms of their G values) of 0.05,0.13, and 0.15 in irradiated benzene, hexane, and cyclohexane with static dielectric constants of 2.3, 1.9, and 2.02, respectively. Holroyd has observed20that the yield of “free ions’’ increases with the static dielectric constant and has given a plot of variation of G(e,-) with the dielectric constants of several liquids and has compared it with the theoretically expected curve.3 The observed G(free ion) of 0.25 for ethyl acetate (D = 6) fits this curve reasonably well.
Acknowledgments. The author wishes to express his thanks to Dr. J. H. Baxendale under whose suggestion and guidance the above study was conducted at the University of Manchester, U.K. The author is indebted to the personnel of the Peterson Laboratories, Manchester for the provision of the facilities used during the pulse radiolysis experiments. The author is thankful to the British Government and the Indian Government for awarding the Colombo Plan Fellowship during which this study was carried out. The author is grateful to Dr. K. N. Rao of Bhabha Atomic Research Centre for his helpful suggestions in the preparation of this paper. References and Notes (1) Based upon the Ph.D Thesis in Chemistry submitted to the Unlversity of Manchester, U.K., in Sept. 1970. (2) (a) E.J. Hart and J. W. Boag, J. Am. Chem. SOC., 84,4090 (1962); (b) I. A. Taub, M. C. Sauer, and L. M. Dorfman, Discuss. Faraday SOC., 36, 206 (1963). (3) G. R. Freeman and J. M. Fayadh, J. Chem. Phys., 43, 86 (1965). (4) W. F. Schmidt and A. 0. Allen, J. Phys. Chem., 72, 3730 (1968). (5) W. F. Schmidt and A. 0. Allen, J. Chem. Phys., 52, 2345 (1970). (6) M. G. Robinson, P. G. Fuochi, and G. R. Freeman, Can. J. Chem., 49,3657 (1971). (7) G. E. Adams, J. H.Baxendale,and R. D. Sedgwick, J. Phys. Chem., 63,854
,.- -- ,.
IIQliQ\
(8)
P.Y. Feng, W. A. Glasson,-andS. A. Marshall, Contract No.AF 33, (616)-
6141. 1960. Proiect No. 7360. D 64. WADD-TR-60-344. (9) J P. Keene, J. Sci. Instrum., 41, 493 (1964). (IO) J. K. Thomas and E. J. Hart, Radiat. Res., 17, 408 (1962)
The Journal of Physical Chemistry, Vol. 80, No. 14, 1976
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 subsequent formation of 0 2 - , was studied. A rate constant ~((cH~)~c(oH)(o~)+oH--(cH~)~c(o-)(o~)+H~o) = (5.2 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 02CH20H12 and (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,12 the 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.
