Theoretical Investigations On The Columnar Structure Of Pillar[5] Arenes,on Solvation Effects In Ionic Liquids And The Study Of Cycloaddition Reactions Involving Strained Dienes

In this thesis,three different topics concerning challenging chemical questions have been investigated using quantum chemical methodology.In the first one,the dependence of the structure of pillar[5]arenes and rotational energy barriers on bulky alkyl group substituents is investigated.Pillar[5]arenes,a type of novel macrocycle containing di-substituted hydroquinone units linked by methylene bridges in para-positions,have attracted extensive attention in supramolecular chemistry as interesting candidates to be used in the preparation of host-guest complexes.Functionalization by means of rim substitution and sustaining an ordered substituent arrangement on both sides of the rim is important for the development of new pillararene-based materials.In order to develop new pillararene materials systematically,the rim inversion process of rotating the hydroquinone units through the pillar[5]arenes has to be controlled.In our investigations we have studied the effect of different types of hydroquinone substituents on the rotational energy profile using density functional theory combined with the hybrid M06-2X functional.Nonpolar alkyl（-CH₃,-CH₂CH₃,-CH₂CH₂CH₃,-CH（CH₃）₂,-CH₂CH₂CH₂CH₃,-CH₂CH=CH₂,-CH₂CH≡CH）substituents on the rotation mechanism on transition state barriers and different local minima were investigated in this study.For the alkyl series,the barrier decreased significantly up to propyl due to increasing stabilizing dispersion interactions while it increased again for n-butyl since the chain did not fit well into the cavity to rotate through.The second topic concerns the investigation about Ionic liquids（ILs）.An ionic liquid（IL）is a salt in the liquid state which has been recognized as novel solvents.They can serve as green alternatives to the use of conventional volatile organic solvents due to its nonvolatile and nonflammable nature.In this work,we performed theoretical calculations using density functional theory for three kinds of ionic liquids systems to investigate their properties:the first is[emim][₄]system,which is composed of 1-ethyl-3-methylimidazolium cation[0)8)4)8)]⁺and ethylsulfate anion[₄]^-;the second is the[emim][1-Ethyl-3-methylimidazolium trifluoroacetate（TFA）]system;the third is the[emim][1-ethyl-3-methylimidazolium trifluoromethanesulfonate（TFO）system.The reason for these choices originated in a collaboration with experimentalists at the Department of Chemical Engineering of the Texas Tech University（USA）.From the monomers to doublet,triplet and quadrupole level,we have investigated the structures,atom and combined charges,electrostatic potential plot and interactions to describe the characteristic association structures in detail.The third topic relates to a study about cycloaddition between strained dienes and o-quinones,which was performed in cooperation with the group of Prof.Zuilhof group.The emergence of click chemistry has had a profound influence on almost all branches of chemical science,inverse electron-demand Diels-Alder reactions which include the tetrazine–trans–cyclooctene（TCO）/cyclopropene click,strain-promoted alkyne-nitrone cycloaddition and the strain promoted oxidation–controlled cyclooctyne–1,2–quinone cycloaddition（SPOCQ）.The distortion/interaction model has been successfully applied to reactions of all types.In this project,we studied theoretically those reactions and analogous reactions using ab initio calculations（SCS-MP2）and single point CCSD（T）for a selected set of reactions.Through the calculation and analysis of activation energies,activation enthalpies,extrapolated correlation energies in different phase and different basis set,we can perform the distortion/interaction analysis and find the rules between these reactions.