- Email: [email protected]
Oxidation kinetics of iron(II) ion with t-butoxyl radical
Radiation Physics and Chemistry 67 (2003) 269–274
Oxidation kinetics of iron(II) ion with t-butoxyl radical Branka Mihaljevic! *, Dus$an Ramem Department of Chemistry, Ruoer Bo$skovi!c Institute, Radiation Chemistry and Dosimetry Laboratory, 10000 Zagreb, P.O. Box 180, Bijeni!cka 54, Croatia
Abstract Rate constant for the oxidation of iron(II) with tert-butoxyl radical (t-BuOd) was measured using laser flash photolysis. t-BuOd radicals were generated by homolytic cleavage of di-tert-butyl peroxide in methanol at room temperature. Overall rate constant of the reactions competing with the reaction of interest was measured in a separate run using diphenylmethanol instead of iron(II). By using known rate constant of the reaction of diphenylmethanol with t-BuOd, the overall rate constant for the decay of t-BuOd radical in the system was calculated. From the relative rate constant of oxidation of iron(II) with t-BuOd radical and t-BuOd radical decay rate constant, absolute rate constant was calculated, 3.4 108 M1 s1. The influence of the medium acidity on the rate constant was considered. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: t-butoxyl radical; Chloroiron(III) complex; Ketyl radical; Quantum yields; Rate constants
1. Introduction Although a major role of transition metal ions in hydroperoxide decomposition and propagation of lipid peroxidation is well known (Halliwell and Gutteridge, 1999; Tang et al., 2000), the oxidation of iron(II) with alkoxyl radicals has not been studied directly. On the other hand, the oxidation of iron and iron complexes by free radicals is of interest for the understanding of mechanisms of electron-transfer processes occurring in biological systems (van Eldik et al., 1994). Our earlier attempt to estimate the rate constant of ferrous iron oxidation with tert-butoxyl radical (t-BuOd) was based on the use of laser flash photolysis to generate t-BuOd radicals and measure the rate constant relative to all other processes of t-BuOd decay (Mihaljevic et al., 1999). It relied on the published values of the rate constants of major processes competing with the reaction of interest. The aim of the present work is to measure rate constant of the oxidation of iron(II) ion with t-BuOd more directly. Microsecond laser flash photolysis was used again to generate t-BuOd. In
*Corresponding author. Fax: 385-1-4680-098. E-mail address: [email protected] (B. Mihaljevi!c).
this way a rate constant of iron(II) oxidation relative to t-BuOd disappearance could be determined in the first step. Absolute rate constant was determined in a separate run using diphenylmethanol (DPM) instead of iron(II) ion to obtain data on the kinetics of t-BuOd disappearance relative to the oxidation of DPM by t-BuOd. It is possible thanks to the known absorption properties and kinetic data for the ketyl radical produced by the reaction of DPM with t-BuOd (Inbar et al., 1981; Small and Scaiano, 1978). The influence of the medium acidity on the rate constant of the oxidation of iron(II) ion with t-BuOd is also considered.
2. Experimental 2.1. Materials Di-tert-butyl peroxide ((t-BuO)2) 99% (technical) by Akzo Chemicals (Amsterdam, The Netherlands) was passed through an alumina column to remove tert-butyl hydroperoxide; the presence of residual tert-butyl hydroperoxide was checked by the spectrophotometric ferric thiocyanate method (Mihaljevic et al., 1996). Diphenylmethanol by Aldrich (Milwaukee, WI, USA)
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00050-1
B. Mihaljevi!c, D. Raz˘em / Radiation Physics and Chemistry 67 (2003) 269–274 10
8
6
3
was sublimated twice before use. All other materials were described in the earlier paper (Mihaljevic et al., 1999). Methanolic solutions of (t-BuO)2 for photolysis typically contained 2.74 M (t-BuO)2, between 3.6 103 and 3.6 102 M Fe2+, 2.3 102 M HCl and 2.3%, v/v (1.1 M) H2O. All freshly prepared reagent solutions and solutions for photolysis were deoxygenated by bubbling with pure nitrogen.
