12559
J. Phys. Chem. 1995,99, 12559-12564
Formation and Redox Properties of Radical Ions of Iodopentafluorobenzene in Aqueous Solution: A Pulse Radiolysis Study Hari Mohan and Jai P. Mittal*9+ Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India Received: February 15, 1995; In Final Form: May 25, 1995@
The transient optical absorption bands (Amax = 290 and 400 nm) formed on reaction of 'OH radicals in neutral aqueous solution of iodopentafluorobenzene are assigned to the phenoxyl radical. The radical decays by second-order kinetics with 2Wd = 4.2 x lo6 s-I and results in the formation of F- and I- [G(F-) = 5.3, G(1-) = 0.21. The rate constant for the reaction of 'OH radical with iodopentafluorobenzene is 1.1 x lo9 dm3 mol-' s-I. In acidic solution ([HC104] < 1 mol dm-3), the nature of the transient optical absorption spectrum remained the same whereas when [HC104] > 1 mol dm-3, the transient optical absorption bands formed at 310 and 660 nm are assigned to the solute radical cation. Cl2'- is unable to undergo electron transfer reaction with C&I whereas S O i - is able to react with a bimolecular rate constant of 3.3 x lo8 dm3 mol-' s-I, and transient bands observed at 290 and 390 nm are assigned to the phenoxyl radical. The solute radical cation is a powerful one-electron oxidant and is able to oxidize a number of organic compounds with high rate constant values. The hydrated electron reacts with C6F51 by a nondissociative electron capture process, and the resulting radical anion undergoes fast protonation to form the cyclohexadienyl radical (Amax = 290 nm). The bimolecular rate constant for the reaction of eaq- with C6FsI was 8 x lo9 dm3 mol-' s-I. Similar transient species is also observed on reaction of C02'- with C6F51. The spectrum of the radical anion is not observed even at pH 14.
Introduction The holes and electrons produced on y-radiolysis of a glassy matrix of 3-methylpentane at 77 K are mobile and thus can react with halogenated aromatic compounds to form radical cations and anions, respectively.'-6 The transient species are identified by the optical absorption'-6 and ESR measurement^.^ At room temperature, these transient species can also be generated and characterized by pulse radiolysis and flash photolysis investigations.8-'6 The 'OH radicals are known to react with alkyl iodides and form OH adduct and radical cations in neutral and acidic solutions, respecti~ely.'~.'~ The reaction of 'OH radicals with aromatic halides is mainly by addition to the benzene ring and forms the hydroxycyclohexadienyl r a d i ~ a l . ' ~ - ~Depending ' upon the nature of substituents and presence of H+, the OH adduct may undergo electron transfer reaction.22 The OH adduct may yield either the benzyl or phenoxyl radical if a loss of proton is possible.23 The oneelectron oxidation of aromatic compounds may also be possible with specific one-electron oxidants such as SOi-, T12+, and Ag2+.20-25Depending upon the presence of electron-donating substituents, the solute radical cation may undergo hydration and deprotonation reactions. The eaq- may react by either dissociative electron capture or intramolecular electron transfer reaction mechanism.26 The nature of the transient species formed on y-radiolysis of perfluorobenzene and its substituted derivatives and their redox properties are of considerable intere~t.'~,'~ But very few investigations have been carried out on derivatives of perflu~robenzene.~~ The nature of transient species formed on pulse radiolysis of aqueous solution of iodobenzene has been reported recently.30 With the objective of understanding the effect of fluorine on the redox properties of transient species formed from perfluoro aromatic compounds, 'Also associated with the Jawahar La1 Nehru Centre for Advanced Scientific Research, Bangalore, India. Abstract published in Advance ACS Abstracts, July 15, 1995. @
0022-3654/95/2099-12559$09.00/0
we have carried out detailed investigations on the reaction of primary radiolytic species of water with iodopentafluorobenzene.
