The following thesis focuses on two areas of chemistry, pi-pi interactions and radical cation dimers. Approximations to the exact solution to the Schrodinger equation are investigated for these types of chemical systems with a variety of theoretical methods. The first chapter provides an introduction to the various quatum mechanical methods used in this research. The second chapter focuses specifically on pi-pi interaction. In this chapter, high quality quantum mechanical methods are used to examine how substituents tune pi-pi interactions between monosubstituted benzene dimers in parallel-displaced geometries. In addition, the role of dispersion and coulombic interactions in these systems is investigated to determine the nature of the substituent effect. In the third chapter radical cation dimers are investigated. Benchmark results with full configuration interaction (FCI) and equation-of-motion coupled-cluster for ionized systems (EOM-IP-CCSD) are presented for prototypical charge transfer species. Conclusions regarding chapters 2 and 3 are presented in the final chapter. This work may form the basis for improved approaches to rational drug design, organic optical materials, and molecular electronics.

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Noncovalent interactions are of pivotal importance in many areas of chemistry, biology, and materials science, and the intermolecular interactions involving aromatic rings in particular, are fundamental to molecular organization and recognition processes. The work detailed in this thesis involves the application of state-of-the-art ab initio electronic structure theory methods to elucidate the nature of pi-pi interactions. The binding energies, and geometrical and orientational preferences of the simplest prototype of aromatic pi-pi interactions, the benzene dimer, are explored. We obtain the first converged values of the binding energies using highly accurate methods and large basis sets. Results from this study predict the T-shaped and parallel-displaced configurations of benzene dimer to be nearly isoenergetic. The role of substituents in tuning pi-pi interaction is investigated. By studying dimers of benzene with various monosubstituted benzenes (in the sandwich and two T-shaped configurations), we surprisingly find that all of the substituted sandwich dimers considered bind more strongly than benzene dimer. We also find that these interactions can be tuned by a modest degree of substitution. Energy decomposition analysis using symmetry-adapted perturbation theory (SAPT) reveals that models based solely on electrostatic effects will have difficulty in reliably predicting substituent effects in pi-pi interactions.

Für die makroskopischen Eigenschaften und Funktionen biologisch relevanter Materie spielen schwache, intra- und intermolekulare Wechselwirkungen dispersiver und elektrostatischer Natur auf molekularem Niveau eine große Rolle. Um diese schwachen Wechselwirkungen zu untersuchen, können Modellsysteme, isoliert in der Gasphase, herangezogen werden. Benzoldimer, ein schwach gebundener Van der Waals Komplex, kann beispielsweise als Modellsystem für dispersive Wechselwirkungen dienen. In der vorliegenden Arbeit werden die strukturellen Eigenschaften und die (interne) Dynamik des Benzoldimers mit Hilfe spektroskopischer Methoden in den Energiebereichen der Rotationen, Vibrationen und elektronischen Übergänge untersucht und im Kontext der Symmetrie diskutiert. Die in dieser Arbeit vorgestellten Experimente tragen zu einem tieferen Verständnis des Benzoldimers bei, jedoch zeigt das Experiment zur internen Dynamik auch, dass eine ausreichende theoretische Beschreibung des Benzoldimers nach wie vor eine Herausforderung darstellt. Schwingungsübergänge hochsymmetrischer Moleküle sind oft optisch inaktiv, können jedoch mit der hier vorgestellten Methode der Symmetrieerniedrigung durch Komplexierung zugänglich gemacht werden, wie am Beispiel des Benzols demonstriert wird. Außerdem wird ein Mechanismus vorgstellt, der kollisionsinduzierte Konformationsänderungen in einem Molekularstrahl beschreibt. Dieses Modell kann generell für Molekularstrahlexperimente an flexiblen Molekülen hilfreich sein, einerseits um die beobachtete Konformationsverteilung zu verstehen, andererseits um die experimentellen Parameter gezielt zu verändern und somit Konformerpopulationen zu manipulieren. Die in dieser Dissertation vorgestellten spektroskopischen Experimente liefern einerseits molekülspezifische Informationen und ermöglichen andererseits, Modelle, die von allgemeiner Bedeutung sind, zu entwickeln. / The macroscopically observable properties and functionalities of biological matter are often determined by weak intra- and intermolecular interactions on the microscopic level. Such weak interactions are for example hydrogen bonding and van der Waals interactions and can be investigated best on isolated model systems in the gas phase. The benzene dimer, for example, is a prototype system to investgate dispersive interactions. The spectroscopic experiments, covering the energy ranges of rotations, vibrations and electronic transitions, presented in this thesis, contribute to a deeper understanding of the benzene dimer. However, from the experiments investigating the internal dynamics it becomes clear that an appropriate theoretical description of the benzene dimer is still a challenge. Vibrational transitions of highly symmetric molecules, as for example of the benzene, are often optically inactive. Here, a method is presented, which exploits symmetry reduction upon complexation and thus allows one to access such modes. Furthermore, a model is proposed describing collision induced conformational interconversion in a molecular beam. This model can be helpful for molecular beam experiments of flexible molecules to understand the observed relative conformational population and to adapt the experimental conditions allowing for the manipulation of the relative conformer abundances. In this thesis, results are presented that allow one on the one hand to deduce molecular specific information and that on the other hand also give a broader insight into phenomena of general importance.