Title: Selected Free Radicals and Critical Intermediates:
Thermodynamic Properties from Theory and Experiment
Chairman: Tibor Bérces
E. Boggs, A. Burcat,
A. Császár, J.
Demaison, R. Janoschek,
M. J. Rossi, B.
Ruscic, J. Stanton,
P. Szalay, P.R.
Westmoreland, and F.
Completion Date: 2003
> for extension see project
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
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
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
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,
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.