Experimental Section Preparation of Solution. Iodopentafluorobenzene (IPFB, C&I) was obtained from Aldrich Chemicals (purity >99%) and used without any further purification. All other chemicals used were of "AnalaR" grade purity. The detailed experimental procedure has been reported p r e v i o u ~ l y . * ~ . ~ ~ Irradiation Procedure. Pulse radiolysis studies have been carried out with 7 MeV electron pulses of 50 ns pulse width obtained from a linear accelerator (Viritech Ltd., England), whose details are described el~ewhere.~'An aerated aqueous solution of KSCN (10 m o l dm-3) was used for measuring the dose delivered per pulse, taking GE = 21 520 dm3 mol-' cm-' per 100 eV at 500 nm for the transient (SCN)i- species.32 The dose delivered per pulse was %16 Gy (1 Gy = 1 J kg-I). Steady state y-irradiation was carried out with a 6oCoy-source. The dose rate as determined by Fricke dosimeter was e 1 0 Gy min-I. Radiolysis of neutral aqueous solution (N2-saturated) results in the formation of three highly reactive species (eaq-, H,and 'OH) in addition to the formation of other inert or less reactive molecular compounds.33
The reaction of 'OH radical in neutral solution was carried out in N20-saturated solution where e%- are quantitatively converted to 'OH radicals (eaq- N20 'OH OHN2) with G('0H) = 5.6 (where G denotes the number of species produced per 100 eV of energy absorbed or micromolar concentration of species per 10 J of energy absorbed). In acidic solutions, the reaction of 'OH radical was studied in 02-saturated solution to scavenge H and eaq- (eaq- f Haq+ 'H H20; 'H 0 2 HOi). The reaction of eaq- was studied in N2-saturated solution
+
-
+
-.
0 1995 American Chemical Society
+
+
+
+
Mohan and Mittal
12560 J. Phys. Chem., Vol. 99, No. 33, 1995
SCHEME 1 121 F
0.02
0 0
a 0.01
WAVELENGTH ( n m )
i
Figure 1. Transient optical absorption spectra obtained on pulse radiolysis of NzO-saturated neutral aqueous solution of C&I (3 x lo-) mol dm-3) (a) 2 and (b) 40 p s after the pulse. (c) Spectrum (a) in the presence of 0.3 mol dm-) ?err-butylalcohol. Inset shows second-order decay of (d) 400 and (e) 290 nm with time.
-
at pH 7 in the presence of tert-butyl alcohol to scavenge 'OH radicals [(CH3)3COH 'OH 'CH2(CH3)2COH H201. Product Analysis. F- was determined by a high-performance ion chromatograph (Dionex-2010) by using a conductivity detector. I- was determined as 13- (A = 352 nm, E = 25 800 dm3 mol-' cm-') spectrophotometrically (Hitachi 330) after extracting the aqueous solution with 12-hexane ([I21 = 1 x 1 0 - ~mol dm-3) solution.34 Stability of Iodopentafluorobenzene. The stability of IPFB in the presence of a high concentration of HC104 was studied on monitoring its optical absorption spectrum, which showed it to be stable. Independent experiments were also carried out to estimate I-, F-, and 12, which might be produced on thermal/ photochemical decomposition of aqueous (neutral and acidic) solution.34 The studies confirmed the stability within the experimental time.
