Theoretical Investigation For Activation Of C-H Bond In Methane By Pd_n~+(n=1-4) In The Gas Phase

The essence of chemical reaction is the breaking of the old bond and the formation of the new bond, namely the bonding electron gets redistributed. In many cases the C-H bond rupture constituted the key step in the catalytic cycle, as a result,their activation has been the attention from the general theoretical chemist and experimental chemists. In recent years, the transition metal catalytic reaction in catalytic chemical industry clusters has caused considerable attention; metal cluster of catalytic activity and the dependencies between the sizes of metal cluster has become a hot spot in the contemporary clusters chemical research. In the transition metal catalyzed reactions, the high-spin state transition metal complexes usually contain multiple unpaired electrons. The electronic of transition metal are extremely likely to occur spin flips and cross of the potential energy surfaces（PES） in a catalyzed reaction due to the high exchange effect among d orbital. This means that even thermal chemical reactions of transition metal compounds are often spin-forbidden in that the reactants and products involved have a different overall electronic spin states. The experiment and theoretical study reported a lot, but little for 4d transition metal. Few theoretical investigations have been performed on relatively small cationic palladium clusters so far.Based on the two-state reactivity theory, this thesis uses the density functional theory（DFT）, coupled cluster theory（CCSD）, and quantum-chemical calculation programs including Gaussian-03 program, Crossing2004, Games program, and NBO5.0 to make a thorough and detailed theoretical analysis on the reaction mechanism of palladium metal cluster cation activation of methane in the gas phase.This thesis consists of four chapters. Chapter one briefly describe the development and application of quantum chemistry, this paper introduces the two state reaction theory and the main work of this article. Chapter two introduces the common theory and calculation method of quantum chemistry, the theoretical knowledge in this paper, the theory research for us to provide the reliable theory basis of quantum chemistry.The chapter three, the mechanisms of the gas phase reactions activation of methane catalyzed by Pd_n⁺（n=1-3） have been investigated using density functionaltheory（DFT） B3 LYP functional in the doublet and quartet states. The results indicate that the reaction takes place more easily along the low-spin potential energy surface for Pd⁺ and Pd₂⁺. Pd₃⁺ is likely excellent mediators for CH₄ activation and this reaction is a typical two-state reaction. The reactions of Pd+ and Pd₂⁺ with methane need endothermic 30.58 and 11.84 kcal/mol, respectively, but the Pd3+activation of methane exothermic 9.05 kcal/mol.The chapter four, we have investigated the mechanisms of reactions that activation of methane catalyzed by Pd₄⁺using density functional theory（DFT）B3LYP functional in the gas phase. We found that this reaction is a typical two-state reaction and compared to the system of Pt4+ activation of CH4 at the same time. The potential energy surfaces（PESs） were characterized in detail using molecular orbital theory and natural bonding orbital（NBO） analysis. Crossing seam were found of the doublet and quartet states by Yoshizawa’ single-point vertical excitation method, and structures and relative energy of crossing point the potential energy surface were determined. The possibility of spin inversion and cross-section situation of potential energy surfaces were discussed, through calculating spin-orbit coupling（SOC）constants and transition probability at the minimum energy crossing point（MECP）.Finally, we determine the optimal reaction path.