Project Details:

Standard potentials of radicals

Project No.:2001-015-1-100
Start date:2002-01-01
End date:
Division:Physical and Biophysical 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 tableshave been prepared, including: Inorganic Standard Potentials, GibbsEnergies of Formation for Radicals, Inorganic Radical pKas,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 presentwe have prepared about 140 evaluation sheets.

Thisproject was presented at a postersession at the IUPAC Congress/GA July 2005
>viewpdf - 185KB<

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

June 2007 - The Task Group met in early June2007 for the fourth and final time. We are now at ~80% completionof the project, and we have a clear vision of the remaining 20%. Onevery important benchmark that we have achieved is a consensus on thevalues of a set of about 10 important reference radical potentials;these reference potentials are essential in order to establish finalrecommendations for the majority of the other recommended potentials.The final set of recommended potentials will consist of several hundredvalues.

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

<project announcement publishedin Chem.Int.24(3) 2002>

Chairman: