Quantum Chemical Study Of The Novel Composite Superacids Based On Superhalogen And Its Catalysis Of Ethylene Hydrogenation

This dissertation mainly includes two parts:1)Theoretical study on the composite structure of superacids based on silaborane superhalogens under the Wade-Mingos rule;2)The ethylene hydrogenation reaction catalyzed by the composite superacids based on the eight-electron rule superhalogens.The first part mainly includes the theoretical calculation of gas-phase acidity(ΔG_acid)and solution phase dissociation constants（p K_a）of the new composite structure superacids and analysis of various influencing factors.The second part mainly includes the theoretical study of the stepwise and concerted mechanism in the ethylene hydrogenation reaction catalyzed by the superacids HMgX₃ and HMg₂X₅（X=F,Cl,Br）.In the first chapter,the research background and significance of super halides and superacids are introduced,including the main characteristics of superhalogen and dfferent electron counting rules,and the application of the new superacids in the field of catalysis is also introduced.The second chapter introduces the calculation method and transition state theory.In the third chapter,based on systematic DFT calculations,silaborane-based superhalogen anions,which obey the Wade–Mingos rule,are shown to be capable of giving rise to superacids via their combination with protons.The calculation results show that in addition to the composite structures H/[Si B₅H₆]and H/[Si B₇H₈],a total of 61 composite structures meet the theoretical criterion of gas phase superacids.The vertical electron detachment energy（VDE）of superhalogens plays a leading role in the acidity of the composite structure.In most cases,the higher the VDE value of the superhalogen part,the stronger the acidity of the entire composite structure.Therefore,choosing superhalogens as conjugate bases to design superacids is a reasonable route.The calculation results also show that increasing the size of the silaborane cage,introducing highly electronegative ligands to replace hydrogen atoms in the cage,and increasing the number of substituents can enhance the acidity of the composite structure.In addition to the gas phase acidity,the solution phase dissociation constant p K_aof the designed structure can also be calculated theoretically by designing the thermodynamic cycle process and combining the dissolution free energy obtained by the SMD model.Taking perchloric acid HCl O₄ as the reference material,the composite structure of silaborane and its derivatives are all superacids in the solution phase.The comparison of the calculation results of the gas phase and the solution phase shows that the order of the strength of the gas phase acidity is different from the corresponding order of the solution phase.The main reason for this difference is that,compared with the composite structure where the substituents are F and Cl,the conjugate acid of the CN-substituted composite structure is stabilized by solvent molecules,which is not conducive to the progress of deprotonation,so the acidity in the solution phase is weakened.Compared with the previously studied carborane system,the acidity of silaborane-based composite superacids is stronger whether in the gas phase or the solution phase.Therefore,silaborane-based superhalogens may have greater potential in designing superacids systems than carborane-based superhalogens.The fourth chapter,we studied the mechanism of the stepwise and concerted hydrogenation of ethylene catalyzed by HMgX₃ and HMg₂X₅（X=F,Cl,Br）.The results show that the energy barriers of both concerted and stepwise reactions are much lower than those of common acids（HF,HCl,HBr）,and the concerted reaction of HMg₂Br₅-2 is the best one.In most cases,the energy barrier of concerted reaction is lower than that of stepwise reaction,and the possibility of concerted reaction is greater.Through further analysis of the influence factors of superacids catalyst,we confirmed that the stronger the acidity of superacids composite structure,the better the catalytic effect.Therefore,we can design a reasonable superacids composite structure to enhance the catalytic ability.