+
+
Results and Discussion Reaction of 'OH Radicals at Neutral pH. Figure la,b shows the time-resolved transient optical absorption spectrum obtained on pulse radiolysis of N20-saturated neutral aqueous solution of IPFB, 2 and 40 ps after the pulse. It exhibits absorption bands with A,,, = 290 and 400 nm. Both the bands decayed with same kinetics, second order, with 2Wd = (4.2 f 1.5) x lo6 s-l, suggesting the presence of only one species. In the presence of 0.3 mol dm-3 tert-butyl alcohol, an efficient 'OH and weak H atom scavenger, the transient absorption at 290 and 400 nm reduced considerably (Figure IC). High GCOH) yield and appreciable decrease in the transient absorption by 'OH radical scavenger may suggest that the transient absorption (Figure la) is mainly due to reaction of *OHradicals with IPFB. The bimolecular rate constant for the reaction of *OHradical with the solute, determined by formation kinetics at 290 nm, was 1.2 x lo9 dm3 mol-' s-'. The absorbance of this band remained constant with solute concentration (6 x 10-4-l.2 x mol dm-3), showing that all the 'OH radicals have reacted with the solute. The extinction coefficient at 400 and 290 nm was thus determined to be 1350 and 2980 dm3 mol-' cm-I, respectively. The rate constant for the reaction of 'OH radicals with IPFB, determined by competition kinetics using KSCN as a standard (~.OH+SCN- = 1.1 x lolo dm3 mol-' s-l), was 1 x lo9 dm3 mol-' s-l. As a result of competition for 'OH radicals, the absorbance at 472 nm due to (SCN)2'- decreased from
14)
0.0853 to 0.0716 with simultaneous increase in absorbance at 290 nm from 0.0106 to 0.0152 on addition of IPFB. This increase at 290 nm should be due to the transient species formed on reaction of 'OH radicals with IPFB. Taking the extinction coefficient at 472 nm due to (SCN)2'- as 7580 dm3 mol-' ~ m - ' the , ~ extinction ~ coefficient at 290 nm was determined to be equal to 2550 dm3 mol-' dm-I. The spectrum (Figure la) is similar to that observed on reaction of 'OH radicals with perfluorobenzene and assigned to the pentafluorophenoxyl radical (C&0.).28 Therefore, in the present case, the transient spectrum (Figure la) is assigned to phenoxy1 radicals formed according to reaction 3 (Scheme 1). The OH adduct (2, Scheme 1) is reported to be unstable due to addition of *OH radicals on the carbon atom which also contains a halogen atom (FA). Such configurations are unstable and undergo very rapid hydrogen halide elimination.28 Steady state y-radiolysis showed the formation of I- and Fwith G values of 0.2 and 5.3, respectively. The combined yield of 1 - F is equal to 5.5, close to GTOH). These studies support the reaction mechanism (Scheme 1). Reaction of *OHRadicals in Acidic Solutions. (a)(HC1041 < I mol dm-3. The nature and the decay kinetics of the transient optical absorption spectrum formed on reaction of 'OH radical with IPFB remained independent of pH from 0 to 10. The contribution of H atom reaction with IPFB, in neutral aqueous solution, is observed to be negligible (Figure l ~ ) Even . ~ at ~ pH = 0 where the H atom yield is quite high [G(H) = 3.21, pulse radiolysis studies showed that the reactivity of H atom with IPFB is quite low. The bimolecular rate constant for the reaction of H atom with perfluorobenzene is reported to be < 1 x lo7 dm3 mol-' S - I . ~ ' Figure 2a shows the transient optical absorption spectrum obtained on pulse radiolysis of Orsaturated mol dm-3) in 1 mol dm-3 aqueous solution of Cp51 (3 x HC104. In aerated acidic solutions, the transient species reacting with IPFB would be HO2' and 'OH radicals. The absorbance of the transient signal at 290 nm decreased appreciably in the presence of 0.3 mol dm-3 tert-butyl alcohol, showing that the spectrum (Figure 2a) is mainly due to reaction of *OHradicals. In N2-saturated solutions, HO2' radicals would not be formed, and the transient optical absorption spectrum was similar to that observed in 02-saturated solutions. HO2' radicals are mild oxidizing agent with redox potential value of 1.O V,37and even stronger oxidizing agents such as Br2'- and C12'- failed to undergo electron transfer with IPFB (see text). These studies suggest that HO2' radicals are nonreactive toward IPFB. Since the nature and the kinetics of the transient optical absorption spectrum observed on reaction of 'OH radicals with IPFB in 1 mol dm-3 HC104 were similar to those observed at neutral pH,
Radical Ions of Iodopentafluorobenzene in Aqueous Solution
J. Phys. Chem., Vol. 99, No. 33, 1995 12561
0.031
WAVELENGTH ( n m 1
Figure 2. Transient optical absorption spectrum obtained on pulse radiolysis of 02-saturated aqueous solution of IPFB (3 x mol dm-)) containing HClOl (a) 1, (b) 4.9, (c) 6.9, (d) 7.8, (e) 9.8, and (Q 7.8 mol dm-3 HC104 alone. Inset shows absorption-time signal at 660 and 310 nm.
