Theoretical Study Of The Reactivity Of Transition Metal Ions With CO₂, N₂O And CH₄ In The Gas Phase

Because they can provide fundamental information about catalytic bond activation a large number of studies both experimental and theoretical have been devoted to the reactions of transition metal atoms or ions with small molecules. Many of these studies concentrated on the activation of hydrocarbons (CH₄, C₂H₄, CH₃OH…). Wide attention was also given to the activation of the C–O and N–O bonds in some important pollutants of the atmosphere (N₂O, CO₂, NO₂, CO and NO,). A large number of experimental chemists have found that transition-metal atoms and their cations have peculiar activations to the N–O, C-O, C-H and C-C bonds of small organic compounds in the gas phase. The theoretical approaches to these reactions also indicated that the transformations of spin multiplicities occur 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)". Today, TSRs have attracted a great deal of interest in the world for the sake of exact reaction mechanisms.In the thesis, the gas phase reactions of La⁺; Ce⁺, Pr⁺ and Ir⁺ with N₂O; CO₂ and CH₄, which were selected as representative systems of the activation of N-O, C-O and C-H bonds of hydrocarbons by bare transition-metal cations, have been examined by using density functional theory (DFT) and CCSD (T) methods with corresponding basis sets to explore the reaction mechanisms of TSR. The Gaussian 03, Gamess and Molpro program package were performed in this thesis.The whole thesis consists of five chapters. Chapter 1 and Chapter 2 describe the progress and application of quantum chemistry as well as the development and the present situation of TSR, and briefly introduce elementary theory and quantum chemistry computation methods. The contents of the two chapters are the basis and background of our studies and offered us with useful and reliable quantum methods. In Chapters 3, 4 and 5, the gas phase reactions of La⁺, Ce⁺, Pr⁺ and Ir⁺ with N₂O, CO₂, CH₄ have been studied carefully using B3LYP methods and the activation of N-O, C-O and C-H bonds have been emphasized, respectively. The potential energy surface crossing behaviors have been discussed in detail. 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. Vibrational frequency calculations and intrinsic reaction coordinate (IRC) methods were used to characterize the reaction path channels on two PESs. Where after, a series of crossing points (CPs) involving the structures and energy values has been located and the spin inversion behaviors by which the reaction system can hop from one PES to another by effective spin-orbital coupling have been inspected. Finally, the energetically more favorable channel was confirmed according to thermodynamic and dynamic data. All of our calculated results in this thesis are in agreement with the early experimental findings.