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Supporting Information Quinone 1 e and 2 e /2 H + Reduction Potentials: Identification and Analysis of Deviations from Systematic Scaling Relationships Mioy T. Huynh, Colin W. Anson, Andrew C. Cavell, Shannon S. Stahl,*, and Sharon Hammes-Schiffer*, Department of Chemistry, University of Illinois at Urbana Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States Department of Chemistry, University of Wisconsin Madison 1101 University Avenue, Madison, Wisconsin 53706, United States *corresponding authors: shs3@illinois.edu and stahl@chem.wisc.edu Table of Contents Experimental Page 1. Experimental 1 e and 2 e /2 H + Reduction Potentials... S2 2. Chart of Quinones used in Benchmarking Study... S3 3. Experimental 2 e /2 H + Reduction Potentials... S4 4. Experimental 1 e Reduction Potentials... S5 5. Cyclic Voltammograms of Quinones... S6 S13 Computational Page 6. Reference (Isodesmic) Reactions... S14 7. Calculated Reduction Potentials Using Different Functionals... S15 8. Table of Hammett Constants (σ)... S16 9. Hammett Correlation Plots for Electron-Donating Quinones... S17 10. Hammett Correlation Plots for Different Substitution Schemes... S18 11. E [Q, 2H + /H2Q], E [Q/Q 2 ], and pka vs. E [Q/Q ]... S19 12. E [Q, H + /HQ ], E [Q/Q 2 ], and pka vs. E [Q/Q ]... S20 13. Analysis of Tautomers for Select Quinones... S21 14. E [Q/Q ] vs. Effective Hammett Constant ( )... S22 15. Experimental and Calculated E [Q/Q ] in Different Solvents... S23 16. Experimental and Calculated E [Q /Q 2 ] in Water... S24 17. Experimental and Calculated pka of H2Q... S25 18. Experimental and Calculated pka of HQ... S26 19. Experimental and Calculated E [Q, 2H + /H2Q] in Water... S27 20. List of Quinones that Deviate from Linear Correlations... S28 21. All Calculated Reduction Potentials and pka Values in Water... S29 S33 22. References... S34

Table S1. Experimental 1e and 2 e /2 H + E Values for Quinones Presented in Figure 1. Complex 1 e E (V vs. Fc 0/+ ) 2 e /2 H + E (V vs. NHE) 1,4-benzoquinone (1) -0.882 0.643 2-phenyl-1,4-benzoquinone (2) -0.866 0.634 2-methyl-1,4-benzoquinone (3) -0.963 0.599 2-tert-butyl-1,4-benzoquinone (4) -0.982 0.579 2-methoxy-1,4-benzoquinone (5) -0.980 0.570 2,6-dimethyl-1,4-benzoquinone (69) -1.035 0.547 2,3-dimethyl-1,4-benzoquinone (25) -1.046 0.553 Trimethyl-1,4-benzoquinone (91) -1.131 0.487 2,6-dimethoxy-1,4-benzoquinone (71) -1.092 0.473 Tetramethyl-1,4-benzoquinone (113) -1.213 0.416 DDQ (134) 0.137 0.887 Tetrafluoro-1,4-benzoquinone (121) -0.377 0.706 2,5-dichloro-1,4-benzoquinone (56) -0.561 0.699 Tetrachloro-1,4-benzoquinone (122) -0.357 0.699 2-chloro-1,4-benzoquinone (12) -0.723 0.676 S2

Chart S1. Quinones Used in Benchmarking Study S3

Table S2. Experimental 2 e /2 H + E Values and Peak-to-peak Separation for Studied Quinones. Complex a E red a E ox E a E 1,4-benzoquinone (1) 545 741 643 104 2-phenyl-1,4-benzoquinone (2) 618 649 633.5 31 2-methyl-1,4-benzoquinone (3) 534 664 599 130 2-tert-butyl-1,4-benzoquinone (4) 538 620 579 82 2-methoxy-1,4-benzoquinone (5) 518 622 570 104 2,6-dimethyl-1,4-benzoquinone (69) 523 570 546.5 47 2,3-dimethyl-1,4-benzoquinone (25) b 469 636 553 167 Trimethyl-1,4-benzoquinone (91) 467 506 486.5 39 2,6-dimethoxy-1,4-benzoquinone (71) 457 489 473 32 Tetramethyl-1,4-benzoquinone (113) 384 447 415.5 63 DDQ (134) 847 926 886.5 79 Tetrafluoro-1,4-benzoquinone (121) 629 782 706 153 2,5-dichloro-1,4-benzoquinone (56) 623 775 699 152 Tetrachloro-1,4-benzoquinone (122) b 647 751 699 104 2-chloro-1,4-benzoquinone (12) 546 805 676 259 1,4-naphthoquinone 265 487 376 222 9,10-Anthraquinone 81 97 89 16 1,8-dichloro-9,10-anthraquinone 47 183 115 136 2,3-dichloro-1,4-naphthoquinone 407 487 447 80 3,5-di-tert-butyl-1,2-benzoquinone 419 741 580 322 4-tert-butyl-1,2-benzoquinone 595 793 694 198 Phenanthrenequinone 390 421 405.5 31 1,2-naphthoquinone 485 559 522 74 1,10-phenanthroline-5,6-dione 515 653 584 138 Tetrachloro-1,2-benzoquinone 812 854 833 42 a Values given in mv vs. NHE. b Species used as the hydroquinone form. S4

