First-principles studies of electronic delocalization in weak interaction systems
Student thesis: Doctoral Thesis
Intermolecular weak interactions widely exist in molecular crystals, in which they act as attractive forces between neighboring molecules. They are weak in strength compared to the interaction within the molecule. But they have obvious effects on the geometric structures and thermal properties of the systems. In the past two decades, C6H6-CX4 (X = H, Br, Cl, F) and C6H6-CHY3 (X = Br, Cl, F) systems have been used as model systems to study weak interaction, which is considered as a CH-π interaction and has been found to play important roles in the physical, chemical, and biological properties of a variety of substances. Different from chemical bonds in rocks or metals, the interactions between benzene and substituted methane have dual natures that both dispersion and electrostatic terms contribute to the interaction energy. At the same time, there is a strong overlap between delocalized π bond of benzene and σ bond of substituted methane within complexes, which are expected to result in an electronic delocalization. However, there is no research reported about the electronic delocalization induced by CH-π interaction within these systems. In this thesis, we systematically investigate the electronic delocalization and molecular orbital deformation within C6H6-CX4 and C6H6-CHY3 systems employing first-principles calculations based on quantum theory and find that the intermolecular orbital interactions between C6H6 and substituted methane molecules are responsible for this phenomenon. In chapter 1, we introduce the fundamentals of intermolecular weak interactions in terms of structures, interaction energies, stabilities, and physical natures. Special attentions are paid on CH-π intermolecular interaction. In chapter 2, we focus on the most popular methods for the computational simulation, including molecular orbital (MO) theory and density functional theory (DFT) used in this work. In chapter 3, using first-principles calculations based on different methods and basis sets, we calculate the electronic distribution and composition of MOs of C6H6-CH4 complex. From HOMO-3 to LUMO+1, the MOs of system are composited by benzene and the electrons are localized on corresponding MOs of benzene. However, the HOMO-4 and LUMO+2 are composited by both benzene and methane and involve obviously electronic delocalization. The first-principles calculations reveal that the electronic delocalization originates from both the π bond of C6H6 and σ bond of CH4 and extends to the center of system. Our results demonstrate that the weak interaction between benzene and methane not only reduces the total energy and forms special configuration of the complex but also affects the composition of MOs and distribution of electrons. In chapter 4, to further understand the electronic delocalization of C6H6-CH4 complex, we calculate the density of states (DOS), projected density of states (PDOS) and overlap population of density of states (OPDOS) and give the diagrams of intermolecular orbital interaction. The calculated results demonstrate an obvious intermolecular orbital interaction between benzene and methane due to the delocalization of π orbital of benzene. The intermolecular interaction of orbitals forms "bonding" and "antibonding" orbitals through orbital overlap and increase the total energy of the complex because of the occupation of antibonding" orbital. Our finding extends the concept of orbital interactions from an intramolecular type to an intermolecular type and provides a penetrative understanding of CH-π hydrogen interaction between benzene and methane. In chapter 5, to further confirm the electronic delocalization and intermolecular orbital interaction in C6H6-CH4 complex and explore the effects of intermolecular orbital interaction on system, we calculate the basis set superposition error (BSSE) corrected interaction energy, dipole moment, DOS, PDOS and OPDOS between C6H6 and substituted methane. Our results show that the C6H6-CHF3 complex has smaller interaction energy than C6H6-CHCl3 and C6H6-CHBr3, though its dipole moment is larger. The analyses of OPDOS indicate there is a strong intermolecular orbital interaction between C6H6 and CHF3. For C6H6-CHCl3 and C6H6-CHBr3, the overlap of orbitals is much weak. The strong intermolecular orbital interaction leads to the smallest interaction energy among C6H6-CHY3 due to the occupation of antibonding orbitals. In chapter 6, a conclusion of the whole thesis will be given. Based on ab initio calculations, we investigate systemically the deep orbital deformation and electronic delocalization induced by CH-π interaction in the concerned complexes and explain the phenomena using intermolecular orbital interaction theory. We summarize the results into the follows. First, we find orbital deformation and electronic delocalization induced by CH-π interaction in week interaction system. Second, we conduct detailed analysis of electronic properties and find that the phenomena we observed are due to intermolecular orbital interactions. Third, to further confirm our results, we extend our studies to benzene and halogen substituted methane systems.
- Electroweak interactions, Intermolecular forces