Next-generation Quantum Theory Of Atoms In Molecules (QTAIM) For Bond Broken And Distinguishing Molecular Isotopomers

In the Chapter 1,the basic background of quantum theory of atoms in molecules?QTAIM?is introduced briefly to facilitate the understanding of the contents.In the Chapter 2,the factors underlying the experimentally observed branching ratio?70:30?of the?1,3-cyclohexadiene?CHD?HT?1,3,5-hexatriene?photochemical ring-opening reaction are investigated.The ring-opening reaction path is optimized by a high-level multi-reference DFT method and the density along the path is analyzed by the quantum theory of atoms in molecules?QTAIM?and stress tensor methods.The performed density analysis suggests that,in both S₁ and S₀electronic states,there exists an attractive interaction between the ends of the fissile?-bond of CHD that steers the ring-opening reaction predominantly in the direction of restoration of the ring.It is suggested that opening of the ring and formation of the reaction product?HT?can only be achieved when there is a sufficient persistent nuclear momentum in the direction of stretching of the fissile bond.As this orientation of the nuclear momentum vector can be expected relatively rare during the dynamics,this explains the observed low quantum yield of the ring-opening reaction.In the Chapter 3,the factor underlying two possible pathways for the Dihydrocostunolide?DHCL?photochemical ring-opening reaction were investigated;the first pathway returned to the ring-closed conformation of the reactant and the second pathway progressed to the ring-opened product.High-level multi-reference DFT methods were used to optimize the two pathways and the density was analyzed using QTAIM and the stress tensor.Oscillations in the chemical character of the fissile bond were found for the first pathway before and after the conical intersection that steered the reaction back to reactant.Conversely,this behavior was absent for the second pathway that led forward to the product.In the Chapter 4,investigation of the hydrogen transfer tautomerization process yielded metallic hydrogen bonds in the benzoquinone-like core of the switch.Bond-path framework sets and?,comprising a three stranded,non-minimal 3-D bond,which included the familiar QTAIM bond-path and two additional paths defining the least and most preferred directions of electron density motion,were used with QTAIM and the stress-tensor respectively.The and?were visualized and uncovered the destabilizing effect on the hydrogen bond of the presence of an Fe atom.The length of and?quantified this effect and the dependence on the position of a fluorine substituent.In the Chapter 5,the H₂O/D₂O/HDO isotopomers of water are presented in terms that enable bond-flexing,bond-twist and bond-anharmonicity to be quantified during the bending?Q1?,symmetric-stretch?Q2?and anti-symmetric-stretch?Q3?normal modes of vibration.Bond-flexing was detected by the presence of curved bonding and the bond-anharmonicity was detected by the presence of motion of the bond critical point?BCP?relative to the oxygen atom.To detect bond-twist a vector-based measure was used,in the form of the bond critical point?BCP?trajectory,constructed in terms of preferred directions of electronic motion,defined by the variation of the position of the BCP during the normal modes of vibration.The BCP trajectories describe the coupling of the intramolecular bending and symmetric-stretch normal modes as well as distinguishing all three isotopomers within the harmonic approximation.The coupling of the bending and symmetric-stretch normal modes is suggested to be facilitated by the absence of bond-twist that would disrupt the coupling between sigma O-H bonds and hydrogen-bonding.Partial coupling was found for the mixed isotopomer HDO.The results are found to be consistent with findings from experiment.In Chapter 6,a conclusion and further work on the basic of this thesis are provided.