Table S3. Experimental 1 e E Values and Peak-to-peak Separation for Studied Quinones. Complex E 1 red a E 1 ox a E 1 a E 1 1,4-benzoquinone (1) -923-840 -881.5 83 2-phenyl-1,4-benzoquinone (2) -902.5-829.5-866 73 2-methyl-1,4-benzoquinone (3) -987-939 -963 48 2-tert-butyl-1,4-benzoquinone (4) -1012.5-951.5-982 61 2-methoxy-1,4-benzoquinone (5) -1011-949 -980 62 2,6-dimethyl-1,4-benzoquinone (69) -920-1007 -1034.5 87 2,3-dimethyl-1,4-benzoquinone (25) -1069-1022 -1045.5 47 Trimethyl-1,4-benzoquinone (91) -1168.5-1093.5-1131 75 2,6-dimethoxy-1,4-benzoquinone (71) -1132.5-1051.5-1092 81 Tetramethyl-1,4-benzoquinone (113) -1255-1171 -1213 84 DDQ (134) 90 184 137 94 Tetrafluoro-1,4-benzoquinone (121) -403.5-350.5-377 53 2,5-dichloro-1,4-benzoquinone (56) -585.5-535.5-560.5 50 Tetrachloro-1,4-benzoquinone (122) -379-334 -356.5 45 2-chloro-1,4-benzoquinone (12) -753.5-692.5-723 61 1,4-naphthoquinone -1130-1010 -1070 120 9,10-Anthraquinone -1335.5-1284.5-1310 51 1,8-dichloro-9,10-anthraquinone -1130-1010 -1070 120 2,3-dichloro-1,4-naphthoquinone -940-840 -890 100 3,5-di-tert-butyl-1,2-benzoquinone -956-887 -921.5 69 4-tert-butyl-1,2-benzoquinone -856-802 -829 54 Phenanthrenequinone -1066.5-1011.5-1039 55 1,2-naphthoquinone -1036-889 -962.5 147 1,10-phenanthroline-5,6-dione -910.5-847.5-879 63 Tetrachloro-1,2-benzoquinone -321-259 -290 62 a Values given in mv vs. Fc 0/+. S5

Figure S1. Cyclic Voltammograms of 1 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S2. Cyclic Voltammograms of 1 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. Figure S3. Cyclic Voltammograms of 2 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S4. Cyclic Voltammograms of 2 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. S6

Figure S5. Cyclic Voltammograms of 4 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S6. Cyclic Voltammograms of 4 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. Figure S7. Cyclic Voltammograms of 5 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S8. Cyclic Voltammograms of 5 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. S7

Figure S9. Cyclic Voltammograms of 71 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S10. Cyclic Voltammograms of 71 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. Figure S11. Cyclic Voltammograms of 3 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S12. Cyclic Voltammograms of 3 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. S8

Figure S13. Cyclic Voltammograms of 25 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S14. Cyclic Voltammograms of 25 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. Figure S15. Cyclic Voltammograms of 69 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S16. Cyclic Voltammograms of 69 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. S9

Figure S17. Cyclic Voltammograms of 91 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S18. Cyclic Voltammograms of 91 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. Figure S19. Cyclic Voltammograms of 113 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S20. Cyclic Voltammograms of 113 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. S10

Figure S21. Cyclic Voltammograms of 12 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S22. Cyclic Voltammograms of 12 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. Figure S23. Cyclic Voltammograms of 56 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S24. Cyclic Voltammograms of 56 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. S11

Figure S25. Cyclic Voltammograms of 121 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S26. Cyclic Voltammograms of 121 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. Figure S27. Cyclic Voltammograms of 122 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S28. Cyclic Voltammograms of 122 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. S12

Figure S29. Cyclic Voltammograms of 134 in 1 M p-tsoh. Scan Rate = 50 mv/s. Figure S30. Cyclic Voltammograms of 134 in CH3CN with 0.1 M TBAPF6. Scan Rate = 50 mv/s. S13

Reference (Isodesmic) Reactions In our implementation, the reference reaction always pertains to the reaction involving our reference species (1). An example of the use of a reference reaction for the calculation of a reduction potential is depicted in Scheme S1. In this case, the reaction of interest is the 1 e reduction of benzoquinone (2) from [2] 0 to [2], and the reference reaction is the oxidation of [1] to [1] 0. Scheme S1. Calculation of Reduction Potentials Using a Reference Reaction [2] 0 + e [2] FE o [1] ref [1] ref 0 [1] ref + [2] 0 0 [1] ref + e o FE ref + [2] o G r The reduction potential for the system of interest (2) can then be expressed as o o Gr o E E ref F where G o r is the free energy change associated with the reaction shown at the bottom of o Scheme S1 and E ref is the reduction potential of the reference species (1), which is known from experimental measurements. A reference reaction can also be used to calculate pka values, which is shown in Scheme S2. In this case, the reaction of interest is the deprotonation of [H2] + to form [2] 0, and the reference reaction is the protonation of [1] 0 to form [H1] +. Scheme S2. Calculation of pka Values Using a Reference Reaction [H2] + [2] 0 + H + ln(10)rtpka + H + [H1] ref ln(10)rtpka,ref 0 [1] ref 0 [1] ref + [H2] [H1] ref + [2] 0 G r o The pka for the system of interest (2) can then be expressed as o Gr p Ka ([H 2 ] ) pka,ref ln(10) RT where G o r is the free energy change associated with the reaction shown at the bottom of Scheme S2 and pk a,ref is the pka value of the reference species [H1] +, which is known from experimental measurements. Note that in practice, we only calculated the pka values for the reduced benzoquinone species using analogous reference reactions. S14