it is also assigned to neutral radical (reaction 3, Scheme 1) formed on elimination of halide ion from the OH adduct. (b) [HC104]> 1 mol dm-3. When [HC104] was more than 1 mol dm-3, pulse radiolysis studies showed the formation of transient bands with A, = 310 and 660 nm (Figure 2). The absorption at these wavelengths increased with HC104 and did not attain a saturation value even when [HCIOI] was 9.8 mol dm-3. The increase in absorbance with HC104 may not represent the true variation of absorbance with HC104 as *OH radical yield would no longer be constant in a high concentration of HC104. The fraction of radiation energy absorbed by HC104 would increase, and the actual concentration of 'OH radicals would decrease with an increase in HC104 concentration. Assignment of Transient Bands. In the presence of a high concentration of HC104, the primary reactive species produced on radiolysis are 'OH, HOz', and Clod radicals. C104 radicals are formed on reaction of WOH radicals with HC104 and absorb in the region of 330 nm.38 Pulse radiolysis of aerated aqueous solution of €IC104 (7.8 mol dm-3) showed small absorption in the 340 nm region (Figure 20. Therefore, absorption observed at 660 and 310 nm could not be due to C104 radicals or any other transient produced from radiolysis of HC104. Pulse radiolysis of an 02-saturated neutral solution of NaC104 (6.6 mol dm-3) containing IPFB (1 x mol dm-3) showed the absence of a transient band with A,,, = 660 nm. The transient bands were not observed in the presence of 0.3 mol dm-3 tertbutyl alcohol, an efficient 'OH radical scavenger. Therefore, the bands should be due to reaction of 'OH radicals with IPFB, in the presence of high H+. If the bands are due to the reaction of 'OH radicals with IPFB (in the presence of high Hf concentration), then similar bands should also be produced in the presence of another acid. Pulse radiolysis of Orsaturated aqueous solution of H2S04 (8 mol dm-3) in the presence of 3 x mol dm-3 IPFB showed absorption bands at 315 and 660 nm. The rate constant for the reaction of the 'OH radical with IPFB in 8 mol dm-3 H2S04 was the same at 315 and 660 nm, and the value was (5.5 -f 0.3) x lo9 dm3 mol-' s-l. The decay and formation kinetics and the nature of the transient spectrum formed in H2S04 were similar to that observed in HC104. These studies confirm that the transient spectrum is due to reaction of *OH radical with IPFI3 in the presence of a high concentration of Hf. The intensity and the lifetime of the transient bands were found to be independent of solute concentration in the range
(0.5-7.5) x mol dm-3, suggesting the bands to be due to a monomeric species. The OH adduct is known to undergo acid-catalyzed elimination of water20,22-24 and form the solute radical cation. The equilibrium (Scheme 1) would shift more toward the radical cation (4, Scheme 1), with increasing concentration of HC104. The intensity of absorption bands at 660 and 310 nm did not reach saturation with increasing concentration of H+; it suggests that the entire fraction of OH adduct is not converted to a solute radical cation even at high concentration of H+. The 660 nm band decayed by first-order kinetics whereas the decay at 310 nm was of mixed order at low concentrations of HC104. It showed better first-order decay at higher concentrations of HC104 and was close to that of 660 nm band in the presence of 9.8 mol dm-3 HC104. The cause for the mixed-order decay at 310 nm could be due to the fact that the OH adduct has appreciable absorption in this region. Therefore, the increased absorption at 3 10 and 660 nm in highly acidic solutions is assigned to the solute radical cation. Effect of Solute Structure. The transient bands at 305 and 630 nm, formed on pulse radiolysis of 02-saturated aqueous solution of iodobenzene (pH = 1S), are assigned to C ~ H S ~ ' + , ~ O whereas in the case of C&I the formation of C&I'+ requires a very high H+ concentration. This could be'due to presence of fluorine, which has a high electron affinity. The 'OH radicalinduced acid-catalyzed oxidation of C6F5I would be difficult and require high H+ concentration. The 'OH induced oxidation of fluorinated alkyl iodides has also required a high H+ concentration as compared to the very low H+ concentration (pH = 2) required for alkyl i o d i d e ~ . I ~The * ~ ~presence of chlorine, an electron affinic group, has also increased the concentration of H+ required for acid-catalyzed oxidation of chloroiodoalkanes. Reaction with Oxidizing Radicals. The Sod*- radical anion is a strong one-electron oxidant with redox potential of 2.43 vs NHE?O The reaction of sod*- with IPFB was studied by following the decay of the 460 nm band of sod- as a function of IPFB concentration. The decay becomes faster showing that reaction 2 is taking place.
