Non-adiabatic ab initio molecular dynamics and the electronically excited states of small hydrocarbon radicals

Abstract

Small hydrocarbon radicals play a key role in combustion chemistry, in hydrocarbon crackers and in certain regions of the atmosphere as well as in the interstellar space. The emergence of such radicals requires high energy environments where the energy can be transfused by heat or by light. Thermally induced processes thereby occur almost exclusively in the electronic ground state of a molecule whereas photoexcited molecules can react in different electronic states resulting in much more complex reaction mechanisms. The subject of this thesis is the investigation of the photophysical processes, the reaction mechanisms and the kinetics of the reactions that occur upon excitation of small hydrocarbon radicals to their first electronically excited state. Furthermore, the characterization and the localization of several electronically excited states of small hydrocarbon radicals was conducted. All the work described in this thesis is based on computational simulations that were carried out in parallel to experimental investigations in our group. In order to calculate the molecular dynamics of photoexcited molecules ab initio, a computer program was written which calculates the movements of the atoms during a photoreaction. Thereby it also determines in which electronic state the molecule is at any time. This program interacts with available quantum chemical software packages in order to obtain the energies, energy gradients and nonadiabatic couplings between the electronic states of the molecule under investigation. Before we inspected the nonadiabatic molecular dynamics of the ethyl radical, C2H5, we simulated the dissociation dynamics of a microcanonical ensemble of ethyl radicals in the electronic ground state with UHF and DFT potentials. The chosen ex-