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Chemistry International
Vol. 23, No.5
September 2001


Selected Free Radicals and Critical Intermediates: Thermodynamic Properties from Theory and Experiment

Accurate thermodynamic data are required in modeling of atmospheric processes, combustion, and other complex chemical systems. However, for most of the free radicals participating in such processes, no reliable thermodynamic properties are available; literature data are contradictory, often depend on a single determination made decades ago using indirect methods, or are based on estimations. Owing to the progress made in developing new direct experimental techniques, reliable thermodynamic properties have become available for more and more free radicals. In spite of this success, a breakthrough and an essential improvement in this field may not be expected in the near future. Fortunately, recent studies using ab initio theoretical methods have shown that an accuracy better than 4 kJ/mol is a realistic aim in the computation of enthalpies of formation.

This analysis of the state of affairs was the motivation that initiated a joint effort of experimentalists and theoreticians to create a set of reliable thermodynamic properties for selected organic free radicals. IUPAC has approved a 2.5-year project for the compilation and critical evaluation of available thermodynamic properties as well as for the computation of accurate data for selected free radicals that are of importance in atmospheric chemistry and/or combustion. Altogether, six experimentalists and seven theoreticians are participating in the project. The interdisciplinary character of this group may prove to be useful in finding the right answer in cases where contradictory thermodynamic properties were published for the same free radical from different laboratories or were obtained by different techniques.

A series of seminal papers, to be published in leading journals of physical chemistry and chemical physics, is expected from this project. The final results of the project will appear in peer-reviewed journals such as the Journal of Physical and Chemical Reference Data or Pure and Applied Chemistry. A web version of the results may be presented in the NIST Chemistry Web Book.

It is expected that the recommended thermodynamic data, the results of compilation and critical data evaluation of organic free radicals, would be widely used by the kineticist community. The major users are probably the modelers dealing with atmospheric chemistry and/or combustion. The anticipated impact is an improvement in the reliability of the predictions of modeling studies. Comments from the chemistry community are welcome and should be addressed to the task group chairman, Prof. Tibor Bérces, Institute for Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri u. 59/67, H1025 Budapest, Hungary; Tel: 36 1 325 7752; Fax 36 1 325 7554; E-mail: See projects/2000/2000-013-1-100.html for project description and update.



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