Number: 2001-015-1-100
Title: Standard potentials of radicals
Task Group
Chairmen: David
M. Stanbury
Members:
D. Armstrong,
R. E. Huie, W.H.
Koppenol, Sergei V.
Lymar, G. Merényi,
Pedi Neta, S.
Steenken, and P.
Wardman
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 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 present
we have prepared about 140 evaluation sheets.
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.
Last update: 20 June 2007
<project announcement published
in Chem.
Int.
24(3) 2002>
Page last modified 20 June 2007.
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