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Physical and Biophysical Chemistry Division (I)


Number: 2000-013-1-100

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

Task Group
: Tibor Bérces

Members: J. E. Boggs, A. Burcat, A. Császár, J. Demaison, R. Janoschek, J.M.L. Martin, M. J. Rossi, B. Ruscic, J. Stanton, P. Szalay, P.R. Westmoreland, and F. Zabel

Completion Date: 2003
> for extension see project #2003-024-1-100

Compilation and critical evaluation of available thermodynamic properties as well as computation of accurate data for selected free radicals which are of importance in atmospheric chemistry and combustion.
(a preliminary version of the present project was 140/9/97)

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 either contradictory, they often depend on a single determination made decades ago using indirect methods, or are based on estimations. Due to the progress made in developing new direct experimental techniques, reliable thermodynamic properties become available for more and more free radicals. In spite of this, a break-through 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 which initiated a joint effort of experimentalists and theoreticians to create a set of reliable thermodynamic properties for selected organic free radicals which are of importance in atmospheric chemistry and combustion.

The lack of well established thermodynamic data for free radicals made 6 experimentalists and 7 theoreticians start a project with the aim of compilation and critical evaluation of available thermodynamic data as well as computation of accurate data for selected free radicals. The interdisciplinary character of the group may prove to be useful in finding the right answer in cases where contradictory thermodynamic properties were published for the same species in the literature.

As a first step of the activity, those organic free radicals were identified which play an important role in atmospheric chemistry and/or in combustion. Next, the thermodynamic properties were selected which should be the subject of compilation, critical data evaluation and computation. For the presentation of the compiled thermodynamic properties a Data Sheet has been elaborated. Simultaneously a number of methodologycal questions of data evaluation have been solved. In order to facilitate communication and exchange of information between task group members, a password-protected web page has been constructed.

It was decided that both very high level computations and less expensive methods should be used to obtain theoretical values for the thermodynamic properties. In addition to enthalpy of formation computations, a consistent set of structural data, vibrational frequencies, heat capacities and entropies are obtained from the use of such methods.

All together 35 key radicals are dealt with. The thermochemical properties discussed are the enthalpy of formation, as well as the heat capacity, integrated heat capacity and entropy of the radicals. One distinguishing feature of the present evaluation is the systematic utilization of available kinetic, spectroscopic and ion thermochemical data as well as high-level theoretical results.

Publication of the results of compilation and critical data evaluation is planned in J. Phys. Chem. Ref. Data, under the heading "IUPAC Critical Evaluation of Thermochemical Properties of Selected Radicals", according to the schedule given below.

Part 1: Manuscript should be ready for publication in June, 2003.
It contains the datasheets for radicals CH, CH2(singlet), CH2(triplet), CH3, CF2, CCl2, C6H5CH2, OH, CH3O, C2H5O, CH3O, CH2OH, NH2.

Part 2: Manuscripts should be ready for publication in October, 2003. It contains the datasheets for radicals C2H, C2H3, C3H3, C3H5, C2H5, i-C3H7, t-C4H9, s-C4H9, CH3O2, HCCO, CH2CHO, C2H5CO, C6H5O, HO2, C6H5, CHO/CDO.

Part 3: Manuscripts should be ready for publication in December, 2003. It contains the datasheets for radicals HOCO, C2H5O2, NO3, HS/DS, CH3S, HS2, CN.

For project extension, see #2003-024-1-100

Last Update: 21 October 2003

<project announcement published in Chem. Int. 23(5) 2001>


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