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Reduction Potentials of One-electron Couples Involving Free Radicals in Aqueous Solution


Peter Wardman
Gray Laboratory of the Cancer Research Campaign,
Mount Vernon Hospital,
Northwood, Middlesex HA6 2RN, UK

J. Phys. Chem. Ref. Data 18: 1637-1755 (1989)


Reduction of an electron acceptor (oxidant), A, or oxidation of an electron donor (reductant), A2–, is often achieved stepwise via one-electron processes involving the couples A/A.– or A.–/A2– (or corresponding prototropic conjugates such as A/AH• or AH•/AH2). The intermediate A.– (AH•) is a free radical. The reduction potentials of such one-electron couples are of value in predicting the direction or feasibility, and in some instances the rate constants, of many free-radical reactions. Electrochemical methods have limited applicability in measuring these properties of frequently unstable species, but fast, kinetic spectrophotometry (especially pulse radiolysis) has widespread application in this area. Tables of ca. 1200 values of reduction potentials of ca. 700 one-electron couples in aqueous solution are presented. The majority of organic oxidants listed are quinones, nitroaryl and bipyridinium compounds. Reductants include phenols, aromatic amines, indoles and pyrimidines, thiols and phenothiazines. Inorganic couples largely involve compounds of oxygen, sulfur, nitrogen and the halogens. Proteins, enzymes and metals and their complexes are excluded.

List of Tables

  1. Reduction potentials of quinones
    1. Benzoquinones
    2. Naphthoquinones
    3. Anthraquinones
    4. Isoindole-4,7-diones
    5. Miscellaneous quinones
  2. Reduction potentials of nitroaryl compounds
    1. Nitrobenzenes
    2. Nitrofurans
    3. 2-Nitroimidazoles
    4. 4-Nitroimidazoles
    5. 5-Nitroimidazoles
    6. Nitroazaindoles
    7. Nitroacridines
    8. Miscellaneous nitroaryl compounds
  3. Reduction potentials of bipyridinium and related compounds
    1. Unbridged 2,2'-bipyridinium compounds
    2. Bridged 2,2'-bipyridinium compounds: derivatives of dipyrido[1,2-a:2',1'-c]pyrazinediium
    3. Bridged 2,2'-bipyridinium compounds: derivatives of 6,7-dihydrodipyrido[1,2-a:2',1'-c]pyrazinediium (`diquat')
    4. Bridged 2,2'-bipyridinium compounds: derivatives of 7,8-dihydro-6H-dipyrido[1,2-a: 2',1'-c][1,4]diazepinediium
    5. Bridged 2,2'-bipyridinium compounds: derivatives of 6,7,8,9-tetrahydrodipyrido[1,2-a: 2',1'-c][1,4]diazocinediium
    6. Miscellaneous 2,2'-bipyridinium compounds
    7. 2,4'-Bipyridinium compounds
    8. Symmetrical 1,1'-disubstituted 4,4'-bipyridinium compounds (viologens, R1 = R1') without additional ring substituents
    9. Symmetrical 1,1'-disubstituted 4,4'-bipyridinium compounds (viologens, R1 = R1') with additional ring substituents
    10. Asymmetrical 1,1'-disubstituted 4,4'-bipyridinium compounds (viologens, R1 = CH3, R1' = variable)
    11. Asymmetrical 1,1'-disubstituted 4,4'-bipyridinium compounds (viologens, R1 not R1' not CH3)
    12. Quaternary derivatives of phenanthrolines, diazapyrenes and diazapentaphenes (see also 3.6.)
  4. Reduction potentials of miscellaneous organic compounds
    1. Aldehydes and ketones
    2. Disulfides (RSSR)
    3. Amides
    4. Pyridinium and related compounds
    5. Phenothiazinium derivatives
    6. Flavins (isoalloxazines) and lumichrome derivatives (alloxazines)
    7. Dioxathiadiazaheteropentalenes
    8. Miscellaneous organic compounds
  5. Reduction potentials of phenoxyl radicals
    1. Phenols
    2. 1,2-Dihydroxybenzenes
    3. 1,3-Dihydroxybenzenes
    4. 1,4-Dihydroxybenzenes (1,4-Hydroquinones)
    5. Trihydroxybenzenes
  6. Reduction potentials of amine, indole, pyrimidine and purine radicals
    1. Aminobenzenes and phenylenediamines
    2. Indoles (IndH)
    3. Pyrimidines
    4. Purines
  7. Reduction potentials of phenothiazine radicals
    1. 10H-Phenothiazine
    2. 10H-Phenothiazines with one ring carbon substituent
    3. 10H-Phenothiazines with two ring carbon substituents
    4. N-Substituted phenothiazines without ring carbon substitution
    5. N-Substituted phenothiazines with one ring carbon substituent
    6. Benzophenothiazines
    7. Phenothiazines with oxidized sulfur
  8. Reduction potentials of radicals from miscellaneous organic compounds
    1. Hydroxy compounds
    2. Dihydronicotinamides
    3. Thiols (RSH)
    4. Pyrazolinones
    5. Peroxy radicals
  9. Reduction potentials of inorganic couples

