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Spin-Polarized Conceptual Density Functional Theory

dinsdag, 11 september, 2012 - 09:30
Campus: Brussels Humanities, Sciences & Engineering campus
Faculteit: Science and Bio-engineering Sciences
Freija De Vleeschouwer

Free radical chemistry constitutes a substantial part of important large-scale industrial
chemical processes. Some examples are radical chain polymerization (resulting in a
whole variety of industrial plastics like for instance polyethylene, polypropylene,
poly(vinyl chloride) and polystyrene), combustion of fuels, pyrolysis and steam
cracking. Radicals are also essential in atmospheric chemistry through the oxidation
of other molecules in the air and in biochemistry through cell damage. The main
obstacle in understanding radical reactions is the high reactivity and short lifetime of
radicals. Quantum chemistry and more specifically Conceptual DFT allows us to
address these issues theoretically and computationally. In this work various chemical
concepts, essential to explain the reactivity of radical systems, are being studied and
in some cases even quantified, resulting in easily accessible scales. The first
investigated concepts are electrophilicity and nucleophilicity. It is stated that radicals
can be regarded as electrophilic or nucleophilic and that the nature of radicals is
certainly of relevance in a variety of radical reactions, as is illustrated in this work.
The definition of the electrophilicity index, as introduced by Parr, was used to
construct an electrophilicity scale of 47 radicals, important in organic chemistry. This
scale is independent of the reaction partner and free from experimental or theoretical
reaction data. Next, the somewhat vague concept of stability was studied. The
intrinsic stability of a radical is intuitively defined as the reactivity of that radical with
respect to a range of different chemical environments, so irrespective of the reaction
partner, the presence of a solvent, and so on. A model is presented that breaks down
Bond Dissociation Enthalpies into parts that only incorporate properties of the
individual radical fragments, that is radical electrophilicity, radical stability and
Pauling electronegativity of the radical center. The resulting scale reproduces several
acknowledged stability sequences from literature, such as the effect of increasing
alkylation and delocalization. In addition, the BDE model is suitable to compute
intrinsic stabilities for other (uncharged) intermediates like biradical and divalent
systems. A third and last studied concept in this work is kinetic (ir)reversibility in
intramolecular radical reactions. This concept is very important when considering side
reaction in radical polymerization. Through the analysis of the spin-polarized Fukui
function and dual descriptor along the reaction path of 4 intramolecular side reactions
in the polymerization of PVC and within the framework of the reaction force concept,
the irreversibi-lity of the 1,2-Cl shift, the 1,5- and the 1,6-backbiting reactions as well
as the reversibility of the 2,3-Cl shift could be explained. Finally, the last Chapter
focuses on intramolecular cyclization reactions, for which the regioselectivity is
rationalized through a special form of the spin-polarized dual descriptor via the
positive orbital overlap between the radical center and one of the carbon atoms of the
radical's double bond, agreeing with the experimental findings.

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