Method Development On Valence Bond Theory And Its Applications

In this paper,new Valence Bond(VB) methods for solvent effects are developed. Sequentially ab initio VB methods are applied to several chemical reactions and small molecules.There are three sections in it.1.Development on VB methodsChapter 3 presents a VB method,called VBSM,to study free energies of solvation by VB methods.The VB calculation takes the polarization free energies which come from VB atomic charges into account in self-consistent field procedure with including first-solvation-shell effects.The VBSM method is applied to the Menshutkin reaction and the degenerate S_N2 reaction of CH₃Cl with Cl^- at the VBSCF,BDO,and BOVB levels.It can be shown that the VBSM method enables us to reveal the solvent effects in both aqueous and nonaqueous solution by ab inito VB computation.In general,the VBSM method is validated as a tool for the study of chemical reactions in solution.In chapter 4,the non-equilibrium solvation version of VBSM method is developed and used in vertical excitation in aqueous solution.Test calculations on the vertical excitation of formaldehyde and acetone show that non-equilibrium VBSM method can deal with the non-equilibrium solvation of chemical process in solution. Furthermore,it can be seen that the dipoles and the wave functions of molecules in aqueous solution are almost unchange under non-equilibrium solvation.2.VB studies of some diatomic moleculesIn chapter 5,the dioxygen molecule is studied by ab initio VB methods,including the spectroscopic data,potential energy curves,and the analysis of the wave functions. VB structures are carefully selected to avoid missing any important structures.For the excited states,sixteen structures also provide quantitative correct description.It can be shown that the origin of the barrier is due to an artifact of calculations that lack dynamic correlation.The study shows that the 'mythical failure' of VB theory in early VB period was due to the lack of quantitative study.In chapter 6,the ground state and the two lowest excited states of NF molecule is studied.Important parameters,including the spectroscopic data,potential energy curves,dipole moment and partial atomic charges are computed.The analysis of the wave functions is also performed.With the analysis of theÏ€andσresonance,the change of bond length,dipole moment,bond energy in different state are discussed.It can be concluded that the n resonance plays an important role in the bonding of in the two lowest excited states of NF molecule.In chapter 7,the ground state C₂ molecule is being studied.The particular bonding between the C atoms is revealed with the selected VB structures.VBCIS method can give the dissociation energies and other physical properties in good agreement with experiment and FCI calculation.Furthermore,the nature of chemical bond in C₂ molecule is discussed.3.VB studies on chemical reactionsIn chapter 8,Hydrogen abstraction reactions of the type X·+H-H'→X-H+H'·,(X =F,Cl,Br,I) are studied by ab initio valence bond methods and VB state correlation diagram(VBSCD) model.Some further approximations lead to a simple formula that expresses the barrier for nonidentity and identity hydrogen abstraction reactions as a function of the bond strengths of reactants and products.In chaper 9,Menshutkin reaction,is studied by VBPCM method and VBSCD model.The influence of the solvent effects are discussed and revealed.It is shown that the reactivity parameters of VBSCD model along with their semiempirical derivations provide together a satisfactory qualitative and quantitative account of the barriers.In chaper 10,the heterolytic cleavage of(CH₃)₃CCl and(CH₃)₃SiCl in aqueous solution are studied by VBPCM method.The calculations reveal the different natures of the C-Cl and Si-Cl bonds and show that the barriers for the heterolysis processes in both cases are controlled by the resonance energy.Another discussed issue is the difficulty to generate silicenium ions in the condensed phase.