10 A(360 nm)
270
4
2
2.2. Methods
3. Results and discussion t-Butoxyl radicals were generated by photodecomposition of di-tert-butyl peroxide. Radicals produced oxidize Fe2+ ions (reaction 2), and in the presence of Cl ions monochloroiron(III) complex ([FeCl]2+) is formed (reaction 3), which absorbs at about 360 nm in acidic methanol solution. Assuming that the rate of the complexation of Fe3+ with Cl in methanolic solution is fast on the experimental time scale, as well as that chloroiron(III) complex is relatively stable on that scale, the following reactions leading to the formation of chloroiron(III) complex take place: hv
t-BuOOt Bu ! 2t-BuOd k2
t-BuOd þ Fe2þ ! t-BuOH þ Fe3þ k3
Fe3þ þ Cl !½FeCl2þ k0
t-BuOd !ðFirst order decaysÞ
ð1Þ ð2Þ ð3Þ
ð4Þ
0 0
5
10
15
20
25
6
(A)
10 t/s
12 10 8 6
3
10 A(535 nm)
Light pulses from a frequency doubled, Q-switched ruby laser by Korad (Santa Monica, CA, USA) (347.1 nm), were used for the production of alkoxyl radicals. All signals were normalized to the corresponding actinometry data. Actinometry was based on laser flash generation of a transient triplet state in 0.001 M deaerated aqueous solution of 4-carboxy benzophenone (Marciniak et al., 1995). The details of the laser flash photolysis setup and of actinometry are given in the earlier paper (Mihaljevic et al., 1999). Aqueous solution of 0.03 M NaNO2 was used for attenuating laser pulses. All measurements were made at an incident photon flow of about P0 E3 105 mol flash1. Absorption spectra of iron(III) ion complex formed after a laser flash were compared with the steady-state spectra taken by Cary 2200 spectrophotometer (Varian). All measurements were made at room temperature.
4 2 0 0
(B)
1
6
2
3
10 t/s
Fig. 1. (A) The absorbance change of chloroiron(III) complex at 360 nm on the experimental time scale ([Fe2+]=9.0 105 M, P0 ¼ 3:0 105 mol flash1 ); (B) The absorbance change of ketyl radical at 535 nm after data processing up to 3 ms ([Ph2CHOH]=0.2 M; P0 ¼ 2:1 105 mol flash1).
From the anticipated mechanism the quantum yield of iron(II) ion oxidation, F(Fe3+) follows as: FðFe3þ Þ ¼ Fðt-BuOd Þk2 ½Fe2þ =ðk0 þ k2 ½Fe2þ Þ
ð5Þ
1=FðFe3þ Þ ¼ 1=Fðt-BuOd Þð1 þ k0 =k2 ½Fe2þ Þ
ð6Þ
Quantum yield F(Fe3+) was determined by measuring absorbance at 360 nm of the formed chloroiron(III) complex about 2 ms after the flash (Fig. 1A), and by calculating the concentration of iron(III) complex using molar absorptivity determined under our experimental conditions, e(360 nm, CH3OH, 20 C)=47007230 M1 cm1. Absorption spectra of the chloroiron(III) complex obtained by laser flash photolysis were identical to the spectra of the same complex obtained by mixing components and measured by spectrophotometry (Mihaljevic et al., 1999).
B. Mihaljevi!c, D. Raz˘em / Radiation Physics and Chemistry 67 (2003) 269–274 20
271
30
17.5 25
15
20
1/Φ(Fe3+)
10
15
3
10 × A(360 nm)
12.5
7.5 10
5 5
2.5
0
0 0
1
2 5
10 × Pabs/mol flash
3 -1
Fig. 2. Absorbance of chloroiron(III) complex at 360 nm as function of the absorbed photon flow measured about 2 ms after the flash; (circles): 4.7 103 M Fe2+; (rhombs): 9.6 103 M Fe2+; full and empty symbols denote 2.7 and 1.8 M (tBuO)2, respectively.
3.1. Determination of the relative rate constant The dependence of the absorbance of chloroiron(III) complex at 360 nm on incident photon flow measured at two different concentrations of Fe2+ ion and di-tertbutyl peroxide, respectively (Fig. 2), shows that conditions are in accordance with the pseudo-first order of reaction (2). By photolysis of solutions that contained 2.7 and 1.8 M di-tert-butyl peroxide, respectively, a plot corresponding to Eq. (6) was obtained: 1/F(Fe 3+)= 0.0616/[Fe2+]+3.4131 (R2 ¼ 0:96) (Fig. 3). Quantum yield F(t-BuOd)=(0.3070.01), follows from the intercept, while relative rate constant of the reaction of t-BuOd radical with Fe2+ ion follows from the ratio of the intercept to slope, krel ¼ k2 =k0 ¼ ð55:570:14Þ M1 : By flash photolysis of solutions that contained the same components except for the increasing hydrochloric acid concentration (Table 1), a slight increase of F(Fe3+) value with increasing acid concentration was observed. On the other hand, increasing acid
0
100
200
300
400
500
(1/[Fe2+]) /dm3 mol-1
Fig. 3. Plot corresponding to Eq. (6) (1/F(Fe3+)=0.062/ [Fe2+]+3.41; R2 ¼ 0:96). Full and empty symbols represent results obtained at 2.7 M and 1.8 M (t-BuO)2, respectively. Error bars denote 715%.
