Supporting Information: The Two Electron, Two Proton Oxidation of ...
Supporting Information: The Two Electron, Two Proton Oxidation of Catechol: Kinetics and Catalysis Qianqi Lin, Qian Li, Christopher Batchelor-McAuley and Richard G. Compton* Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom
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Simulation for Apparent Parameters, kapp and E0app. In order to focus on the 2H+2e– process in catechol oxidation, only the chemically reversible cyclic voltammograms at low pH range 1.0 ~ 5.0 were studied quantitively. Apparent parameters are introduced to the approximate, simplified mechanism proposed in Scheme SI1, where the apparent electrochemical rate constants (kapp) were set at 10 cm s–1 corresponding to the electrochemically fully reversible limit. Apparent redox potentials (E0app) can be derived via simulation using DIGISIM (Version 3.03b, BASi, West Lafayette, IN, US), a commercial software based on a fully implicit finite difference method suggested by Rudolph.1-2 The equilibrium constant for disproportionation (Keq) is generated by DIGISIM for the two E0app values. The disproportionation rate constant (kf) of SH was set at the diffusion controlled limit (~ 1010 dm3 mol-1 s-1). Transfer coefficients (α) of 0.5 and equality of all diffusion coefficients are assumed within our model.
Scheme SI1. Two one-electron transfers of catechol (H2C) along with the oxidation products, o-benzoquinone (Q) and the semiquinone species (SH). To estimate the diffusion coefficient (D) of catechol, cyclic voltammetry was run in 5 mM catechol pH 2.9 buffered solution supported with 0.1 M KCl on a carbon microelectrode (not shown). The potential was swept from 0 to 0.9 V (vs. SCE) and reversed to 0 V at a scan rate of 20 mV· s–1. The steady-state limiting current (Iss) observed at 0.9 V was analysed by using Eqn.(1) where the value of n is 2 for catechol oxidation. The resulting diffusion coefficient is (1.1 ± 0.1) × 10-5 cm2s-1. The high concentration of catechol examined here minimises capacitative current. The use of microelectrode limiting currents circumvents the effect from the associated electron transfer kinetics or separation between two redox potentials, which may occur in measurement via the use of a glassy carbon macroelectrode.3-4 2
The value of diffusion coefficient is consistent with the previous literature reported values which widely range from 4.8 × 10-6 to 1.3 × 10-5 cm2s-1.5-8 The aim within simulations was to identify the dependence of peak potentials upon the separation between E01,app and E02,app. Utilising the mechanism above, the experimental voltammetric response at variable scan rates (16 to 400 mV· s–1) can be simulated. A set of well-fitting data was obtained with the error of less than 2% in peak potential and 9% in peak current (not shown). Figure SI1 depicts a respectable but not perfect level of agreement between the simulated voltammetries (red circle, peak potentials in particular) and the experimental data (black line) for catechol redox in pH solutions ranging from pH 1.0 to pH 5.0 at a scan rate of 144 mV· s–1. The approximation in parts derives from the replacement of the ‘scheme of squares’ by the mechanism in Scheme SI1. The simulated values of E0app are listed in Table SI1. It can be seen from Figure SI2 that E01,app is ca. 1.0 V more negative than
E02,app. Table SI1. Simulated parameters for catechol redox in different pH solutions: apparent redox potentials (E01,app and E02,app) and apparent electrochemical rate constants (k1,app and k2,app). pH
E01,app (V vs. SCE)
k1,app (cm· s–1)
E02,app (V vs. SCE)
k2,app (cm· s–1)
1.0
– 0.020
10
0.980
10
2.0
– 0.079
10
0.921
10
2.9
– 0.145
10
0.892
10
4.0
– 0.165
10
0.803
10
5.0
– 0.21
10
0.755
10
3
30
(a)
Current / µA
20 10 0 -10 -20 -0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
0.8
1.0
0.8
1.0
Potential / V vs. SCE
30
(b)
Current / µA
20 10 0 -10 -20 -0.4
-0.2
0.0
0.2
0.4
0.6
Potential / V vs. SCE
30
(c)
Current / µA
20 10 0 -10 -20 -0.4
-0.2
0.0
0.2
0.4
0.6
Potential / V vs. SCE
4
30
(d)
Current / µA
20 10 0 -10 -20 -0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential / V vs. SCE
30
(e)
Current / µA
20 10 0 -10 -20 -0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential / V vs. SCE
Figure SI1. Comparison of experimental (line) and simulated (circle) cyclic voltammograms of 0.5 mM catechol in pH buffered solutions supported with 0.1 M KCl and saturated with nitrogen. Simulations were optimised by adjusting apparent redox potentials, E01,app and E02,app. Apparent electrochemical rate constants, k1,app and k2,app were set at 10 cm· s–1. (a) pH 1.0, (b) pH 2.0, (c) pH 2.9, (d) pH 4.0 and (e) pH 5.0. Scan rate = 144 mV· s–1. The most important conclusion is that such a large potential inversion at low pH was found from similar work on p-benzoquinone9-12 and anthraquinone derivatives3, with the values of ca. 0.7 V and 0.31 V respectively. The lines of best fit of both E01,app and E02,app yield gradients of near – 50 ± 10 mV/pH. In the light of Eqn. (2), these simulated results corroborate the proposed 1H+1e– within each net step over pH ranging from 1.0 to 5.0, recognising the approximation of the ‘scheme of squares’ by Scheme SI1.
