A Density Functional Theory Study Of The Spin-forbidden Reactivity Of The Activation Of CH₄ By Transition Metal

Methane activation by transition metal has been a topic of growing interest during the past decade, due to economic interest in the methane conversion and chemistry. Interesting insights into the details and mechanisms of the activation process can be gained by studies of gas-phase reactions of metal ions and clusters. The activation of C-H bonds is a key step in numerous syntheic reactions and catalytic processes. Methane, as one of the most important chemicals, has been widely used in chemical synthesis, hydrogen production, and energy production. As such, methane activation has been studied extensively by both experimentalists and theorists. However, the activation of C-H bond in methane is a unique challenge among the hydrocarbons due to its large bond energy (the C-H bond energy is about 440 kJ/mol). Early investigations have shown that neutral metal atoms are often reacive under electronic excitation with small alkanes. Recent studies reveal that many transition metals including lanthanides and actinides are good C-H insertion agents in reactions with small alkanes and often form high-oxidation-state complexes with a carton-metal multiple bond following H migration.Methane activation mediated by W or Ta/ Ta- have generated new class of small insertion products and high-oxidation-state complexes with multiple carbon-metal bonds and was found to be a spin-forbidden process. According to the principle of two-state reactivity reaction, the activation of C-H bonds of methane by bare transition metal atom or cation had been examined using density functional theory to explore the reaction mechanisms.The whole paper consists of four chapters. Chapter 1 mainly reviews the progress and application of quantum chemistry as well as the development and the present situation of two-state reactivity. The second chapter summarizes the theoretical background of this thesis, mianly contained the potential energy surface, the natural bond orbital and spin-orbit coupling mechanism etc. The contents of two chapters were the basis and background of our studies and offer us with useful and reliable quantum methods.In chapter 3 and 4, the gas phase reactions of W or Ta/ Ta- with CH4 have been investigated using Density Functional Theory and the activation of the C-H bonds of the methane have been emphasized. We discuss detaily the crossing points between two PESs of different spin multiplicities in the reaction pathway to better understand the spin inversion processes involved in the reaction. Firstly, all molecular geometries on each spin were fully optimized and vibrational frequency calculations on the potential energy surface of different spin multiplicities in the reaction pathway by 6-311++G (3df, 3pd) basis set and corresponding pseudopotential. Secondlly, the intrinsic reaction coordinate (IRC) was then calculated to probe the reaction path and check if the correct transition state was located. Then, on the basis of the Hammond postulate, the crossing points (CPs) between states of different spin multiplicities has been selected by means of the procedure used by Yoshizawa et al. For the sake of comparison, the mathematical algorithm to MECPs developed by Harvey et al. had also been employed. Finally, The spin-orbit coupling is calculated between electronic states of different multiplicities at the crossing points (MECPs) to estimate the intersystem crossing probabilities, and the probability of hopping from one surface to the other in the vicinity of the crossing region is calculated by the Landau-Zener type model. The energetically more favorable channel was confirmed according to thermodynamic and dynamic date.