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Last update: 28 July 2008

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Project: Standard potentials of radicals

Number:	2001-015-1-100
Start:	01 January 2002

Project of: Physical and Biophysical Chemistry Division

and

Inorganic Chemistry Division

Objective

To critically evaluate the standard potentials of inorganic and organic radicals in the literature, to recommend values, and to identify reduction potentials for further experimentation.

Description

Radicals play an important role in many chemical transformations. There are presently two fields where more detailed knowledge of the thermodynamic properties of radicals would be extremely useful. The first is biomedicine. The discovery of superoxide dismutase and nitrogen monoxide as a messenger has led to an explosive growth in articles in which one-electron oxidations and reductions are explored. Organic radicals play an important role in the treatment of cancers. The other is atmospheric chemistry where modelling of reactions requires accurate reduction potentials. There are presently two compilations ^(1,2) which are now both more than ten years old and in need of updating. We plan to compile new data that has been published since 1989, set up a thermodynamic network and develop in this fashion values for standard potentials that are internally consistent. Per radical we will prepare a data sheet as found in the JANAF tables. The relation of a particular radical to neighbouring (+/- one electron) stable compounds will be shown in oxidation state diagrams. We will start with radicals for which accurate data are known in water and then extend our studies to radicals that have been studied in organic solvents, or for which only estimated data are available.

References
⁽¹⁾ Stanbury, D.M. (1989) Reduction potentials involving inorganic free radicals in aqueous solution. Adv. Inorg. Chem., 33, 69-138.
⁽²⁾ Wardman, P. (1989) Reduction potentials of one-electron couples involving free radicals in aqueous solution. J. Phys. Chem. Ref. Data, 18, 1637-1755.

Progress

Feb 2005 - So far, a set of evaluations and seven summary tables have been prepared, including: Inorganic Standard Potentials, Gibbs Energies of Formation for Radicals, Inorganic Radical pK_as, Hemicolligation Equilibrium Constants, Organic Standard Potentials, Radical Henry's Law Constants, and Inorganic Radical Equilibrium Constants. Linked to these tables are the individual evaluation sheets. At present we have prepared about 140 evaluation sheets.

This project was presented at a poster session at the IUPAC Congress/GA July 2005
>view pdf - 185KB<

One particularly challenging task is to obtain a least-squares optimization for a thermochemical network that links the properties of about 50 radicals (primarily inorganic). We have completed evaluations for about half of these radicals and found that we have a robust method for performing the least-squares optimization.

June 2007 - The Task Group met in early June 2007 for the fourth and final time. We are now at ~80% completion of the project, and we have a clear vision of the remaining 20%. One very important benchmark that we have achieved is a consensus on the values of a set of about 10 important reference radical potentials; these reference potentials are essential in order to establish final recommendations for the majority of the other recommended potentials. The final set of recommended potentials will consist of several hundred values.

The publication will consist of a few pages of text and several Tables of recommended data. The recommended data will consist of standard potentials involving radicals in aqueous solution, radical pKa's, and related equilibrium constants. This publication will be web-enhanced; the web version will be identical to the printed version, with the exception that each recommendation will have a link to the underlying references, comparisons, and discussion.