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E0app
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3
E02,app
E01,app 1
2
3
4
5
6
pH
Figure SI2. A plot of apparent redox potentials, E01,app (■) and E02,app (●), against pH. The lines of best fit yield the gradient for E01,app and E02,app being near – 50 ± 10 mV/pH.
REFERENCES (1) Rudolph, M. A Fast Implicit Finite Difference Algorithm for the Digital Simulation of Electrochemical Processes. J. Electroanal. Chem. Interfacial Electrochem. 1991, 314, 13-22. (2) Rudolph, M. Digital Simulations with the Fast Implicit Finite Difference (Fifd) Algorithm: Part Ii. An Improved Treatment of Electrochemical Mechanisms with SecondOrder Reactions. J. Electroanal. Chem. 1992, 338, 85-98. (3) Batchelor-McAuley, C.; Li, Q.; Dapin, S. M.; Compton, R. G. Voltammetric Characterization of DNA Intercalators across the Full Ph Range: Anthraquinone-2,6Disulfonate and Anthraquinone-2-Sulfonate. J. Phys. Chem. B 2010, 114, 4094-4100. (4) Batchelor-McAuley, C.; Compton, R. G. Voltammetry of Multi-Electron Electrode Processes of Organic Species. J. Electroanal. Chem. 2012, 669, 73-81. (5) Lawrence, N. S.; Davis, J.; Compton, R. G. Electrochemical Detection of Thiols in Biological Media. Talanta 2001, 53, 1089-1094. (6) de Cássia Silva Luz, R.; Damos, F. S.; de Oliveira, A. B.; Beck, J.; Kubota, L. T. Development of a Voltammetric Sensor for Catechol in Nanomolar Levels Using a Modified Electrode with Cu(Phen)2(Tcnq)2 and Pll. Sensor Actuat. B: Chem. 2006, 117, 274-281. (7) Raoof, J. B.; Ojani, R.; Nematollahi, D.; Kiani, A. Digital Simulation of the Cyclic Voltammetry Study of the Catechols Electrooxidation in the Presence of Some Nitrogen and Carbon Nucleophiles. Int. J. Eletrochem. Sc. 2009, 4, 810-819. (8) Di Fusco, M.; Favero, G.; Mazzei, F. Polyazetidine-Coated Microelectrodes: Electrochemical and Diffusion Characterization of Different Redox Substrates. J. Phys. Chem. B 2011, 115, 972-979. (9) Ilan, Y. A.; Czapski, G.; Meisel, D. The One-Electron Transfer Redox Potentials of Free Radicals. I. The Oxygen/Superoxide System. Biochimica et Biophysica Acta (BBA) Bioenergetics 1976, 430, 209-224. (10) Bailey, S. I.; Ritchie, I. M.; Hewgill, F. R. The Construction and Use of Potential-Ph Diagrams in Organic Oxidation-Reduction Reactions. Journal of the Chemical Society, Perkin Transactions 2 1983, 645-652. 6
(11) Laviron, E. Electrochemical Reactions with Protonations at Equilibrium: Part X. The Kinetics of the P-Benzoquinone/Hydroquinone Couple on a Platinum Electrode. J. Electroanal. Chem. Interfacial Electrochem. 1984, 164, 213-227. (12) Laviron, E. Electrochemical Reactions with Protonations at Equilibrium: Part Xii. The 2e−, 2h+ Homogeneous Isotopic Electron Exchange Reaction (Nine-Member Square Scheme). J. Electroanal. Chem. Interfacial Electrochem. 1984, 169, 29-46.