Figure S31. Plots correlating calculated and experimental 1 e reduction potentials in acetonitrile (top) and 2 e /2 H + reduction potentials in water, (bottom) using different functionals. The structures of the quinones studied are shown in Chart S1. S15

Table S4. Hammett Constants (σ) for Substituents a,b Substituent σ p σ m σ p H 0.00 0.00 0.00 C6H5 0.01 0.06 0.02 CH3 0.17 0.07 0.17 C(CH3)3 0.20 0.10 0.13 OCH3 0.27 0.12 0.26 N(CH3)2 0.83 0.16 0.12 NH2 0.66 0.16 0.15 CH2CH3 0.15 0.07 0.19 OH 0.37 0.12 0.37 OCH2CH3 0.24 0.10 0.28 F 0.06 0.34 0.03 Cl 0.23 0.37 0.19 Br 0.23 0.39 0.25 SH 0.15 0.25 n/a SiH3 0.10 0.05 n/a CHO 0.42 0.35 1.03 COOCH3 0.45 0.37 0.75 CF3 0.54 0.43 0.65 CN 0.66 0.56 1.00 COOH 0.45 0.37 0.77 SO3 0.35 0.30 0.58 NO2 0.78 0.71 1.27 COCH3 0.38 0.50 0.84 a All Hammett constants (σ) were taken from Ref 1. b The bolded Hammett constants were used for the correlation plots. The σm values were used for halogen substituents, the σp values were used for substituents capable of conjugating with the reaction center, and the σp values were used for all other substituents. S16

Figure S32. Plots of the 1 e reduction potentials and pka values as functions of the sum of the effective Hammett constants ( p) for the electron-donating quinones shown in Chart S1 for which experimental values are known. The selected quinones and corresponding values are provided in Tables S5-S9. S17

Figure S33. Plots of the 2 e /2 H + reduction potentials (darkly filled circles), the sum of the pka values (lightly filled circles), and the average of the two 1 e reduction potentials (open circles) as functions of the sum of the effective Hammett constants ( ) for different quinone substitution schemes. The quinones studied are in Chart 2 of the main text. S18

Figure S34. Plots of the E [Q/Q ] reduction potential versus (a) the 2 e /2 H + reduction potentials, (b) the average of the two 1 e reduction potentials, and (c) the sum of the two pka values for all of the quinones given in Chart 2 of the main text. S19

Figure S35. Plots of the E [Q/Q ] reduction potential versus (a) the 2 e /1 H + reduction potentials (hydride transfer), (b) the average of the two 1 e reduction potentials, and (c) the pka values for all of the quinones given in Chart 2 of the main text. S20

Figure S36. Analysis of tautomers for select quinones with NH2, OH, SH, and CO2H substituents for the neutral quinone (Q), quinone radical anion (Q ), and quinone dianion (Q 2 ) states. Relative free energies in kcal/mol are given in parentheses when more than one tautomer for a given state is given. For the CO2H substituents, the dianion species exhibit spontaneous intramolecular proton transfer for hydrogen-bonding conformations. Quinones with multiple protic substituents could display enhanced effects but are unlikely to alter the qualitative trends for the 1 e versus 2 e /2 H + reduction potentials. S21

Figure S37. Correlations between the 1 e reduction potentials, E [Q/Q ], of quinones 1 134 and their effective Hammett constants ( ). The gray data points were used to generate the linear fit, and the colored data points were found to exhibit deviations from these linear fits and are defined in the legend, with the specific substituents in each group given in the SI (Table S10). S22

Table S5. Experimental and Calculated E [Q/Q ] in Different Solvents Water Acetonitrile Complex Expt. E a Calc. E a Expt. E b Calc. E b Lit. E b,c 1,4-benzoquinone (1) 0.099 d 0.099 e 0.881 0.881 e 0.851 2-phenyl-1,4-benzoquinone (2) n/a 0.072 0.866 0.906 0.842 2-methyl-1,4-benzoquinone (3) 0.023 d 0.007 0.963 0.972 0.928 2-tert-butyl-1,4-benzoquinone (4) 0.032 f 0.041 0.982 1.017 0.958 2-methoxy-1,4-benzoquinone (5) n/a 0.039 0.980 1.018 n/a 2,6-dimethyl-1,4-benzoquinone (69) 0.080 g 0.088 1.034 1.060 1.01 2,3-dimethyl-1,4-benzoquinone (25) 0.074 d 0.090 1.0455 1.048 n/a Trimethyl-1,4-benzoquinone (91) 0.165 d 0.211 1.131 1.164 n/a 2,6-dimethoxy-1,4-benzoquinone (71) n/a 0.154 1.092 1.109 1.05 Tetramethyl-1,4-benzoquinone (113) 0.250 d,h 0.310 1.213 1.295 1.175 2,3-dichloro-5,6-dicyano-1,4- n/a 1.096 0.137 0.123 n/a benzoquinone (134) Tetrafluoro-1,4-benzoquinone (121) n/a 0.638 0.377 0.339 0.358 2,5-dichloro-1,4-benzoquinone (56) n/a 0.372 0.5605 0.597 0.535 Tetrachloro-1,4-benzoquinone (122) n/a 0.564 0.3565 0.402 0.34 2-chloro-1,4-benzoquinone (12) n/a 0.242 0.723 0.732 0.602 1,4-naphthoquinone 0.140 i 0.171 1.070 1.150 1.029 Anthraquinone 0.445 0.440 1.310 1.475 1.259 1,8-dichloroanthraquinone 0.035 0.426 1.070 1.386 n/a 2,3-dichloronaphthoquinone n/a 0.063 0.890 0.908 n/a 1,8-dinitroanthraquinone n/a 0.051 n/a 1.024 n/a 3,5-di-tert-butyl-1,2-benzoquinone 0.010 j 0.052 0.9215 1.011 n/a 4-tert-butyl-1,2-benzoquinone n/a 0.119 0.829 0.876 n/a Phenanthrenequinone 0.124 i 0.155 1.039 1.149 n/a 1,2-naphthoquinone 0.089 i 0.060 0.9625 1.040 n/a 1,10-phenanthroline-5,6-dione n/a 0.033 0.879 0.951 n/a Tetrachloro-1,2-benzoquinone n/a 0.672 0.290 0.312 n/a a Units of V vs. NHE. b Units of V vs. Fc 0/+. c Ref. 2. d Ref. 3. d This is the reference reaction and agrees by construction. e Ref. 4. f Ref. 5. g Refs. 6 and 7. h Ref. 8. i Ref. 9. S23

