||Chapter 1 Transition structures of titanium mediated epoxidation The non-local density functional calculations for the transition structures of titanium mediated epoxidation were carried out by using acyclic, penta-coordinate and hexa-coordinate models. By this study, the orientation of the ethylene approach, the function of the chiral tartrate ester groups and also the effect of bulky alkyl hydroperoxide are addressed, which are in agreement with experimental findings. Chapter 2 Transition structures of hydride transfer reactions involving NAD+/NADH models Hydride transfer reactions of four NAD+/NADH model systems have been studied with ab initio molecular orbital calculations. There is a strong preference for a syn or stacking approach of the two pyridine rings in the transition structures. The pyridine rings are slightly puckered into boat conformations in the transition structures. 3-amide, 3-thioamide and 3-cyano groups are used to study the effect of these groups on the hydride transfer reaction of NAD+/NADH models. When in a cis conformation, the 3-amide group of nicotinamide slightly increases the activation energy for the hydride transfer. When the group is in a trans conformation, it significantly reduces the activation energy for the hydride transfer. There is a preference for the trans amide group to be out-of-plane, with the carbonyl group directed toward the transferring hydride. By examining the charge variation and the energetic, the electrostatic interaction is believed to be the major stabilization effect which governs the conformational preference for the transition structures of the hydride transfer reaction. Chapter 3 Substituent effects on the bond dissociation energy of phenols and anisoles - A density functional study The origin of the substituent effect of the bond dissociation energy (BDE) of phenol and anisole systems were studied by local density functional calculations, which gives satisfactory results compared to experiments. Both ground state effect and radical effect contribute to the bond dissociation energy of O-H and O-CH3. The major contribution to the radical effect can be well correlated by two factors, namely, polar effect and spin delocalizatian effect which are represented by charge variation (Aδ) and spin variation on the radical center (δs), respectively. By this study, the substituent effect on the BDE of phenol and anisoles can be well understood.