Quantum Chemical Calculations on ESR, Core Excitations, and Isotope Effects in Molecular Systems

In this thesis, quantum chemical calculations are undertaken mostly in order to interpret experimental results, but also to learn about computational techniques, their performance and their limitations. In paper I, the ionization-cleavage process of alkenes is investigated and two pathways are followed, one of initial cleavage and subsequent ionization and on the opposite, the other one of initial ionization and subsequent cleavage. The calculations reveal that ionization is best described by a vertical process, which is much faster than the relaxation of the molecule to its ionized structural minimum. Further, in paper II, the core hole excited state of ammonia is investigated and found to dissociate in an ultra-fast manner nicely explained by the calculated potential energy surface showing a very low barrier for dissociation. In paper III, the static and dynamic structures of two halogenated dimethyl ether radical cations are studied in ESR experiments, and it is found that, while the chlorinated molecule remains unaffected, the fluorinated molecule undergoes a dissociation or association reaction before the measurement takes place, the resulting fragments are searched for but not identified decisively. In paper IV, the stability of Jahn-Teller distorted selectively deuteriated benzene radical cation isotopomers is investigated by ESR experiments and density functional theory calculations. The temperature dependence, between 4.2 K and 77 K, of the ESR spectra is explained. Finally, in paper V, the hydrogen inversion in aziridine and methyl and dimethyl substituted aziridines is investigated. The rate constants and kinetic isotope effects are calculated using various techniques of transition state theory and tunneling correction methods