Table S6. Experimental and Calculated E [Q /Q 2 ] in Water Complex Expt. E a Calc. E a 1,4-benzoquinone (1) 0.023 b 0.023 c 2-tert-butyl-1,4-benzoquinone (4) 0.112 d 0.096 2-methoxy-1,4-benzoquinone (5) 0.085 b 0.049 Tetramethyl-1,4-benzoquinone (113) 0.540 b 0.271 a Units of V vs. NHE. b Ref. 10. c This is the reference reaction and agrees by construction. d Ref. 4. S24

Table S7. Experimental and Calculated pka of H2Q in Water Complex Expt. pk a Calc. pk a 1,4-benzoquinone (1) 9.85 a 9.85 b 2-methyl-1,4-benzoquinone (3) 10.00 c 10.52 2-tert-butyl-1,4-benzoquinone (4) 10.60 d 10.40 2-methoxy-1,4-benzoquinone (5) 9.91 a 9.53 2,6-dimethyl-1,4-benzoquinone (69) 10.35 a 9.98 2,3-dimethyl-1,4-benzoquinone (25) 10.43 a 10.21 Trimethyl-1,4-benzoquinone (91) 10.80 a 11.04 Tetramethyl-1,4-benzoquinone (113) 11.18 c 11.21 2,5-dichloro-1,4-benzoquinone (56) 7.90 a 5.78 2-chloro-1,4-benzoquinone (12) 8.90 a 7.28 1,4-naphthoquinone 9.35 c 7.97 3,5-di-tert-butyl-1,2-benzoquinone 10.05 e 6.28 a Ref. 11. b This is the reference reaction and agrees by construction. c Ref. 12. d Ref. 4. e Ref. 9. S25

Table S8. Experimental and Calculated pka of HQ in Water Complex Expt. pk a Calc. pk a 1,4-benzoquinone (1) 11.40 a 11.40 b 2-methyl-1,4-benzoquinone (3) 11.60 c 11.39 2-tert-butyl-1,4-benzoquinone (4) 13.00 d 12.25 2-methoxy-1,4-benzoquinone (5) 11.90 a 11.27 2,6-dimethyl-1,4-benzoquinone (69) 12.40 a 11.81 2,3-dimethyl-1,4-benzoquinone (25) 12.60 a 12.64 Trimethyl-1,4-benzoquinone (91) 12.90 a 12.18 Tetramethyl-1,4-benzoquinone (113) 12.95 c 12.89 2,5-dichloro-1,4-benzoquinone (56) 10.00 a 6.21 2-chloro-1,4-benzoquinone (12) 11.00 a 9.21 1,4-naphthoquinone 11.02 c 10.28 a Ref. 11. b This is the reference reaction and agrees by construction. c Ref. 12. d Ref. 4. S26