C,F,I
+ SO,*- - c~F,I'++
(2)
The bimolecular rate constant for this reaction, determined from the slope of linear plot of kobs with IPFB concentration, was 3.3 x IO8 dm3 mol-' s-l. Time-resolved studies (10 p s after
Mohan and Mittal
12562 J. Phys. Chem., Vol. 99,No. 33, 1995 TABLE 1: Reaction of C&sI'+ with Various Solutes mol dm-3, [HClOJ = 7.8 mol dm-3, A ([CsF51] = 6 x
0.Oh
= 660 nm)
rate constant reaction
C6F5I" $. BrC6F5I" + CICsF5I" + SCNC6FsI'+ + (CH3)zS C$5I'+
+ (CH3S)z
(dm3 mol-'
S-I)
3.2 x 109 1.7 x 109 3.5 x lo9 2.0 x 109 1.5 x lo9
transient species (Amu) Br2'- (355 nm) Cl2'- (345 nm) (SCN)2'- (480 nm) [(CH3)2Sl2'+(460 nm) (CH,SSCH3)'+ (450 nm)
the pulse) showed the formation of bands with Amax = 290 and 390 nm. The spectrum does not match that of the solute radical cation (Figure 2). Therefore, it could not be assigned to C&r+. The solute radical cation is stable only at high H+concentration. C6F51.+formed at neutral pH may undergo hydration to yield the OH adduct (Scheme 1). This OH adduct may then decay to the phenoxy1 radical, which is already shown to have a similar absorption spectrum. Based on G(SO4*-) = 3.3, the extinction coefficient of the transient band with Amax = 290 nm was determined to be 2060 dm3 mol-' cm-'. The decay of C12'- does not become faster in the presence mol dm-3), of a small concentration of IPFB (5 x suggesting that electron transfer from IPFB to C1z'- is not taking place. As expected, the decay of Br2'- was also not affected by addition of a small concentration of C6F51. From these studies it appears that the redox potential value for the formation of solute radical cations (C&I'+) should be between the redox potential value of the C12'-/2Cl- and so4'-/so42-couple. Therefore, the C6F5I'+/C&I couple should be a strong oneelectron oxidant and should have a redox potential value between 2.09 and 2.43 V vs NHE. Redox Studies. (a) C6F,-P+. The transient band of C&I'+ (A, = 660 nm) formed on pulse radiolysis of 02-saturated aqueous solution of C&I (7.7 x mol dmW3)in 7.8 mol dm-3 HClO4 was observed to decay faster in the presence of a small concentration of Br-, (0.4-2.5) x mol dm-3. The bimolecular rate constant, determined from the decay at 660 nm, was 2.6 x lo9 dm3 mol-] SKI, Time-resolved studies showed the formation of a transient band with Amax = 355 nm. The bimolecular rate constant determined from the growth at 355 nm was 3.6 x lo9 dm3 mol-' s-l, close to that determined from the decay of the 660 nm band. The transient band of CsF5I'+ was also observed to decay faster in the presence of a small concentration of C1-, (1-4.5) x mol dm-3. The bimolecular rate constant determined from the slope of linear plot of kobs vs C1- concentration was 1.7 x lo9 dm3 mol-' s-I. Time-resolved studies showed the formation of a new band with "A = 340 nm. The rate constant was observed to remain independent of solute concentration, mol dm-3. Similar redox studies have also been (3-6) x carried out with a number of other additives (Table 1) and support the earlier conclusion that the CsFsI'+/C&I couple is a strong one-electron oxidant. (b) c6FdfO'/c6F5O'. The transient band Of C6F4Io'/c&o' at 290 nm formed on pulse radiolysis of NzO-saturated neutral aqueous solution of C6F5I remained unaffected in the presence of a small concentration of I- (1 x mol dm-3), showing that electron transfer is not possible, Therefore, its redox potential value must be lower than that of the 12'-/21- couple (1.03 V vs NHE).40 In acidic solution (1 mol dm-3 HC104), the decay of the transient optical absorption band (A = 290 nm) also remained unaffected in the presence of a small concentration of SCN- (1 x mol dr~--~). Reaction with eaq-. The rate constant for the reaction of eaq- with IPFB was determined on monitoring the decay of eaq-,
