Elucidating Intricate Solvation Phenomena via <em>In Silico</em> Experiments

Abstract

In this thesis, complicated solvation phenomena that are impossible to observe via laboratory experiments are comprehensively investigated by molecular dynamics (MD) simulations. The OH radical effectively cleanses the atmosphere and is also used in wastewater cleaning processes. Regenerated keratin is a biodegradable material that has been recently used as an ingredient in bioplastics. <br /> Within the last 15 years, only representative <em>ab initio</em> MD (AIMD) simulation studies with the generalized gradient approximation (GGA) functionals can be found for the OH radical in aqueous environments. However, these aforementioned functionals include the self-interaction error (SIE) that leads to artificial delocalization of unpaired electrons. Thus, a comprehensive, representative and accurate Born-Oppenheimer (BO) MD simulation study of OH*-wn (w=water, n=1-5) clusters and OH*-w31 was performed by employing hybrid functionals and the accurate diffuse basis set DZVP-MOLOPT-SR-GTH. With the aid of this highly accurate BOMD simulation, the absence of the long-debated hemibonded configuration is shown. Furthermore, this thesis reveals that a less accurate basis set such as the 6-31G* basis set and a too low simulation temperature can also lead to the occurrence of the hemibonded structure. Since these two features were also used in recent AIMD simulation studies, this thesis paves the way for a more accurate description of this system type and thus sets a clear milestone within the whole topic. Additionally, a BOMD simulation of pure liquid water, involving 32 water molecules and simulated at the B3LYP-D3 and PBE0-TC-LRC-D3 levels of theory, showed very precise structural features that were in very good agreement with highly accurate experimental data. <br /> The former BOMD simulations of the OH radical in aqueous systems by using the B3LYP-D3/DZVP-MOLOPT-SR-GTH level of theory were further used to calculate infrared spectra via the maximally localized Wannier function (MLWF) scheme and the radical Voronoi tessellation method. First, the infrared spectrum of pure liquid water with 32 water molecules showed the characteristic bulk phase. Additionally, both methods also lead to exactly the same OH radical stretching vibration band for all clusters, which concludes the absence of delocalized electrons. Moreover, structures with a short O*-O distance, such as in the traditional hemibonded structure, and structures with a larger O*-O distance between 270 to 300 pm were found within several OH*-wn clusters simulations and also within the OH*-w31 simulations. However, a comparison between MLWF and radical Voronoi tessellation infrared spectra for the clusters at the B3LYP-D3/DZVP-MOLOPT-SR-GTH level of theory and also for the bulk phase at the PBE0-TC-LRC-D3/DZVP-MOLOPT-SR-GTH level of theory conveyed that these structures do not consist of delocalized electrons and were therefore not real hemibonded structures. These results communicate that in future not only the O*-O distance must be taken into account, but also the H*O*-O and the O*-O-H angle to detect real hemibonded structures that lead to delocalized electrons. <br /> The thesis closes with the solute molecule keratin in ionic liquids. Thus, hoof keratin was regenerated for the first time by an ionic liquid and the regenerated material was characterized via differential scanning calorimetry, Fourier Transform infrared spectroscopy and scanning electron microscopy. The MD simulation results within this thesis conveyed by RDFs that the anion plays a major role in the dissolution process and additionally, spatial distribution functions (SDFs) showed how the anion and the cation work together to dissolve the keratin. The whole thesis shall guide MD simulation studies of complicated system types in future.