A QTAIM And Stress Tensor Perspective Of Reaction Pathways

In this thesis, we made use of the “quantum theory of atoms in molecules” method to investigate the reaction pathways. One system is the pathway of tetrasulfur tetranitride S4N4 with large-amplitude motions, and the other one is the pathway of(H2O)5 on potential energy surface. All the data we get from Gaussian 09 package and AIMALL. The dissertation contains five chapters, we have a brief introduction following:In chapter 1, we have a brief introduction of history and development of quantum chemistry. Then some fundamental concepts of QTAIM are introduced, which can help readers have a general understanding of it.In chapter 2 and chapter 3, we explore the S4N4 within the framework of QTAIM. The nature of the bonding interactions including the intramolecular S--S interactions in tetrasulfur tetranitride, S4N4 are probed by performing very large amplitude vibrations of all of the eighteen normal modes of vibration. The QTAIM and stress tensor point properties are then investigated and found to be highly dependent on the mode of vibration. A considerable degree of metallicity ξ(rb) is found for the S--S and S-N bonding interactions. A unique bonding feature is found for a small amplitude vibration of the most anharmonic mode of this investigation, mode 2, where the S--S bond critical point(BCP) transforms from a closed-shell S--S BCP to a shared-shell S-S BCP. We find seventeen new unique QTAIM topologies for the molecular graphs corresponding to the eighteen modes of vibration along with seven ‘missing’ topologies that are mapped onto a spanning 2-D Quantum topology Phase Diagram(QTPD). In addition, eleven unique topologies existing on 3-D QTPDs are found due to the presence of non-nuclear attractors(NNAs). We use the stress tensor eigenvalues to explain the invariance of the numbers and types of QTAIM critical points. The applicability of both the stiffness S and stress tensor stiffness Sσ are also explored. Two new bond measures are introduced, a polarizability P and the stress tensor polarizability Pσ which are derived from the stiffness S and stress tensor stiffness Sσ respectively.In chapter 4, using the quantum theory of atoms in molecules(QTAIM), directed spanning quantum topology phase diagrams for eleven(H2O)5 reaction-pathways and five unique Poincaré–Hopf solutions are generated on the MP2 potential energy surface. Two new energy minima that were predicted from earlier work include the first(H2O)5 conformer with a 3-DQT quantum topology. A two-way exchange of chemical character between the sigma O-H and hydrogen-bonds is found using the total local energy density, which is consistent with the variation of the frequencies of the lower edge of the stretching band. A large gap in the stretching region provides evidence for the energetic preference of the pentamer ring. The QTAIM derived stiffness and the polarizability are found to be line with physical intuition from the energy density and ellipticity values, but the stress tensor stiffness and corresponding polarizability are not.In chapter 5, a conclusion and future work based on this thesis are displayed in this part.