0.0 2
I
25 0
350 WAVELENGTH
L50
550
(nm)
Figure 3. Transient optical absorption spectrum obtained on pulse radiolysis of Nz-saturated aqueous solution ([tert-butyl alcohol] = 0.3 mol dm-,, [IPFB] = 1 x mol dm-3) 2 ,us after the pulse (a) at neutral pH and (b) at pH = 14. Inset decay of eaq-at 700 nm (c) in the absence and (d) in the presence of C851 (1 x loT3mol dm-3).
SCHEME 2
F
formed on pulse radiolysis of Nz-saturated neutral solution ([rertbutyl alcohol] = 0.3 mol dm-3, A,, = 700 nm) for various concentrations of E", (1-10) x mol dm-3. The decay of ea¶- becomes faster and of first order, showing its reaction with IPFB (inset of Figure 3). The first-order rate constant ( k b s ) was found to increase linearly with solute concentration. The bimolecular rate constant determined from the slope of the linear plot of /cobs vs solute concentration was 8 x lo9 dm3 mol-' s-I. Time-resolved studies showed the formation of a transient band with A,,, = 290 nm and broad absorption in the region of 400 nm (Figure 3a). The bimolecular rate constant, determined from the growth at 290 nm, was 5.6 x lo9 dm3 mol-' s-', close to the value determined from the decay at 700 nm. The transient band (A = 290 nm) was observed to decay by second-order kinetics with 2Wd = 5 x lo6 s-'. The extinction coefficient of the transient band with Amax = 290 nm was estimated to be 6840 dm3 mol-' cm-' [assuming G(eas-) = 2.71. Steady state analysis showed a very small yield of F- [G(F-) = 0.21, as compared to the yield of ea¶- (2.7). Even I- could not be detected. This suggests that the reaction of eaq- with IPFB is nondissociative. It is reported that benzene and fluorinesubstituted benzene radical anion undergo rapid protonation to yield the cyclohexadienyl radical at lower Therefore, the transient spectrum (Figure 3a) is assigned to C6FsI-H formed on protonation of C6FsI- (Scheme 2). The transient spectrum (Figure 3b) obtained on pulse radiolysis of Nz-saturated aqueous solution of IPFB (1 x mol dm-3, [tert-butyl alcohol] = 0.3 mol dm-3, pH = 14) was similar to that observed at neutral pH. The formation (8.2 x lo9 dm3 mol-' s-l) and decay kinetics (2kk1 = 6.2 x lo6 s-I) were also similar to that observed at neutral pH. Steady state analysis of y-irradiated solution of IPFB (1 x mol dm-3, [tert-butyl alcohol] = 0.3 mol dm-3, pH = 14) also showed the absence of I- as the
Radical Ions of Iodopentafluorobenzene in Aqueous Solution
J. Phys. Chem., Vol. 99, No. 33, 1995 12563
TABLE 2: Characteristics of the Transient Species Formed on Reaction of Primary Radiolytic Species of Water with IPFB
A,,,,,(nm)
reaction
+ C6FsI + 'OH + H+ ([H+] 1 M) C6F51+ 'OH + H+ ([H+] > 1 M) C&I + eaq- (pH = 7) C6F51 'OH
C&I a
E
(dm3 mol-'
290
formation (dm3 mol-' s-l)
decay (s-l)
G(F-)
G(1-)
transient species
2760 1260 2810
400 290 400 310 660 290 290
+ eaq- (pH = 14)
S-I)
6840
Second-order decay.
