Quantum Chemical Study Of The Novel Composite Superacid Based On Superhalogen

We systematically studied the composite superacids constructed from several types of supehalogen obeying the eight-electron rule or the Wade-Mingos rule.The main content is,based on the deprotonation process,their gas-phase acidity and dissociation constants?p K_a?were calculated and analyzed by high-level methods.Firstly,we used the superhalogen[Mg_nCl_2n+1]^-?n=1-3?based on the eight-electron rule as a building block to construct a series of superacid composites together with Br?nsted acid HX?X=F,Cl,Br?.The gas-phase acidity(?G_acid)and influences of various factors on the superacid properties of these composites were analyzed in detail.We revealed the relationship between?G_acidcid of the composites and the VDE of the superhalogen part.Subsequently,starting from closo-carborated-based and their derivatives based on Wade-Mingos rule,we designed a series of superacid composites.The gas-phase acidity(?G_acid)and dissociation constant?pK_a?of these composites were comprehensively explored.We revealed the effect of the number of boron atoms,the type and number of substituents on the acidity of the system.The relationship between the gas-phase acidity and liquid dissociation constant of the system and the VDE value of the superhalogen were investigated.The consistency between the order of the acidity determined in the gas phase and in solution is systematically explored.The first chapter present the background and significance of the research topics,including the characteristics of superhalogen,the composites constituted by superhalogen and the importance of the research of novel superacid.The second chapter mainly describe the theoretical methods used in this thesis as well as the computational details.In the third chapter,we systematically probed the properties of a series of superacid composites composed of superhalogen[Mg_nCl_2n+1]^-?n=1-3?and Br?nsted acids?HF,HCl,HBr?.The superhalogen based on eight-electron ruled was selected,ranging from mononuclear to polynuclear structures.The gas-phase acidity of all composites were systematically studied,as indicated by high-level CCSD?T?results,where the designed composites is expected to be an candidates to the superacid.The influence of different factors on the gas-phase acidity of the superacid were explored.The influences of various factors on the superacid properties of these composites were analyzed in detail.Our results demonstrated that the acidity of these superacids are mainly determined by the superhalogen components and the connection between the VDE value of superhalogen and the gas-phase acidity of superacid is revealed.Therefore,it is probable to design novel composite superacid with enhanced property through the regulation of the superhalogen component.it is highly probable that an effective guide to the related experimental research could be provided by the theory.Comparing different methods,we found that the ab-initio method MP2 and the functional?B97XD functional could be capable of providing results approaching the accuracy at the CCSD?T?level.In the fourth chapter,the properties of a series of superacid composites constructed from closed carboranes and their derivatives[CB_nHX_n]^-?n=5-13;X=H,F,Cl,CN?and proton hydrogen based on Wade-Mingos rules were discussed in detail.Based on the results of?B97XD functional,the gas-phase acidity(?G_acid)of these composites was systematically studied.The results show all the composites here fulfill the theoretical criterion for superacid except for the three composites of HCB₅H₆,HCB₆H₇,and HCB₇H₈.The influence of different factors on the gas-phase acidity of the superacid were discussed.The acidity of the superacids were significantly improved by introducing of highly electronegative ligand and increasing of size of cages and the number of ligand.We displayed the relationship between?G_acidcid of the composites and the VDE values of the corresponding superhalogen.Designing a reasonable thermodynamic cycle,the dissociation constant of superacids in liquid phase were systematically explored by the combination of the SMD model and solvent?DCE?DMSO and H₂O?.We found that the designed superacid composites exhibits the ability of dissociating under solution,that is to say,possessing strong acidity.Meanwhile,The connection between the VDE values in the gas and liquid phase of carboranes and their derivatives and the liquid acidity?pK_a?of superacids were revealed.Then,we systematically explored the consistency of the order of the strengths of acids determined by the criterion of gas phase and liquid phase.We found that the order of the strength of the superacids determined by the criterion of gas phase were consistent with liquid phase for the same type of composites.However,the cyano-substituted composites structure possess a large difference between the order of the strength of gas phase and liquid phase for different types of composites.Further analysis show that it may be due to the larger dipole moment of the cyano-substituted composite,that is to saying,the interaction between the conjugate acid and the solvent molecule is lager,which enhance the effect of stabilization of the solvent molecule on the conjugate acid,and weaken the ability of deprotonation of conjugate acid in solution.