concentration decreased the relative rate constant, while in strongly acid medium F(t-BuOd) increased. Dependence of the rate constant of the reaction of Fe2+ with t-BuOd on acid concentration in our work is similar to the dependence of the rate constant of the reaction of Fe2+ with dOH in aqueous solution (2.3 108 M1 s1 at pH=1, Jayson et al., 1972; 4.3 108 M1 s1 at pH=3, Christensen and Sehested, 1981). However, it is known that the rates of both b-cleavage and addition of alkoxyl radicals relative to hydrogen abstraction by alkoxyl radicals increase in strongly acid solution (Avila et al., 1993; Gilbert et al., 1981). We did not observe any large decrease of the relative rate constant on going from 0.023 to 0.062 M HCl, but b-cleavage accelerated by protonation of t-BuOd in strongly acid medium would be expected in methanolic solution containing 0.24 M HCl causing the relative rate constant to decrease. The presence of tert-butyl radical-cation, t-BuOdH+ in acid medium could increase F(Fe3+) due to its greater electrophilic character than that of
B. Mihaljevi!c, D. Raz˘em / Radiation Physics and Chemistry 67 (2003) 269–274
272
Table 1 Quantum yield F(Fe3+) (in solutions containing [Fe2+]=1.6 102 M, [(t-BuO)2]=2.7 M, [MeOH]=12.3 M (2.3% H2O v/v)), the relative rate of reaction (2) krel and quantum yield F(t-BuOd) as function of the concentration of hydrochloric acid; P0 B3 105 mol flash1 [HCl] (M1)
e(FeCl2+) (M1 cm1)
F(Fe3+)
krel (M1)
F(t-BuOd)
k2 (M1 s1)
2.3 102 6.2 102 2.4 101
4700 4830 6360
0.1170.01 0.1470.02 0.1870.02
(55.570.14) 48.5 15.7
(0.3070.01) 0.31 0.92
3.4 108 3.0 108 9.8 107
3.2. Determination of the rate constant k2 The rate constant for oxidation of Fe2+ with t-BuOd radicals was calculated using the relative rate constant and the overall rate constant for disappearance of t-BuOd radicals, k0 : The rate constant k0 was determined under the same experimental conditions as in the measurements of the quantum yield F(Fe3+), with the exception that DPM was used instead of the Fe2+ in the solution. The reaction of t-BuOd radical with DPM (7) gives ketyl radicals with the absorbance maximum at 535 nm (Inbar et al., 1981) and the rate constant k7 is known (Small and Scaiano, 1978) k7
4
3
2
1
0
d
t-BuOd þ Ph2 CHOH ! t-BuOH þ Ph2 C OH
5
1/Φ(Ph2C· OH)
t-BuOd radical (Cookson et al., 1976). On the basis of our results, at the half-life of the oxidation of Fe2+ with t-BuOd followed by the formation of chloroiron(III) complex, t1/2o1 ms (Fig. 1A), the rate constant for the formation of complex between iron(III) and chloride in our system ([HCl]=0.023 M, 2.3% H2O, v/v) would be k3>3 107 M1 s1. As compared with F(t-BuOd)=0.3 in less acid solutions, the value of F(t-BuOd)=0.9 was obtained in strongly acid solution containing 0.24 M HCl. The increase of F(t-BuOd) with increasing acid concentration suggests a catalytic effect of acid on the decomposition of (t-BuO)2.
ð7Þ
0
2
4
6 3
8
-1
(1/[Ph2CHOH])/dm mol
k0
t-BuOd ! ðFirst order decaysÞ (4) The value of FðPh2 Cd OHÞ is given by
Fig. 4. Plot corresponding to Eq. (9) ð1=FðPh2 Cd OHÞ=0.52/ [Ph2CHOH]+0.58; R2 ¼ 0:97).
d
FðPh2 C OHÞ ¼ Fðt-BuOd Þk7 ½Ph2 CHOH=ðk0 þ k7 ½Ph2 CHOHÞ ð8Þ or written as reciprocal, we have d
1=FðPh2 C OHÞ ¼ 1=Fðt-BuOd Þð1 þ k0 =k7 ½Ph2 CHOHÞ
ð9Þ
Quantum yield FðPh2 Cd OHÞ was calculated by measuring the absorbance at 535 nm 0.5 ms after the flash (Fig. 1B) and using the known molar absorptivity of
ketyl radical determined in water e(535 nm, H2O, 20 C)=3220 M1 cm1 (Bensasson and Land, 1971). On the basis of molar absorptivities of ketyl radical in water and in cyclohexane, Bensasson and Land (1971) have concluded that the effect of solvent on molar absorptivity of ketyl radical is negligible. From the intercept of the plot according to Eq. (9) (Fig. 4), quantum yield F(t-BuOd)=1.7 follows. Using the known rate constant k7=6.9 106 M1 s1 (Small and Scaiano, 1978), the rate constant of the overall rate of disappearance of t-BuOd radicals in the system was
B. Mihaljevi!c, D. Raz˘em / Radiation Physics and Chemistry 67 (2003) 269–274
obtained from the ratio of intercept to slope, k0=6.2 106 s1. This rate constant is very close to the rate constant used earlier, k0=5.4 106 s1 (Mihaljevic et al., 1999), which was estimated using the known values of the rate constants for the reactions leading to the disappearance of t-BuOd. The largest contribution to the overall rate of the radical disappearance is b-cleavage of t-BuOd (in water, kb= 1.4 106 s1, Erben-Russ et al., 1987). Using the known value for the reaction of t-BuOd with MeOH (2.9 105 M1 s1, Paul et al., 1978) and the rate constant for the reaction of t-BuOd with (t-BuO)2 (1.6 105 M1 s1, Small and Scaiano, 1978), from the rate constant k0 obtained in our work we calculated the rate of b-cleavage of t-BuOd in methanol, kb=2.2 106 s1. On the basis of the known stability