Thermodynamic properties such as enthalpies of formation, dissociation energies, and transition energies can be a challenge to determine for both experimental and theoretical chemistry. For main group elements, experimental thermodynamic data are widely available with low uncertainty with reproducibility with different experiments. However, the availability of such data is limited for the lower part of the periodic table. For heavy transition metals, lanthanide and actinide, it can be challenging to determine experimentally thermodynamic data due to the many low-lying states close to the ground state, relativistic effects, and stability of the elements. In many cases, the experimental properties are often extrapolated from other compounds and direct measurement does not exist or may lead to very large experimental uncertainties and/or inconsistencies. To aid in the investigation of such complex systems, computational chemistry can be utilized. Many ab initio and density functional theory methods among others, have been utilized to investigate heavy element complexes.In this thesis, ab initio based methods called composite approaches are introduced and employed to investigate the enthalpy of formation of 5d elements as well as the dissociation energies of lanthanide sulfide, selenide and halide species. Moreover, different density functional methods, the most widely used computational chemistry approach, were applied to investigate the 5d thermodynamic data. However, as mentioned earlier, some of the heavy elements can show low-lying excited states and as such, single-reference wavefunction methods can struggle to accurately describe the correct behavior of the molecule. In these cases, multireference methods such as complete active space self-consistent field or multireference configuration interaction were utilized. Such methods have been applied to investigate the ground and excited state of the LuF molecule and used to calculate the spin-orbit coupling and determine the multireference character of a set of lanthanide diatomics.Ab initio composite approaches, as well as density functional theory have also been utilized to study the enthalpy of formation of per- and polyfluoroalkyl substances (PFAS). These substances are often called "zombie chemicals" since they do not degrade in the environment due to their strong C-F bonds. They have been found in many different environments from soil samples, water, Antarctic ice and many other places around the globe. In this thesis, the PFAS gas phase enthalpies of formation are investigated using the correlation consistent Composite Approach ccCA approach as well as density functional theory and coupled-cluster methods.Finally, the interaction between light and matter can also be investigated using theoretical methods. In this case, the time-dependent Schrodinger equation needs to be solved. While many different methods exist, in this thesis, the time-dependent configuration interaction method is used to study the effect of a photon to the electron dynamics of small molecules, in particular for the ionization effects.
Read