Table S9. Experimental and Calculated E [Q, 2H + /H2Q] in Water Complex Expt. E a Calc. E a,b Lit. E a 1,4-benzoquinone (1) 0.643 0.690 c 0.699 d 2-phenyl-1,4-benzoquinone (2) 0.6335 0.635 n/a 2-methyl-1,4-benzoquinone (3) 0.599 0.636 0.644 d 2-tert-butyl-1,4-benzoquinone (4) 0.579 0.602 n/a 2-methoxy-1,4-benzoquinone (5) 0.570 0.571 n/a 2,6-dimethyl-1,4-benzoquinone (69) 0.5475 0.553 n/a 2,3-dimethyl-1,4-benzoquinone (25) 0.553 0.564 0.588 d Trimethyl-1,4-benzoquinone (91) 0.4865 0.485 0.527 d 2,6-dimethoxy-1,4-benzoquinone (71) 0.473 0.493 0.514 d Tetramethyl-1,4-benzoquinone (113) 0.4155 0.422 0.48 e 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (134) 0.887 0.887 n/a Tetrafluoro-1,4-benzoquinone (121) 0.706 0.731 n/a 2,5-dichloro-1,4-benzoquinone (56) 0.699 0.718 0.721 d Tetrachloro-1,4-benzoquinone (122) 0.699 0.684 0.680 e 2-chloro-1,4-benzoquinone (12) 0.676 0.706 0.712 d 1,4-naphthoquinone 0.377 0.409 0.470 d Anthraquinone 0.0885 0.098 n/a 1,8-dichloroanthraquinone 0.276 0.115 n/a 2,3-dichloronaphthoquinone 0.447 0.402 n/a 3,5-di-tert-butyl-1,2-benzoquinone 0.580 0.639 n/a 4-tert-butyl-1,2-benzoquinone 0.694 0.681 n/a Phenanthrenequinone 0.4055 0.377 n/a 1,2-naphthoquinone 0.522 0.489 n/a 1,10-phenanthroline-5,6-dione 0.584 0.441 n/a Tetrachloro-1,2-benzoquinone 0.833 0.833 0.827 d a Units of V vs. NHE. b Calculated using equation (2) in main text. c Reference reaction, agrees by construction. d Ref. 13. e Ref. 14. S27

Table S10. All Quinones that Deviate from Linear Correlations a Hydrogen- Hydrogen- Sterically Bonding (H2Q) b Bonding (Q) c Halogenated Charged Bulky d 16 20 11 21 29 17 42 12 43 73 20 64 13 65 92 21 86 33 87 95 23 108 34 109 103 38 130 35 131 114 39 55 117 42 56 119 43 57 120 45 77 125 60 78 128 61 79 131 64 99 65 100 67 101 82 121 82 122 86 123 87 89 104 105 108 109 111 126 127 130 131 133 a The reduction potentials and pka values are given in Table S11. b The substituent acts as a hydrogen bond acceptor to the hydroquinone (H2Q) OH. c The substituent acts as a hydrogen bond donor to the neutral quinone (Q) C=O. d The substituents were chosen by examining the hydroquinone structures for either a nonplanar six-membered quinone core or protons which were oriented out of the plane of the planar quinone core. S28

Table S11. All Calculated Reduction Potentials and pka Values in Water a R2 R3 R5 R6 E b E b E b pka pka pka E a E b,c [Q/Q ] [Q /Q 2 ] [HQ/HQ ] [HQ ] [H2Q] [HQ] [Q, H + /HQ ] [Q, 2H + /H2Q] 1 H H H H 0.000 0.099 d,e 0.023 d,e 0.450 e,g 11.40 e,h 9.85 e,h 4.10 e,i 0.398 e 0.690 e 2 C6H5 H H H -0.010 0.072 0.052 0.415 10.88 8.50 4.66 0.384 0.635 3 CH3 H H H -0.170 0.007-0.030 0.349 11.39 10.52 4.90 0.325 0.636 4 C(CH3)3 H H H -0.200-0.041-0.096 0.320 12.25 10.40 5.13 0.294 0.602 5 OCH3 H H H -0.260-0.039-0.049 0.309 11.27 9.53 5.14 0.289 0.571 6 N(CH3)2 H H H -0.830-0.221-0.144 0.124 12.32 9.60 7.72 0.182 0.466 7 NH2 H H H -0.660-0.193-0.117 0.042 11.16 9.50 8.39 0.175 0.456 8 CH2CH3 H H H -0.150-0.025-0.068 0.321 11.71 10.32 5.05 0.300 0.605 9 OH H H H -0.370 0.013-0.025 0.333 11.03 9.64 4.89 0.320 0.605 10 OCH2CH3 H H H -0.280-0.070-0.069 0.300 11.51 9.13 5.18 0.271 0.541 11 F H H H 0.340 0.231 0.153 0.559 9.32 7.43 2.37 0.467 0.687 12 Cl H H H 0.370 0.242 0.195 0.595 9.21 7.28 2.35 0.491 0.706 13 Br H H H 0.390 0.243 0.191 0.618 9.80 5.58 2.51 0.507 0.672 14 SH H H H 0.150 0.110 0.086 0.436 10.31 8.85 4.31 0.403 0.665 15 SiH3 H H H 0.100 0.156 0.070 0.493 10.50 7.89 3.27 0.423 0.657 16 CHO H H H 1.030 0.393 0.362 0.635 9.23 8.60 4.54 0.650 0.905 17 COOCH3 H H H 0.750 0.339 0.260 0.594 11.32 7.81 5.60 0.635 0.866 18 CF3 H H H 0.540 0.365 0.263 0.716 9.13 5.03 1.39 0.584 0.733 19 CN H H H 1.000 0.479 0.401 0.853 8.32 3.11 0.60 0.686 0.778 20 COOH H H H 0.770 0.597-0.068 0.621 10.16 8.02 5.15 0.799 0.881 21 SO3 H H H 0.580 0.184 0.160 0.504 11.17 9.25 5.27 0.502 0.776 22 NO2 H H H 1.270 0.613 0.688 1.007 6.15 3.56 0.69 0.833 0.938 23 COCH3 H H H 0.840 0.276 0.299 0.573 11.92 8.08 7.2 0.640 0.879 24 C6H5 C6H5 H H -0.020 0.012 0.008 0.381 11.12 9.06 4.73 0.339 0.607 25 CH3 CH3 H H -0.340-0.090-0.133 0.297 12.64 10.21 5.28 0.262 0.564 26 C(CH3)3 C(CH3)3 H H -0.400-0.385-0.249 0.099 12.33 10.40 6.36 0.047 0.355 27 OCH3 OCH3 H H -0.520-0.048 0.065 0.404 10.97 7.77 5.17 0.333 0.563 28 N(CH3)2 N(CH3)2 H H -1.660-0.301-0.117 0.236 11.11 9.43 5.07 0.119 0.398 29 NH2 NH2 H H -1.320-0.172-0.144 0.101 10.46 7.85 6.24 0.152 0.384 30 CH2CH3 CH2CH3 H H -0.300-0.113-0.118 0.257 11.94 10.51 5.53 0.238 0.549 S29