of ketyl radical in acid solutions (pKa=8.2), as well as the fact that bcleavage of t-BuOd is a preferred mode of disappearance of t-BuOd in strongly acid solution, hydrogen abstraction from diphenylmethanol by t-BuOd (7) in a moderately acid medium should not be significantly affected by acid concentration (Gilbert et al., 1981). As can be seen from Table 1, in contrast to the strongly acid solutions (0.24 M HCl), relative rate constants and quantum yields did not significantly change with increasing acid concentration in solutions for photolysis, revealing the comparable rate constants of the oxidation of Fe2+ with t-BuOd. From the relative rate constant krel of the reaction of t-BuOd radicals with Fe2+ (2) and the measured value of the rate constant k0 ; the rate constants of the reaction of t-BuOd with Fe2+ were determined (k2 ¼ kre k0 ) at 2.3 102 and 6.2 102 M HCl, as 3.4 108 and 3.0 108 M1 s1, respectively. In the formation of ferric ion complex, reactions by radicals other than t-BuOd are possible. These radicals include primarily methyl (dCH3), formed through b-cleavage of t-BuOd radical; however, it should be considered negligible on our time scale 6 1 1 (k(dCH3+Fe(CN)4 s ; Steenken and 6 )=5 10 M Neta, 1982). Contributions of solvent (dCH2OH) and peroxide (t-BuOOt-Bud)-derived radicals to ferric ion complex formation are expected to be even less probable than that of methyl on the basis of the lower reactivity as compared with the reactivity of methyl (Paul et al., 1978). However, contributions of methyl (k(dCH3+ Fe(H2O)5Cl2+)>5 106 M1 s1, Bakac, 2001), and especially hydroxymethyl radical (k(dCH2OH+Fe3+)= 8 107 M1 s1, Buxton and Green, 1978), are more likely to the slow ferric ion complex reduction, which was indeed observed as a slow decrease of the absorbance of ferric ion complex at 360 nm (Fig. 1A). Similarity of the rate constant of the oxidation of Fe2+ with t-BuOd radical obtained in our work with the rate constant of the oxidation of Fe2+ with dOH in water (Jayson et al., 1972; Christensen and Sehested, 1981) suggests a similar reaction mechanism. According
273
to the studies of the mechanism of reaction of divalent transition metal ions and complexes with aliphatic free radicals (van Eldik et al., 1994) in which complex formation between metal ion and free radicals can be treated mechanistically in a similar way as developed for conventional ligand substitution processes, the high rate constant of Fe2+ oxidation with t-BuOd radical indicates that an outer-sphere mechanism is operating.
References Avila, D.V., Brown, C.E., Ingold, K.U., Lusztyk, J., 1993. Solvent effects on the competitive b-scission and hydrogen atom abstraction reactions of the cumyloxyl radical. J. Am. Chem. Soc. 115, 466–470. Bakac, A., 2001. Effects of halide ions and carbon monoxide on the reactions of iron(III) with alkyl radicals. Croat. Chem. Acta 74, 633–640. Bensasson, R., Land, E.J., 1971. Triplet–triplet extinction coefficients via energy transfer. Trans. Faraday Soc. 67, 1904–1915. Buxton, G.V., Green, J.C., 1978. Reactions of some simple a- and b-hydroxyalkyl radicals with Cu2+ and Cu+ ions in aqueous solution. J. Chem. Soc. Faraday Trans. I. 74, 697–714. Christensen, H., Sehested, K., 1981. Pulse radiolysis at high temperatures and high pressures. Radiat. Phys. Chem. 18, 723–731. Cookson, P.G., Davies, A.G., Roberts, B.P., Tse, M.-W., 1976. Photolysis of di-t-butyl peroxide under acid conditions. J. Chem. Soc. Chem. Commun. 937–938. Erben-Russ, M., Michel, C., Bors, W., Saran, M., 1987. Absolute rate constants of alkoxyl radical reactions in aqueous solution. J. Phys. Chem. 91, 2362–2365. Gilbert, B.C., Marshall, P.D.R., Norman, R.O.C., Pineda, N., Williams, P.S., 1981. Electron spin resonance studies. Part 61 The generation and reactions of the t-butoxyl radical in aqueous solution. J. Chem. Soc. Perkin II, 1392–1400. Halliwell, B., Gutteridge, J.M.C., 1999. In: Free Radicals in Biology and Medicine, 3rd Edition. Oxford Science Publications, Oxford. Inbar, S., Linschitz, H., Cohen, S.G., 1981. Quenching, radical formation, and disproportionation in the photoreduction of 4-carboxybenzophenone by 4-carboxybenzhydrol, hydrazine, and hydrazinium ion. J. Am. Chem. Soc. 103, 7323–7328. Jayson, G.G., Parsons, B.J., Swallow, A.J., 1972. Oxidation of ferrous ions by hydroxyl radicals. J. Chem. Soc., Faraday Trans. 1. 68, 2053–2058. Marciniak, B., Hug, G.L., Bobrowski, K., Kozubek, H., 1995. Mechanism of 4-carboxybenzophenone-sensitized photooxidation of methionine-containing dipeptides and tripeptides in aqueous solution. J. Phys. Chem. 99, 13560–13568. Mihaljevic, B., Katusin-Razem, B., Razem, D., 1996. The reevaluation of the ferric thiocyanate assay for lipid hydroperoxides with special considerations of the mechanistic aspects of the response. Free Radical Biol. Med. 21, 53–63.