Content of the Tables


The Tables fall into 3 distinct groups. Tables 1 to 4 present reduction potentials of organic oxidants, in the form E(A/A.–) where A is a stable ground state and A.– the radical produced on one-electron reduction. Tables 5 to 8 present reduction potentials of the radicals obtained upon one-electron oxidation of organic reductants, in the form E(A.–/A2–) where A2– represents a stable reductant and A.– the radical (disregarding prototropic state, of course). Table 9 presents reduction potentials of inorganic species, but without separation into groups where the radical is either reductant or oxidant.

The systematic names for many of the compounds are complex, and (except for inorganic couples) rather than arrange alphabetically, compounds in Tables 1 to 8 are subdivided into related groups. Within each group, compounds are generally listed in related sub-groups with increasing element count (C,H,N etc.) in substituents defining order where appropriate. With the structures, the various groupings should be reasonably clear. Multiple entries for any one couple appear in order of publication year.

The tables contain several data items organized as paragraphs in the following order, if they are present:
(1) The reference from which the data was obtained.

(2) The reduction potential of ground state or radical, as appropriate, all referring to one-electron reduction and all vs. the standard hydrogen electrode. These potentials are all mid-point potentials, Em and in many, although not all cases, may be used as estimates for standard potentials, E0. Whether a measured or calculated value for E as tabulated equates or approximates to a standard potential depends largely upon the possible or known occurrence of prototropic equilibria involving either reductant, or oxidant.
(3) Ionic strength frequently influences measured equilibrium constants or kinetics, an approximate ionic strength is given to which the experiments relate. The expression: ->0 appears if the experimental values were extrapolated to zero ionic strength.
(4) The pH of measurement (or to which the calculation refers, where appropriate). The reduction potential, ionic strength and pH are on the same line, several lines are used if there are multiple sets of data.
(5) These lines may be followed by a line indicating chosen reference values or values that are questionable or superceded.

(6) The reference compound used in the electron-transfer equilibrium, and
(7) the reference potential assumed in the calculation of E.
(8)Since many values were derived from radiation-chemical experiments in which either one-electron oxidation or reduction was selected by using scavengers the co-solute (scavenger) is given, to help describe the experiment.

(9) Comments and notes on the experimental method used. Except where electrochemical methods were used most of the values were obtained by measurement of the concentrations of radicals and ground states at equilibrium. A minority were determined from the kinetics of approach to equilibrium. Either may appear in parentheses where the data were secondary to, i.e. merely supported, the calculation of DeltaE. if only concentrations and/or kinetics appears in the comments, then the method involved monitoring fast electron-transfer equilibria following generation of radicals by pulse radiolysis, before the radical species disappear by other routes.

Alterations to Published Values

In general, only correction to s.h.e. (where appropriate) has been made to the original data. Where a value seems questionable, this is indicated, usually with an explanatory note in the Comments/method. A recommended value is also indicated. Many of the values may be immediately corrected by the reader using new recommendations or new values for reference potentials as they become available, since the Table indicates the reference couple and value assumed in the original work. Such corrections will be relatively minor and presentation of original data seemed preferable to making minor changes which will themselves by subject to revision as refinements to reference potentials are published.

Inorganic Couples: Standard States

The user is reminded that the standard state for a substance is that existing in its normal state at standard temperature and pressure, i.e. for gases such as oxygen it is 1 atmosphere partial pressure. For calculations of equilibrium constants where concentrations are appropriate, the Nernst equation should be used to calculate a reduction potential corresponding to unit concentration.

Abbreviations and Symbols.


Introduction to Reduction Potentials of One-electron Couples.

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