31 OH OH H H -0.740 0.041 0.028 0.370 10.28 6.38 4.43 0.339 0.527 32 OCH2CH3 OCH2CH3 H H -0.560-0.086 0.137 0.373 10.61 8.14 6.55 0.340 0.581 33 F F H H 0.680 0.374 0.282 0.706 6.70 4.90-0.55 0.526 0.671 34 Cl Cl H H 0.740 0.342 0.320 0.726 6.52 4.70-0.44 0.524 0.663 35 Br Br H H 0.780 0.330 0.315 0.699 7.20 4.90 0.63 0.536 0.681 36 SH SH H H 0.300 0.112 0.851 0.271-4.47 7.50 5.24 0.349 0.571 37 SiH3 SiH3 H H 0.200 0.191 0.237 0.589 7.98 6.60 1.96 0.450 0.645 38 CHO CHO H H 2.060 0.658 0.835 1.064 6.60 1.08 2.65 0.942 0.974 39 COOCH3 COOCH3 H H 1.500 0.445 0.417 0.732 9.33 5.39 3.92 0.707 0.866 40 CF3 CF3 H H 0.540 0.365 0.263 0.716 9.13 5.03 1.39 0.584 0.733 41 CN CN H H 2.000 0.886 0.856 1.210 1.47-0.09-4.60 0.914 0.912 42 COOH COOH H H 1.540 0.770 0.125 0.819 7.75 4.71 3.30 0.766 0.817 43 SO3 SO3 H H 1.160 0.184 0.265 0.535 12.68 6.70 8.04 0.600 0.798 44 NO2 NO2 H H 2.540 0.983 1.378 1.460-2.21-3.63-3.68 1.115 1.007 45 COCH3 COCH3 H H 1.680 0.421 0.433 0.833 8.86 3.34 2.02 0.689 0.788 46 C6H5 H C6H5 H -0.020 0.041 0.104 0.404 9.42 9.55 4.27 0.351 0.634 47 CH3 H CH3 H -0.340-0.092-0.081 0.348 12.55 9.77 5.21 0.285 0.574 48 C(CH3)3 H C(CH3)3 H -0.400-0.193-0.193 0.201 12.78 11.35 6.04 0.185 0.520 49 OCH3 H OCH3 H -0.520-0.146-0.233 0.120 10.91 11.01 4.86 0.133 0.459 50 N(CH3)2 H N(CH3)2 H -1.660-0.602-0.284-0.043 12.53 12.18 8.39-0.072 0.288 51 NH2 H NH2 H -1.320-0.614-0.360-0.233 10.43 9.96 8.21-0.178 0.116 52 CH2CH3 H CH2CH3 H -0.300-0.172-0.168 0.214 12.11 11.02 5.56 0.188 0.514 53 OH H OH H -0.740-0.142-0.108 0.237 10.85 9.76 4.94 0.196 0.485 54 OCH2CH3 H OCH2CH3 H -0.560-0.285-0.190 0.099 11.07 9.98 6.11 0.090 0.385 55 F H F H 0.680 0.344 0.270 0.691 6.82 5.35-0.36 0.509 0.667 56 Cl H Cl H 0.740 0.372 0.356 0.751 6.21 5.78-0.56 0.547 0.718 57 Br H Br H 0.780 0.377 0.352 0.744 6.91 5.43 0.21 0.569 0.730 58 SH H SH H 0.300 0.100 0.136 0.486 8.46 8.35 2.46 0.368 0.615 59 SiH3 H SiH3 H 0.200 0.194 0.151 0.545 9.20 7.78 2.46 0.445 0.675 60 CHO H CHO H 2.060 0.628 0.569 0.953 8.80 7.61 2.23 0.858 1.083 61 COOCH3 H COOCH3 H 1.500 0.490 0.398 0.841 11.54 9.20 3.99 0.786 1.058 S30

