Theoretical Study Of Activation C-H Bond Of Methane By Ti, Zr, Hf In The Gas Phase

Recently, experimental chemists have found that transition-metal atoms M and their oxide cations MO~+ have peculiar activations to the C-H, C-C and C-O bonds of small organic compounds in the gas phase. Theoretical approaches to these reactions also indicated that often the reactants and products had ground states of different spin multiplicities, and the transformations of spin multiplicities occurs frequently in thermal reactions. Namely, the reactions did not obey"spin conservation law". Usually, more than one state is involved in reaction process, which ensures the whole reaction always proceeds on the low-energy potential energy surface (PES). Such a phenomenon is called"two-state reactivity (TSR)". However, the spin inversion itself was often neglected. The assumptions of either strict spin conservation or its complete neglect prevailed until 1994, and the necessity to explicitly consider surface hopping as a mechanistic step in organometallic chemistry was accepted after the violations of spin conservation along the reaction path were observed by high-tech experimental instruments. This was in part due to the restriction of experimental conditions in the early phases, and more importantly, the dominant status of spin conservation along the reaction path. Today, TSRs have attracted a great deal of interest in the world for the sake of exact reaction mechanisms. Today, Chemists had become increasingly interested in TSR in the world.In the thesis, the spin-forbidden reaction of gas-phase molecular activation by trans-mental will be studied at the density functional theory using the high-level basis. Using the mathematical algorithm proposed by Harvey, we obtain the minimum energy crossing point (MECP) of different potential energy surfaces (PESS), and then calculate the spin-orbit coupling matrix at this point, aim at understanding the impact of vibration for the spin-orbit coupling. Based on Andrews' experimental results, the gas phase reactions of Ti, Zr, Hf with CH4, which were selected as representative systems of the activation of C-H bonds of methane by bare transition-metal atoms, had been examined using density functional theory (DFT) to explore the reaction mechanisms of TSR.The whole paper consists of four chapters. Chapter 1, described the theoretical basis, application. At the same time, the development and the current situation of two-state reactivity (TSR) are explained. In Chapter 2, simply introduced elementary theory, mainly contained spin-orbit coupling mechanism and rules for intersystem crossing. The contents of the two chapters were the basis and background of our studies and offer us with useful and reliable quantum methods.In Chapter 3, 4, The mechanism of the spin-forbidden reaction Ti, Zr, Hf with CH4 on three potential energy surfaces(PESs) has been investigated at the CCSD/6-311++G(3df,3pd)//B3LYP/6-311++G(3df,3pd) levels. Firstly, for each reaction system, all molecular geometries were fully optimized on respective ground state and the lowest excited state PESs by high-level quantum chemistry calculation methods. Secondly, the reaction path channels have been studied on two potential energy surfaces (PESs). Thirdly, the PES crossing dramatically affecting reaction efficiency and the reaction rate has been studied by means of the Hammond postulate and the intrinsic reaction coordinate (IRC) approach used by Yoshizawa et.al. and a series of crossing points (CPs) involving the structures and energy values have been located. Fourthly, for the sake of comparison, the mathematical algorithm to MECPs developed by Harvey et al. had also been employed. The spin-orbit coupling (SOC) of this MECP had also been located. Finally, the energetically more favorable channel was confirmed according to thermodynamic and dynamic data. The results show that the above reactions are typical two-state reactivities, the singlet and triplet potential energy surface occur a crossing before the transition state, it largely affects the reaction efficiency and reaction rate. However, the calculated SOC constant is small, so that the happening of the intersystem crossing is difficulty.