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1
Simulation for Apparent Parameters, kapp and E0app. In order to focus on the 2H+2e– process in catechol oxidation, only the chemically reversible cyclic voltammograms at low pH range 1.0 ~ 5.0 were studied quantitively. Apparent parameters are introduced to the approximate, simplified mechanism proposed in Scheme SI1, where the apparent electrochemical rate constants (kapp) were set at 10 cm s–1 corresponding to the electrochemically fully reversible limit. Apparent redox potentials (E0app) can be derived via simulation using DIGISIM (Version 3.03b, BASi, West Lafayette, IN, US), a commercial software based on a fully implicit finite difference method suggested by Rudolph.1-2 The equilibrium constant for disproportionation (Keq) is generated by DIGISIM for the two E0app values. The disproportionation rate constant (kf) of SH was set at the diffusion controlled limit (~ 1010 dm3 mol-1 s-1). Transfer coefficients (α) of 0.5 and equality of all diffusion coefficients are assumed within our model.
Scheme SI1. Two one-electron transfers of catechol (H2C) along with the oxidation products, o-benzoquinone (Q) and the semiquinone species (SH). To estimate the diffusion coefficient (D) of catechol, cyclic voltammetry was run in 5 mM catechol pH 2.9 buffered solution supported with 0.1 M KCl on a carbon microelectrode (not shown). The potential was swept from 0 to 0.9 V (vs. SCE) and reversed to 0 V at a scan rate of 20 mV· s–1. The steady-state limiting current (Iss) observed at 0.9 V was analysed by using Eqn.(1) where the value of n is 2 for catechol oxidation. The resulting diffusion coefficient is (1.1 ± 0.1) × 10-5 cm2s-1. The high concentration of catechol examined here minimises capacitative current. The use of microelectrode limiting currents circumvents the effect from the associated electron transfer kinetics or separation between two redox potentials, which may occur in measurement via the use of a glassy carbon macroelectrode.3-4 2
The value of diffusion coefficient is consistent with the previous literature reported values which widely range from 4.8 × 10-6 to 1.3 × 10-5 cm2s-1.5-8 The aim within simulations was to identify the dependence of peak potentials upon the separation between E01,app and E02,app. Utilising the mechanism above, the experimental voltammetric response at variable scan rates (16 to 400 mV· s–1) can be simulated. A set of well-fitting data was obtained with the error of less than 2% in peak potential and 9% in peak current (not shown). Figure SI1 depicts a respectable but not perfect level of agreement between the simulated voltammetries (red circle, peak potentials in particular) and the experimental data (black line) for catechol redox in pH solutions ranging from pH 1.0 to pH 5.0 at a scan rate of 144 mV· s–1. The approximation in parts derives from the replacement of the ‘scheme of squares’ by the mechanism in Scheme SI1. The simulated values of E0app are listed in Table SI1. It can be seen from Figure SI2 that E01,app is ca. 1.0 V more negative than
E02,app. Table SI1. Simulated parameters for catechol redox in different pH solutions: apparent redox potentials (E01,app and E02,app) and apparent electrochemical rate constants (k1,app and k2,app). pH
E01,app (V vs. SCE)
k1,app (cm· s–1)
E02,app (V vs. SCE)
k2,app (cm· s–1)
1.0
– 0.020
10
0.980
10
2.0
– 0.079
10
0.921
10
2.9
– 0.145
10
0.892
10
4.0
– 0.165
10
0.803
10
5.0
– 0.21
10
0.755
10
3
30
(a)
Current / µA
20 10 0 -10 -20 -0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
0.8
1.0
0.8
1.0
Potential / V vs. SCE
30
(b)
Current / µA
20 10 0 -10 -20 -0.4
-0.2
0.0
0.2
0.4
0.6
Potential / V vs. SCE
30
(c)
Current / µA
20 10 0 -10 -20 -0.4
-0.2
0.0
0.2
0.4
0.6
Potential / V vs. SCE
4
30
(d)
Current / µA
20 10 0 -10 -20 -0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential / V vs. SCE
30
(e)
Current / µA
20 10 0 -10 -20 -0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential / V vs. SCE
Figure SI1. Comparison of experimental (line) and simulated (circle) cyclic voltammograms of 0.5 mM catechol in pH buffered solutions supported with 0.1 M KCl and saturated with nitrogen. Simulations were optimised by adjusting apparent redox potentials, E01,app and E02,app. Apparent electrochemical rate constants, k1,app and k2,app were set at 10 cm· s–1. (a) pH 1.0, (b) pH 2.0, (c) pH 2.9, (d) pH 4.0 and (e) pH 5.0. Scan rate = 144 mV· s–1. The most important conclusion is that such a large potential inversion at low pH was found from similar work on p-benzoquinone9-12 and anthraquinone derivatives3, with the values of ca. 0.7 V and 0.31 V respectively. The lines of best fit of both E01,app and E02,app yield gradients of near – 50 ± 10 mV/pH. In the light of Eqn. (2), these simulated results corroborate the proposed 1H+1e– within each net step over pH ranging from 1.0 to 5.0, recognising the approximation of the ‘scheme of squares’ by Scheme SI1.
