Theoretical Study On The Activation Of Methane By Transition Metal Complexes MX~+(M=Fe,Ru,Os;X=F,Cl,Br,I)

Methane,which is abundant gaseous molecule on the earth,is a kind of promising fuel.Since 1750,the concentration of methane has increased by 150%in the atmosphere.It is accounting for 20%of all the greenhouse gases and leading to the global warming effect.But it is a challenging task for collecting and storing gaseous methane in standard conditions.Moreover,converting menthane into more useful chemical material has been the difficulties for scientists to overcome all this time,and there is still a long way to go to activate methane at ambient conditions.This thesis studies the activation reaction of transition metal cations complexes and methane at theoretical level by the quantum mechanical method-Density Functional Theory.After calculating the related optimized intermediates and transition states,we have depicted the Potential Energy Surfaces.Then we use Intrinsic Reaction Coordinate analysis to verify whether transition states connect the reactants and products properly.Natural Bond Orbital and Frontier Molecular Orbital analysis for configuration can obtain the Natural Bond Orbital populations,Natural Population Analysis charges and atomic orbital coefficients.We could know the essence of reaction from mechanism,and provide the theoretical basis for experiments.The first part is the activation of methane by transition metal complexes Fe X⁺（X=F,Cl,Br,I）.The calculation shows that the entire reaction takes place on the ground state potential energy surface.In the process of eliminating HX,the reactivity of activating methane gradually decreases with the order of F,Cl,Br,I.In the process of the elimination of H₂,the catalytic performance of Fe X⁺（X=F,Cl,Br,I）with the addition of four ligands does not differ significantly.For the reaction mechanism,in the order of X=F,Cl,Br,and I,the H atom transfer mechanism is from the Proton-Coupled Electron Transfer mechanism to the Hydrogen Atom Transfer mechanism,and finally to the Hydride Transfer mechanism.For the dehydrogenation process,the mechanism is Hydride Transfer.Part II is the interaction between Ru X⁺（X=F,Cl,Br,I）and methane.Results show that there is PESs crossing existing in the adding of ligand Cl,Br and I,and several reasonable MECPs are found.Activation performance of ligand F is much better in the process of eliminating HX,and dehydrogenation is more favorable in the use of ligand Cl and Br.The reaction mechanism is from Proton-Coupled Electron Transfer to Hydrogen Atom Transfer in order of F,Cl,Br and I,and finally Hydride Transfer.With regard to dehydrogenation,F and I ligands are Hydride Transfer mechanism,Cl and Br ligands are Hydrogen Atom Transfer mechanism.Part III studied the reaction of Os X⁺（X=F,Cl,Br,I）and methane.There is only PESs crossing existing in the adding of ligand I in the whole process.With regard to the process of eliminating of HX and H₂,ligand I is relatively better according to Gibbs free energies.The machanism is also similar to Fe X⁺and Ru X⁺in the same group.When HX is eliminated,reaction mechanism changes from Proton-Coupled Electron Transfer to Hydride Transfer in order of F,Cl,Br and I,And it is Hydride Transfer mechanism in the elimination of H₂.The results about the thesis will further perfect the reaction mechanism of activation of methane by transition metal,and also explain ligand effect of halogen on activating methane.Finally providing certain theoretical basis for subsequent experiments.