62 CF3 H CF3 H 1.080 0.614 0.487 0.959 5.47 3.05-2.59 0.712 0.803 63 CN H CN H 2.000 0.814 0.720 1.149 2.88 0.80-4.47 0.852 0.876 64 COOH H COOH H 1.540 0.997-0.252 0.901 10.43 8.22 2.96 0.812 0.924 65 SO3 H SO3 H 1.160 0.307 0.270 0.637 10.50 9.82 4.21 0.599 0.889 66 NO2 H NO2 H 2.540 0.981 0.975 1.362 3.51 1.57-3.12 1.081 1.128 67 COCH3 H COCH3 H 1.680 0.463 0.363 0.718 11.04 11.39 4.96 0.739 1.076 68 C6H5 H H C6H5-0.020 0.019 0.070 0.364 10.15 8.59 5.09 0.345 0.599 69 CH3 H H CH3-0.340-0.088-0.095 0.241 11.81 9.98 6.05 0.258 0.553 70 C(CH3)3 H H C(CH3)3-0.400-0.192-0.274 0.124 13.18 10.49 6.37 0.157 0.467 71 OCH3 H H OCH3-0.520-0.154-0.123 0.148 11.96 9.37 7.30 0.215 0.493 72 N(CH3)2 H H N(CH3)2-1.660-0.468-0.255-0.017 13.48 10.16 9.38 0.037 0.338 73 NH2 H H NH2-1.320-0.345-0.265-0.143 10.99 8.97 8.86 0.020 0.285 74 CH2CH3 H H CH2CH3-0.300-0.142-0.143 0.199 11.73 10.19 5.86 0.204 0.506 75 OH H H OH -0.740-0.034-0.060 0.263 10.68 8.43 5.14 0.269 0.518 76 OCH2CH3 H H OCH2CH3-0.560-0.173-0.167 0.164 11.66 8.91 5.98 0.175 0.438 77 F H H F 0.680 0.382 0.286 0.679 7.34 4.21 0.61 0.551 0.675 78 Cl H H Cl 0.740 0.389 0.350 0.745 7.24 3.34 0.49 0.584 0.683 79 Br H H Br 0.780 0.387 0.358 0.776 8.25 3.97 1.10 0.616 0.734 80 SH H H SH 0.300 0.135 0.149 0.439 8.77 4.94 3.78 0.402 0.548 81 SiH3 H H SiH3 0.200 0.203 0.148 0.569 10.10 4.77 2.90 0.474 0.615 82 CHO H H CHO 2.060 0.634 0.673 0.990 7.67 4.76 2.23 0.880 1.021 83 COOCH3 H H COOCH3 1.500 0.518 0.437 0.775 8.86 6.76 3.07 0.740 0.939 84 CF3 H H CF3 1.080 0.620 0.496 1.025 7.67 0.40-1.34 0.785 0.797 85 CN H H CN 2.000 0.815 0.734 1.285 6.61-3.25-2.79 0.970 0.874 86 COOH H H COOH 1.540 0.988-0.106 0.809 7.75 6.01 3.04 0.788 0.847 87 SO3 H H SO3 1.160 0.302 0.269 0.614 9.73 8.00 3.82 0.574 0.810 88 NO2 H H NO2 2.540 0.944 1.081 1.488 3.03-1.87-3.93 1.102 1.047 89 COCH3 H H COCH3 1.680 0.375 0.513 0.740 9.33 6.95 5.42 0.720 0.926 90 C6H5 C6H5 C6H5 H -0.030-0.024 0.014 0.334 11.12 8.93 5.64 0.324 0.588 91 CH3 CH3 CH3 H -0.510-0.211-0.192 0.162 12.18 11.04 6.10 0.158 0.485 92 C(CH3)3 C(CH3)3 C(CH3)3 H -0.600-0.560-0.468-0.088 14.69 10.41 8.18-0.079 0.229 S31

93 OCH3 OCH3 OCH3 H -0.780-0.213-0.010 0.233 10.97 8.19 6.78 0.213 0.455 94 N(CH3)2 N(CH3)2 N(CH3)2 H -2.490-0.699-0.262-0.136 15.34 13.41 13.13-0.027 0.370 95 NH2 NH2 NH2 H -1.980-0.556-0.361-0.163 11.14 8.41 7.71-0.129 0.120 96 CH2CH3 CH2CH3 CH2CH3 H -0.450-0.223-0.205 0.125 12.45 11.38 6.79 0.154 0.491 97 OH OH OH H -1.110-0.079-0.030 0.246 9.78 7.06 5.05 0.235 0.444 98 OCH2CH3 OCH2CH3 OCH2CH3 H -0.840-0.290 0.048 0.236 11.02 8.80 7.75 0.205 0.465 99 F F F H 1.110 0.499 0.405 0.824 5.21 2.88-1.95 0.606 0.691 100 Cl Cl Cl H 1.170 0.472 0.472 0.877 5.20 2.46-1.73 0.626 0.698 101 Br Br Br H 0.450 0.462 0.477 0.848 5.86 2.61-0.50 0.643 0.720 102 SH SH SH H 0.450 0.117 0.217 0.511 8.12 2.82 3.07 0.407 0.491 103 SiH3 SiH3 SiH3 H 0.300 0.233 0.272 0.611 7.51 4.61 1.69 0.475 0.611 104 CHO CHO CHO H 3.090 0.796 0.978 1.257 6.25 3.20 1.46 1.072 1.167 105 COOCH3 COOCH3 COOCH3 H 2.250 0.586 0.559 0.938 9.35 6.91 2.85 0.849 1.053 106 CF3 CF3 CF3 H 1.620 0.845 0.748 1.292 4.12-1.47-5.16 0.918 0.875 107 CN CN CN H 3.000 1.178 1.122 1.553-0.55-5.62-7.93 1.134 0.968 108 COOH COOH COOH H 2.310 1.149-0.065 1.060 8.42 5.92 2.01 0.929 0.966 109 SO3 SO3 SO3 H 1.740 0.256 0.353 0.646 12.19 8.01 7.16 0.665 0.902 110 NO2 NO2 NO2 H 3.810 1.261 1.510 1.701-3.92-4.13-7.22 1.269 1.147 111 COCH3 COCH3 COCH3 H 2.520 0.557 0.518 0.935 11.08 1.13 3.96 0.865 0.898 112 C6H5 C6H5 C6H5 C6H5-0.040-0.084 0.009 0.367 10.74 9.48 4.61 0.281 0.561 113 CH3 CH3 CH3 CH3-0.040-0.084 0.009 0.367 10.74 9.48 4.61 0.281 0.561 114 C(CH3)3 C(CH3)3 C(CH3)3 C(CH3)3-0.800-1.107-0.804-0.388 15.08 12.05 7.96-0.509-0.153 115 OCH3 OCH3 OCH3 OCH3-1.040-0.229 0.111 0.370 9.42 8.27 4.96 0.220 0.465 116 N(CH3)2 N(CH3)2 N(CH3)2 N(CH3)2-3.320-0.629-0.322-0.253 11.40 11.54 10.15-0.138 0.203 117 NH2 NH2 NH2 NH2-2.640-0.571-0.456-0.197 10.88 7.45 6.41-0.192 0.028 118 CH2CH3 CH2CH3 CH2CH3 CH2CH3-0.600-0.372-0.347 0.066 13.23 11.89 6.16 0.032 0.384 119 OH OH OH OH -1.480-0.077-0.039 0.295 8.14 6.62 2.41 0.183 0.379 120 OCH2CH3 OCH2CH3 OCH2CH3 OCH2CH3-1.120-0.305 0.238 0.388 10.93 8.01 8.31 0.290 0.527 121 F F F F 1.360 0.638 0.531 0.986 2.90 2.07-4.87 0.670 0.731 122 Cl Cl Cl Cl 1.480 0.564 0.588 1.003 2.93 0.73-4.18 0.663 0.684 123 Br Br Br Br 1.560 0.539 0.581 0.960 3.37 2.03-3.12 0.660 0.720 S32