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Mihaljevic, B., Medved-Rogina, B., Razem, D., 1999. Rate constant for the reaction of the t-butoxyl radical with Fe(II) ion. Radiat. Phys. Chem. 55, 593–597. Paul, H., Small Jr., R.D., Scaiano, J.C., 1978. Hydrogen abstraction by tert-butoxy radicals. A laser photolysis and electron spin resonance study. J. Am. Chem. Soc. 100, 4520–4527. Small Jr., R.D., Scaiano, J.C., 1978. Absolute rates of hydrogen abstraction by tert-butoxy radicals. J. Am. Chem. Soc. 100, 296–298. Steenken, S., Neta, P., 1982. Oxidation of substituted alkyl 3 radicals by IrCl2 6 , Fe(CN)6 , and MnO4 in aqueous
solution. Electron transfer versus chlorine transfer from IrCl2 6 . J. Am. Chem. Soc. 104, 1244–1248. Tang, L., Zhang, Y., Qian, Z., Shen, X., 2000. The mechanism of Fe2+-initiated lipid peroxidation in liposomes: the dual function of ferrous ions, the roles of the pre-existing lipid peroxides and the lipid peroxyl radical. Biochem. J. 352, 27–36. van Eldik, R., Cohen, H., Meyerstein, D., 1994. Ligand interchange controls many oxidations of divalent first-row transition metal ions by free radicals. Inorg. Chem. 33, 1566–1568.
Oxidation kinetics of iron(II) ion with t-butoxyl radical Branka Mihaljevic! *, Dus$an Ramem Department of Chemistry, Ruoer Bo$skovi!c Institute, Radiation Chemistry and Dosimetry Laboratory, 10000 Zagreb, P.O. Box 180, Bijeni!cka 54, Croatia
Abstract Rate constant for the oxidation of iron(II) with tert-butoxyl radical (t-BuOd) was measured using laser flash photolysis. t-BuOd radicals were generated by homolytic cleavage of di-tert-butyl peroxide in methanol at room temperature. Overall rate constant of the reactions competing with the reaction of interest was measured in a separate run using diphenylmethanol instead of iron(II). By using known rate constant of the reaction of diphenylmethanol with t-BuOd, the overall rate constant for the decay of t-BuOd radical in the system was calculated. From the relative rate constant of oxidation of iron(II) with t-BuOd radical and t-BuOd radical decay rate constant, absolute rate constant was calculated, 3.4 108 M1 s1. The influence of the medium acidity on the rate constant was considered. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: t-butoxyl radical; Chloroiron(III) complex; Ketyl radical; Quantum yields; Rate constants
1. Introduction Although a major role of transition metal ions in hydroperoxide decomposition and propagation of lipid peroxidation is well known (Halliwell and Gutteridge, 1999; Tang et al., 2000), the oxidation of iron(II) with alkoxyl radicals has not been studied directly. On the other hand, the oxidation of iron and iron complexes by free radicals is of interest for the understanding of mechanisms of electron-transfer processes occurring in biological systems (van Eldik et al., 1994). Our earlier attempt to estimate the rate constant of ferrous iron oxidation with tert-butoxyl radical (t-BuOd) was based on the use of laser flash photolysis to generate t-BuOd radicals and measure the rate constant relative to all other processes of t-BuOd decay (Mihaljevic et al., 1999). It relied on the published values of the rate constants of major processes competing with the reaction of interest. The aim of the present work is to measure rate constant of the oxidation of iron(II) ion with t-BuOd more directly. Microsecond laser flash photolysis was used again to generate t-BuOd. In
*Corresponding author. Fax: 385-1-4680-098. E-mail address: [email protected] (B. Mihaljevi!c).
this way a rate constant of iron(II) oxidation relative to t-BuOd disappearance could be determined in the first step. Absolute rate constant was determined in a separate run using diphenylmethanol (DPM) instead of iron(II) ion to obtain data on the kinetics of t-BuOd disappearance relative to the oxidation of DPM by t-BuOd. It is possible thanks to the known absorption properties and kinetic data for the ketyl radical produced by the reaction of DPM with t-BuOd (Inbar et al., 1981; Small and Scaiano, 1978). The influence of the medium acidity on the rate constant of the oxidation of iron(II) ion with t-BuOd is also considered.
