QTAIM For The Relative Stability Of The Conical Intersections And The Ring-opening Reaction

In the charpter 1,it mainly introduces the theoretical and computational background that used in this paper's research work,including quantum theory of atoms in molecules?QTAIM?,density functional theory?DFT?,the spin-restricted ensemble-referenced Kohn-Sham?REKS?method,Gaunssian and AIMAll software.The theoretical background of QTAIM,two important concepts?critical points and bond path?,and the relevant important parameters for studying the properties of BCP and bond path are introduced in detail.In the charpter 2,the factors influencing the relative stability of the S₁/S₀ conical intersections of the penta-2,4-dieniminium cation?PSB3?were studied by 3-D bond-path analysis.A balanced treatment of the covalent and ionic contributions to the ground and excited states originating from torsion about double bonds is known to be strongly dependent on the presence of dynamic electron correlation.We undertake an analysis of the minimum energy pathways corresponding to deactivation of the first excited singlet state of PSB3.In doing so,we consider torsion about the three double bonds including other intramolecular degrees of freedom,such as the bond length alternation.The 3-D bond-path analysis provides a new‘bond-localized orbital-like'directional interpretation of bonding.Therefore,we present a more sophisticated method of determination of the degree of covalent and ionic contributions known to be responsible for altering the relative stability of the S₁/S₀ conical intersections.The results presented suggest that the commonly used simplified multi-reference methodologies that often result in incorrect predictions for the excited state deactivation reaction mechanism.In the charpter 3,the photochemical ring-opening reactions of oxirane were studied with QTAIM.The conical intersections corresponding to the C-O and C-C ring opening were optimized and the reaction paths traversing these intersections were obtained.Investigation of the C-O ring opening revealed that when traversing the lowest energy conical intersection,the reaction path returns to the closed ring geometry.The C-O path traversing the intersection featuring torsion of terminal CH2 group however,led to a ring-opened geometry,an H-shift and the formation of acetaldehyde that can undergo further dissociation.The observation of different reaction paths was explained by the 3-D paths from quantum theory of atoms in molecules?QTAIM?that defined the most preferred direction of electronic motion that precisely tracked the mechanisms of bond breaking and formation throughout the photo-reactions.In the charpter 4,we investigated the[1.1.1]propellane molecule using the 3-D bond-path framework set B and the stress tensor B_?within the quantum theory of atoms in molecules?QTAIM?.The controversial axial bond was determined to be a charge-shift bond comprising ssbvitaoiglfnfnudnieifiesnsc gsaa nnitSds ab<q o unna1ed.nw ti Tmfimheeeetda a lisilnuinrcfl eiutt eeyorn fmc tcehsha taoo rffgc etou-hrnsrehee lixafapttx eeibacdolt e ndwbdliioytnnh gdl,o vtowahnle u betposhone ld aor fnis eztitaighfbfheinlb ieotbsyrsoi nnPSdg.Consistency of these results was found at the MP2,CCSD and B3LYP theory levels.In the charpter 5,a brief summary of the work during the postgraduate period and further work carried out.