M. Waroquier

A simple algorithm is presented to derive accurately the exchange-correlation potential in the density functional theory (DFT) from the electron density. The method, which can be used with any physically acceptable density as input, is applied here to the densities in atoms obtained from high-level Green’s function calculations. The resulting potentials show the correct asymptotic behavior and the characteristic intershell peaks. We illustrate the possible use of these potentials in fitting procedures for new functionals, by investigating the HCTH functional [F. A. Hamprecht, A. J. Cohen, D. J. Tozer, and N. C. Handy, J. Chem. Phys. 109, 6264 (1998)]. The potentials derived from Green’s function one-body densities provide a microscopic foundation for present-day functionals in DFT, and may therefore be helpful in the ultimate goal of constructing functionals on a fully ab initio basis.

The thermal degradation process of some new polycarbonates is investigated from an experimental and theoretical point of view, in order to obtain insight into the microscopic aspects that influence the reaction mechanism and the process of thermolysis. In particular, attention is focussed on the influence of the type of substituents in the polymer chain on the degradation temperature. A series of novel polycarbonates were designed differing from each other by the groups attached at the alpha and beta carbon atoms. Thermal behavior was characterized by thermogravimetrical analyses. The polymers undergo rapid and complete thermolysis at different degradation temperatures depending on the structure. The degradation products were separated by gas chromatography and analyzed by mass spectroscopy. The ratio of formed dienes in the product distribution depends on the heating rate. Furthermore, density functional theory calculations were performed on a series of model compound systems for the polycarbonates under study, in particular carbonate systems differing by the groups attached at the alpha and beta carbons. The study proves that the thermal degradation route can be controlled by tailoring the polymer backbone structure. Moreover, ab initio calculations provide further insight into the microscopic ingredients that govern the degradation process and associated reaction rates.

Thermal elimination reactions in carbonates are investigated from a theoretical point of view with density functional theory methods to obtain insight into the reaction mechanism and structural factors that influence the kinetics. Carbonate systems are good model systems for the kinetics of thermal degradable polycarbonates. Special attention is given to the influence of para-substituents placed either at Cα or Cβ. The results enable prediction of ρ values of the Hammett equation and confirm earlier experimental data.

Ab initio density functional theory calculations are presented on cyclization reactions of polyaromatics involved in coke formation during the thermal cracking of hydrocarbons. During coke formation, cyclization can take place at various sites, differing from each other by the local polyaromatic structure. This local structure also determines the minimum number of carbon atoms that must be added to allow the formation of a new ring. Kinetic parameters are calculated for the various ring-closure reactions by means of transition state theory. The activation energy is largely affected by the local structure of the polycyclic aromatic hydrocarbon, whereas the frequency factor varies significantly in terms of the length of the attached alkyl chain. The calculations, as presented, give a microscopic insight into the mechanisms that contribute to barrier formation and to the value of the frequency factor. The relative importance of cyclization at different sites, under conditions typical for an industrial cracking unit, is studied on the basis of the calculated rate constants at various temperatures. The results suggest that the nature of coke formation is autocatalytic: the larger the macroradicals, the faster the subsequent reactions that lead to further growth of the polyaromatic surface. This type of calculation is the first step towards the development of structural relations for the kinetic parameters in terms of the local structure of the coke matrix.