2. Experimental 2.1. Materials Di-tert-butyl peroxide ((t-BuO)2) 99% (technical) by Akzo Chemicals (Amsterdam, The Netherlands) was passed through an alumina column to remove tert-butyl hydroperoxide; the presence of residual tert-butyl hydroperoxide was checked by the spectrophotometric ferric thiocyanate method (Mihaljevic et al., 1996). Diphenylmethanol by Aldrich (Milwaukee, WI, USA)
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00050-1
B. Mihaljevi!c, D. Raz˘em / Radiation Physics and Chemistry 67 (2003) 269–274 10
8
6
3
was sublimated twice before use. All other materials were described in the earlier paper (Mihaljevic et al., 1999). Methanolic solutions of (t-BuO)2 for photolysis typically contained 2.74 M (t-BuO)2, between 3.6 103 and 3.6 102 M Fe2+, 2.3 102 M HCl and 2.3%, v/v (1.1 M) H2O. All freshly prepared reagent solutions and solutions for photolysis were deoxygenated by bubbling with pure nitrogen.
10 A(360 nm)
270
4
2
2.2. Methods
3. Results and discussion t-Butoxyl radicals were generated by photodecomposition of di-tert-butyl peroxide. Radicals produced oxidize Fe2+ ions (reaction 2), and in the presence of Cl ions monochloroiron(III) complex ([FeCl]2+) is formed (reaction 3), which absorbs at about 360 nm in acidic methanol solution. Assuming that the rate of the complexation of Fe3+ with Cl in methanolic solution is fast on the experimental time scale, as well as that chloroiron(III) complex is relatively stable on that scale, the following reactions leading to the formation of chloroiron(III) complex take place: hv
t-BuOOt Bu ! 2t-BuOd k2
t-BuOd þ Fe2þ ! t-BuOH þ Fe3þ k3
Fe3þ þ Cl !½FeCl2þ k0
t-BuOd !ðFirst order decaysÞ
ð1Þ ð2Þ ð3Þ
ð4Þ
0 0
5
10
15
20
25
6
(A)
10 t/s
12 10 8 6
3
10 A(535 nm)
Light pulses from a frequency doubled, Q-switched ruby laser by Korad (Santa Monica, CA, USA) (347.1 nm), were used for the production of alkoxyl radicals. All signals were normalized to the corresponding actinometry data. Actinometry was based on laser flash generation of a transient triplet state in 0.001 M deaerated aqueous solution of 4-carboxy benzophenone (Marciniak et al., 1995). The details of the laser flash photolysis setup and of actinometry are given in the earlier paper (Mihaljevic et al., 1999). Aqueous solution of 0.03 M NaNO2 was used for attenuating laser pulses. All measurements were made at an incident photon flow of about P0 E3 105 mol flash1. Absorption spectra of iron(III) ion complex formed after a laser flash were compared with the steady-state spectra taken by Cary 2200 spectrophotometer (Varian). All measurements were made at room temperature.
4 2 0 0
(B)
1
6
2
3
10 t/s
Fig. 1. (A) The absorbance change of chloroiron(III) complex at 360 nm on the experimental time scale ([Fe2+]=9.0 105 M, P0 ¼ 3:0 105 mol flash1 ); (B) The absorbance change of ketyl radical at 535 nm after data processing up to 3 ms ([Ph2CHOH]=0.2 M; P0 ¼ 2:1 105 mol flash1).
From the anticipated mechanism the quantum yield of iron(II) ion oxidation, F(Fe3+) follows as: FðFe3þ Þ ¼ Fðt-BuOd Þk2 ½Fe2þ =ðk0 þ k2 ½Fe2þ Þ
ð5Þ
1=FðFe3þ Þ ¼ 1=Fðt-BuOd Þð1 þ k0 =k2 ½Fe2þ Þ
ð6Þ
Quantum yield F(Fe3+) was determined by measuring absorbance at 360 nm of the formed chloroiron(III) complex about 2 ms after the flash (Fig. 1A), and by calculating the concentration of iron(III) complex using molar absorptivity determined under our experimental conditions, e(360 nm, CH3OH, 20 C)=47007230 M1 cm1. Absorption spectra of the chloroiron(III) complex obtained by laser flash photolysis were identical to the spectra of the same complex obtained by mixing components and measured by spectrophotometry (Mihaljevic et al., 1999).
B. Mihaljevi!c, D. Raz˘em / Radiation Physics and Chemistry 67 (2003) 269–274 20
271
30
17.5 25
15
20
1/Φ(Fe3+)
10
15
3
10 × A(360 nm)
12.5
7.5 10
5 5
2.5
0
0 0
1
2 5
10 × Pabs/mol flash
3 -1
Fig. 2. Absorbance of chloroiron(III) complex at 360 nm as function of the absorbed photon flow measured about 2 ms after the flash; (circles): 4.7 103 M Fe2+; (rhombs): 9.6 103 M Fe2+; full and empty symbols denote 2.7 and 1.8 M (tBuO)2, respectively.
