Theoretical Study Of Activation H-H σ Bond, C-H And C-C Bonds Of Hydrocarbons By CrO₂～+ In The Gas Phase

Recently, theoretical approaches to gas-phase transition-metal chemistry had indicated that often the reactants, possible intermediates, and products had ground states of different spin multiplicities. Namely, the reactions did not obey "spin conservation law". Examples are the reduction of water by the early transition-metal cations Sc⁺—V⁺, and oxidations involving the metal dioxide cations VO₂⁺ and CrO₂⁺. However, the spin inversion itself was often neglected, and it had even been argued that "the notion of spin-forbidden ' of organometallic reactions in the condensed phase was inappropriate", Although the necessity to explicitly consider surface hopping as a mechanistic step in organometallic chemistry was pointed out more than 10 years ago, the assumptions of either strict spin conservation or its complete neglect prevailed until 1994. This was in part due to difficulties in the appropriate theoretical descriptions of spin crossover in polyatomic metal compounds and, more importantly, the lack of unambiguous experimental examples for a violation of spin conservation along the reaction path, which provoked the necessity to tackle "two-state reactivity (TSR)". Today, Chemists have become increasingly interested in TSR in the world.In the paper, the gas phase reaction of chromium dioxide cation CrO₂⁺(²A₁/⁴A" ) with H₂, CH₄ and C₂H₄ are selected as the representative systems of activation H-H bond, C-H bond, and C-C bond of hydrocarbons. We have been carefully studied them using quantum methods and have obtained some interesting results On the basis of the molecular orbital theory, the tradition transition state theory as well as quantum chemistry theory, the systems choosed have been investigated using Density Functional Theory (DFT) Methods, the Moller-Plesset correlation energy correction MPn, the coupled cluster CCSD (T) calculations, the Fragment Molecular Orbital (FMO) analysis and the Natural Bond Orbital analysis. The structures of the reagents, the reaction products and the transition states along the reaction paths have been optimized. Furtherly, the reaction surfaces, the spectrum datum, the thermodynamic datum as well as the information of orbitals have been obtained, which have been used to disscussed deeply the reaction mechanism.The whole paper consists of five chapters. Chapter 1 describes the progeess and application of quantum chemistry as well as the development and the present situation of two-state reactivity (TSR), and briefly views the situation of the study of gas phase activation of H-Hσbond, C-H and C-C bonds of hydrocarbons by transition metals (M, M⁺, and MO⁺) in the last 10-15 years. In Chapter 2, introduces elementary theory and quantum chemistry computation methods, which mainly contained the reaction surface, crossing rules of the potential energy surfaces, tradition transition state theory, 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 4 ,5, 6, the reactions of CrO₂⁺(²A₁/⁴A" ) with H₂, CH₄, and C₂H₄ have been studied carefully using UB3LYP methods and the activation of H-H, C-H and C-C bonds of hydrocarbons by CrO₂⁺ have been emphasized The involving potential energy surface crossing has been discussed detailedly. Firstly, the formation and character of the reactant concomplexes have been discussed using the electronic charge transfer, the NBO analysis and the molecular orbital theory. 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. Finally, the orbital interaction analysis of activation H-Hσbond, C-H bond, and C-C double bond have been carried out by fragment molecular orbital (FMO).