5
E0app
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3
E02,app
E01,app 1
2
3
4
5
6
pH
Figure SI2. A plot of apparent redox potentials, E01,app (■) and E02,app (●), against pH. The lines of best fit yield the gradient for E01,app and E02,app being near – 50 ± 10 mV/pH.
REFERENCES (1) Rudolph, M. A Fast Implicit Finite Difference Algorithm for the Digital Simulation of Electrochemical Processes. J. Electroanal. Chem. Interfacial Electrochem. 1991, 314, 13-22. (2) Rudolph, M. Digital Simulations with the Fast Implicit Finite Difference (Fifd) Algorithm: Part Ii. An Improved Treatment of Electrochemical Mechanisms with SecondOrder Reactions. J. Electroanal. Chem. 1992, 338, 85-98. (3) Batchelor-McAuley, C.; Li, Q.; Dapin, S. M.; Compton, R. G. Voltammetric Characterization of DNA Intercalators across the Full Ph Range: Anthraquinone-2,6Disulfonate and Anthraquinone-2-Sulfonate. J. Phys. Chem. B 2010, 114, 4094-4100. (4) Batchelor-McAuley, C.; Compton, R. G. Voltammetry of Multi-Electron Electrode Processes of Organic Species. J. Electroanal. Chem. 2012, 669, 73-81. (5) Lawrence, N. S.; Davis, J.; Compton, R. G. Electrochemical Detection of Thiols in Biological Media. Talanta 2001, 53, 1089-1094. (6) de Cássia Silva Luz, R.; Damos, F. S.; de Oliveira, A. B.; Beck, J.; Kubota, L. T. Development of a Voltammetric Sensor for Catechol in Nanomolar Levels Using a Modified Electrode with Cu(Phen)2(Tcnq)2 and Pll. Sensor Actuat. B: Chem. 2006, 117, 274-281. (7) Raoof, J. B.; Ojani, R.; Nematollahi, D.; Kiani, A. Digital Simulation of the Cyclic Voltammetry Study of the Catechols Electrooxidation in the Presence of Some Nitrogen and Carbon Nucleophiles. Int. J. Eletrochem. Sc. 2009, 4, 810-819. (8) Di Fusco, M.; Favero, G.; Mazzei, F. Polyazetidine-Coated Microelectrodes: Electrochemical and Diffusion Characterization of Different Redox Substrates. J. Phys. Chem. B 2011, 115, 972-979. (9) Ilan, Y. A.; Czapski, G.; Meisel, D. The One-Electron Transfer Redox Potentials of Free Radicals. I. The Oxygen/Superoxide System. Biochimica et Biophysica Acta (BBA) Bioenergetics 1976, 430, 209-224. (10) Bailey, S. I.; Ritchie, I. M.; Hewgill, F. R. The Construction and Use of Potential-Ph Diagrams in Organic Oxidation-Reduction Reactions. Journal of the Chemical Society, Perkin Transactions 2 1983, 645-652. 6
(11) Laviron, E. Electrochemical Reactions with Protonations at Equilibrium: Part X. The Kinetics of the P-Benzoquinone/Hydroquinone Couple on a Platinum Electrode. J. Electroanal. Chem. Interfacial Electrochem. 1984, 164, 213-227. (12) Laviron, E. Electrochemical Reactions with Protonations at Equilibrium: Part Xii. The 2e−, 2h+ Homogeneous Isotopic Electron Exchange Reaction (Nine-Member Square Scheme). J. Electroanal. Chem. Interfacial Electrochem. 1984, 169, 29-46.
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