3.1. Determination of the relative rate constant The dependence of the absorbance of chloroiron(III) complex at 360 nm on incident photon flow measured at two different concentrations of Fe2+ ion and di-tertbutyl peroxide, respectively (Fig. 2), shows that conditions are in accordance with the pseudo-first order of reaction (2). By photolysis of solutions that contained 2.7 and 1.8 M di-tert-butyl peroxide, respectively, a plot corresponding to Eq. (6) was obtained: 1/F(Fe 3+)= 0.0616/[Fe2+]+3.4131 (R2 ¼ 0:96) (Fig. 3). Quantum yield F(t-BuOd)=(0.3070.01), follows from the intercept, while relative rate constant of the reaction of t-BuOd radical with Fe2+ ion follows from the ratio of the intercept to slope, krel ¼ k2 =k0 ¼ ð55:570:14Þ M1 : By flash photolysis of solutions that contained the same components except for the increasing hydrochloric acid concentration (Table 1), a slight increase of F(Fe3+) value with increasing acid concentration was observed. On the other hand, increasing acid
0
100
200
300
400
500
(1/[Fe2+]) /dm3 mol-1
Fig. 3. Plot corresponding to Eq. (6) (1/F(Fe3+)=0.062/ [Fe2+]+3.41; R2 ¼ 0:96). Full and empty symbols represent results obtained at 2.7 M and 1.8 M (t-BuO)2, respectively. Error bars denote 715%.
concentration decreased the relative rate constant, while in strongly acid medium F(t-BuOd) increased. Dependence of the rate constant of the reaction of Fe2+ with t-BuOd on acid concentration in our work is similar to the dependence of the rate constant of the reaction of Fe2+ with dOH in aqueous solution (2.3 108 M1 s1 at pH=1, Jayson et al., 1972; 4.3 108 M1 s1 at pH=3, Christensen and Sehested, 1981). However, it is known that the rates of both b-cleavage and addition of alkoxyl radicals relative to hydrogen abstraction by alkoxyl radicals increase in strongly acid solution (Avila et al., 1993; Gilbert et al., 1981). We did not observe any large decrease of the relative rate constant on going from 0.023 to 0.062 M HCl, but b-cleavage accelerated by protonation of t-BuOd in strongly acid medium would be expected in methanolic solution containing 0.24 M HCl causing the relative rate constant to decrease. The presence of tert-butyl radical-cation, t-BuOdH+ in acid medium could increase F(Fe3+) due to its greater electrophilic character than that of
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Table 1 Quantum yield F(Fe3+) (in solutions containing [Fe2+]=1.6 102 M, [(t-BuO)2]=2.7 M, [MeOH]=12.3 M (2.3% H2O v/v)), the relative rate of reaction (2) krel and quantum yield F(t-BuOd) as function of the concentration of hydrochloric acid; P0 B3 105 mol flash1 [HCl] (M1)
e(FeCl2+) (M1 cm1)
F(Fe3+)
krel (M1)
F(t-BuOd)
k2 (M1 s1)
2.3 102 6.2 102 2.4 101
4700 4830 6360
0.1170.01 0.1470.02 0.1870.02
(55.570.14) 48.5 15.7
(0.3070.01) 0.31 0.92
3.4 108 3.0 108 9.8 107
3.2. Determination of the rate constant k2 The rate constant for oxidation of Fe2+ with t-BuOd radicals was calculated using the relative rate constant and the overall rate constant for disappearance of t-BuOd radicals, k0 : The rate constant k0 was determined under the same experimental conditions as in the measurements of the quantum yield F(Fe3+), with the exception that DPM was used instead of the Fe2+ in the solution. The reaction of t-BuOd radical with DPM (7) gives ketyl radicals with the absorbance maximum at 535 nm (Inbar et al., 1981) and the rate constant k7 is known (Small and Scaiano, 1978) k7
4
3
2
1
0
d
t-BuOd þ Ph2 CHOH ! t-BuOH þ Ph2 C OH
5
1/Φ(Ph2C· OH)
t-BuOd radical (Cookson et al., 1976). On the basis of our results, at the half-life of the oxidation of Fe2+ with t-BuOd followed by the formation of chloroiron(III) complex, t1/2o1 ms (Fig. 1A), the rate constant for the formation of complex between iron(III) and chloride in our system ([HCl]=0.023 M, 2.3% H2O, v/v) would be k3>3 107 M1 s1. As compared with F(t-BuOd)=0.3 in less acid solutions, the value of F(t-BuOd)=0.9 was obtained in strongly acid solution containing 0.24 M HCl. The increase of F(t-BuOd) with increasing acid concentration suggests a catalytic effect of acid on the decomposition of (t-BuO)2.