124 SH SH SH SH 0.600 0.111 0.279 0.526 4.96 3.76 0.69 0.342 0.453 125 SiH3 SiH3 SiH3 SiH3 0.400 0.247 0.322 0.675 5.89 3.35-0.16 0.459 0.558 126 CHO CHO CHO CHO 4.120 0.873 1.005 1.319 5.40 4.14 0.01 1.099 1.221 127 COOCH3 COOCH3 COOCH3 COOCH3 3.000 0.744 0.680 1.064 6.66 4.84 0.09 0.909 1.052 128 CF3 CF3 CF3 CF3 2.160 0.972 0.902 1.397 0.00-3.53-8.45 0.937 0.833 129 CN CN CN CN 4.000 1.48 1.430 1.832-6.21-8.33-13.09 1.271 1.025 130 COOH COOH COOH COOH 3.080 1.278 0.068 1.143 5.82 4.56 0.03 0.970 0.980 131 SO3 SO3 SO3 SO3 2.320 0.084 0.348 0.613 11.15 10.14 6.60 0.546 0.846 132 NO2 NO2 NO2 NO2 5.080 1.613 1.662 1.939-6.64-7.09-11.40 1.441 1.231 133 COCH3 COCH3 COCH3 COCH3 3.360 0.663 0.657 0.914 3.66 3.27-0.76 0.768 0.865 134 CN CN Cl Cl 2.740 1.096 1.079 1.461-2.05-4.82-8.60 1.027 0.884 a This data table is also provided as an Excel spreadsheet. b Units of V vs. NHE. c Calculated using equation (2) in Main Text. d Ref. 3. e This is the reference reaction and agrees by construction. f Ref. 10. g Ref. 15. h Ref. 11. i Ref. 16. S33

References (1) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165. (2) Frontana, C.; Vázquez-Mayagoitia, Á.; Garza, J.; Vargas, R.; González, I. J. Phys. Chem. A 2006, 110, 9411. (3) Ilan, Y. A.; Czapski, G.; Meisel, D. Biochim. Biophys. Acta 1976, 430, 209. (4) Dohrmann, J. K.; Bergmann, B. J. Phys. Chem. 1995, 99, 1218. (5) Meisel, D.; Fessenden, R. W. J. Am. Chem. Soc. 1976, 98, 7505. (6) Wood, P. M. FEBS Lett. 1974, 44, 22. (7) Wardman, P. Free Radic. Res. Commun. 1991, 14, 57. (8) Butler, J.; Hoey, B. M. Free Radic. Biol. Med. 1986, 2, 77. (9) Jovanovic, S. V.; Kónya, K.; Scaiano, J. C. Can. J. Chem. 1995, 73, 1803. (10) Steenken, S.; Neta, P. J. Phys. Chem. 1982, 86, 3661. (11) Bishop, C. A.; Tong, L. K. J. J. Am. Chem. Soc. 1965, 87, 501. (12) Baxendale, J. H.; Hardy, H. R. Trans. Faraday Soc. 1953, 49, 1140. (13) Evans, D. H. Chapter XII-1. Carbonyl Compounds in Encyclopedia of Electrochemistry; Eds.: Bard, A. J.; Marcel Dekker, Inc.: New York, 1978; pp. 198-204. (14) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961. (15) Lind, J.; Shen, X.; Eriksen, T. E.; Merenyi, G. J. Am. Chem. Soc. 1990, 112, 479. (16) Adams, G. E.; Michael, B. D. Trans. Faraday Soc. 1967, 63, 1171. S34