stable end product. Therefore, even at this pH, the radical anion undergoes fast protonation and dehalogenation is not taking place. The radical anion of C6F6, C6F6-, has been shown to have a pK value of 12.4.27 Iodine substitution must have increased the pK value of the equilibrium (Scheme 2). At pH = 14, the absorption in 350-400 nm region was slightly higher, which may be due to stabilization of a fraction of C6FsI-. The nature of transient species formed on reaction of primary reactive species of water with C6F51is shown in Table 2. Reaction of Reducing Species. The C02'- radical anion formed on reaction of WOH radicals with HCOO- is a strong reducing agent with redox potential value of -1.9 V.4' The transient optical absorption spectrum obtained on pulse radiolysis of N2O-saturated neutral aqueous solution of HCOO- (4 x mol dn~-~) in the presence of PFB (1 x mol dnr3) exhibits an absorption band with A,, = 290 nm and broad absorption in the 400 nm region. It is similar to the transient spectrum obtained on reaction of eaq- with IPFB at neutral pH. Therefore, it is also assigned to C6FsI.H formed on hydration of radical anion (reactions 3 and 4). C6FsI
+ C0,'-
-
C6F51-
-
+C02
(3)
+H,O
C6FsI-
-on-
C6F$H
(4)
The formation rate constant, determined from the growth at 290 nm in the concentration range of (1-10) x mol dm-3, was 1.4 x lo9 dm3 mol-' s-I. Steady state analysis for F- could not be carried out in the presence of a large excess of HCOO-, and I- was not observed in the y-irradiated samples. Therefore, the reaction of COiis also nondissociative in nature. The cyclohexadienyl radical formed on hydration of radical anion may be decaying by the disproportionation mechanism. The a-hydroxyisopropyl radical, (CH3)2'COH (E" = - 1.05 eV), formed on pulse radiolysis of neutral N2O-saturated solution of 2-propanol (0.5 mol dm-3), was unable to react with IPFB, suggesting that the redox potential value for the C&YC&I- couple is more negative than -1.05 V. Therefore, C6F51- is a strong reducing agent with reduction potential value between -1.1 and -1.9 V. C6FsI-H (Am,, = 290 nm), formed on pulse radiolysis of N2saturated solution of C&I (3 x mol dm-3, [fert-butyl alcohol] = 0.2 mol dm-3), was able to reduce MV2+ ((3-8) x lop5mol dm-3) with a bimolecular rate constant of 2 x 1Olo dm3 mol-' s-l.
Conclusions The hydroxyl radical, in neutral aqueous solution of C&I, forms the phenoxy1 radical (Amax = 290 and 400 nm) and F-/I-. In acidic solution, the solute radical cation (Amax = 310 and 660 nm) is formed. The solute radical cation can also be generated on reaction of SO4*- with C6FsI. The solute radical cation is a powerful one-electron oxidant with redox potential
value between 2.1 and 2.4 V. The hydrated electron reacts with C&I by a nondissociative process. The radical anion undergoes fast protonation and is converted to the cyclohexadienyl radical (A,,,, = 290 nm). The radical anion of C ~ F S isI a strong reducing agent with reduction potential value between - 1.1 and -1.9 V.
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