ð7Þ
0
2
4
6 3
8
-1
(1/[Ph2CHOH])/dm mol
k0
t-BuOd ! ðFirst order decaysÞ (4) The value of FðPh2 Cd OHÞ is given by
Fig. 4. Plot corresponding to Eq. (9) ð1=FðPh2 Cd OHÞ=0.52/ [Ph2CHOH]+0.58; R2 ¼ 0:97).
d
FðPh2 C OHÞ ¼ Fðt-BuOd Þk7 ½Ph2 CHOH=ðk0 þ k7 ½Ph2 CHOHÞ ð8Þ or written as reciprocal, we have d
1=FðPh2 C OHÞ ¼ 1=Fðt-BuOd Þð1 þ k0 =k7 ½Ph2 CHOHÞ
ð9Þ
Quantum yield FðPh2 Cd OHÞ was calculated by measuring the absorbance at 535 nm 0.5 ms after the flash (Fig. 1B) and using the known molar absorptivity of
ketyl radical determined in water e(535 nm, H2O, 20 C)=3220 M1 cm1 (Bensasson and Land, 1971). On the basis of molar absorptivities of ketyl radical in water and in cyclohexane, Bensasson and Land (1971) have concluded that the effect of solvent on molar absorptivity of ketyl radical is negligible. From the intercept of the plot according to Eq. (9) (Fig. 4), quantum yield F(t-BuOd)=1.7 follows. Using the known rate constant k7=6.9 106 M1 s1 (Small and Scaiano, 1978), the rate constant of the overall rate of disappearance of t-BuOd radicals in the system was
B. Mihaljevi!c, D. Raz˘em / Radiation Physics and Chemistry 67 (2003) 269–274
obtained from the ratio of intercept to slope, k0=6.2 106 s1. This rate constant is very close to the rate constant used earlier, k0=5.4 106 s1 (Mihaljevic et al., 1999), which was estimated using the known values of the rate constants for the reactions leading to the disappearance of t-BuOd. The largest contribution to the overall rate of the radical disappearance is b-cleavage of t-BuOd (in water, kb= 1.4 106 s1, Erben-Russ et al., 1987). Using the known value for the reaction of t-BuOd with MeOH (2.9 105 M1 s1, Paul et al., 1978) and the rate constant for the reaction of t-BuOd with (t-BuO)2 (1.6 105 M1 s1, Small and Scaiano, 1978), from the rate constant k0 obtained in our work we calculated the rate of b-cleavage of t-BuOd in methanol, kb=2.2 106 s1. On the basis of the known stability of ketyl radical in acid solutions (pKa=8.2), as well as the fact that bcleavage of t-BuOd is a preferred mode of disappearance of t-BuOd in strongly acid solution, hydrogen abstraction from diphenylmethanol by t-BuOd (7) in a moderately acid medium should not be significantly affected by acid concentration (Gilbert et al., 1981). As can be seen from Table 1, in contrast to the strongly acid solutions (0.24 M HCl), relative rate constants and quantum yields did not significantly change with increasing acid concentration in solutions for photolysis, revealing the comparable rate constants of the oxidation of Fe2+ with t-BuOd. From the relative rate constant krel of the reaction of t-BuOd radicals with Fe2+ (2) and the measured value of the rate constant k0 ; the rate constants of the reaction of t-BuOd with Fe2+ were determined (k2 ¼ kre k0 ) at 2.3 102 and 6.2 102 M HCl, as 3.4 108 and 3.0 108 M1 s1, respectively. In the formation of ferric ion complex, reactions by radicals other than t-BuOd are possible. These radicals include primarily methyl (dCH3), formed through b-cleavage of t-BuOd radical; however, it should be considered negligible on our time scale 6 1 1 (k(dCH3+Fe(CN)4 s ; Steenken and 6 )=5 10 M Neta, 1982). Contributions of solvent (dCH2OH) and peroxide (t-BuOOt-Bud)-derived radicals to ferric ion complex formation are expected to be even less probable than that of methyl on the basis of the lower reactivity as compared with the reactivity of methyl (Paul et al., 1978). However, contributions of methyl (k(dCH3+ Fe(H2O)5Cl2+)>5 106 M1 s1, Bakac, 2001), and especially hydroxymethyl radical (k(dCH2OH+Fe3+)= 8 107 M1 s1, Buxton and Green, 1978), are more likely to the slow ferric ion complex reduction, which was indeed observed as a slow decrease of the absorbance of ferric ion complex at 360 nm (Fig. 1A). Similarity of the rate constant of the oxidation of Fe2+ with t-BuOd radical obtained in our work with the rate constant of the oxidation of Fe2+ with dOH in water (Jayson et al., 1972; Christensen and Sehested, 1981) suggests a similar reaction mechanism. According
273
to the studies of the mechanism of reaction of divalent transition metal ions and complexes with aliphatic free radicals (van Eldik et al., 1994) in which complex formation between metal ion and free radicals can be treated mechanistically in a similar way as developed for conventional ligand substitution processes, the high rate constant of Fe2+ oxidation with t-BuOd radical indicates that an outer-sphere mechanism is operating.
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