Theoretical Studies On The Connection Between Structure And Acidic Strength Of The SO₃H Functionalied Ionic Liquids

Sulfonic acid functionalized hydrosulfate ionic liquid has been applied to organic synthesis and catalysis as the study on acid ionic liquid go thoroughly,and it may take place of the traditional inorganic acid catalysis as the trend goes. The strength of the acid has a direct relation with its ability of catalysis. This work aims to study the relation between the inner structure of sulfonic acid functionalized hydrosulfate ionic liquid and its acid strength,and combines the experiment results testify the general theory about this relation. And therefore the outcome of this work can serve as part of the foundation of the study of structure-effect of ionic liquid to direct the synthesis of ionic liquid and adjustment of the acid strength.Imidazole,pyridine and quaternary ammonium salt with sulfonic acid functionalized propyl group and butyl group,a total of 6 different types of hydrosulfates were studied and the structure,acid content,nuclear magnetism and acid strength were characterized.The structures of the amphoteric compound, cation, and ionic liquids of 6 hydrosulfates were optimized. And the frequency analysis and energy calculation were conducted by using Density Functional Theory（DFT） in 6-311++G（d,p）. The interaction energy,hydrogen bond and charges were analyzed. In the research hydrogen bond was found to perform important function in stabilizing the configuration by lowing energy of the system. The most stable configuration of ionic liquid was in the form of ion-H₂SO₄ interaction formed by the interaction of anion and functionalized sulfate group.with hydrogen.Through the study of the Imidazole, pyridine and quaternary ammonium salt ionic liquids and the hydrogen,charge distribution in them,the acid strength sequence was determined to be Imidazole > pyridine > quaternary ammonium salt. This is because the 7H in imidazole and the 8H in pyridine are more positively charged to make imidazole and pyridine easier to ionize. The quaternary ammonium salt is the weakest in acid strength because of the hydrogen bond limits the ionization. Along with the increasing of carbon numbers the acid strength is lower. This is due to the electrophilic property of imidazole and pyridine that even the charge distribution and limit the ionization of sulfate group. At the same time, the hydrogen bond in O-H..O is shortened and the energy increased. It is also not conducive to the dissociation of hydrogen atoms.The theory matched with the acid strength experiment results which is shown as follow[C₃SO₃HMIM]HSO₄>[C4SO3HMIM] HSO₄>[C₃SO₃HPy]HSO₄>[C4SO3HPy]HSO₄>[C₃SO₃HTMA]HSO₄>[C4SO3HMIM]HSO₄.In conclusion,the acid strength can be increased by shorting the carbon chain,destroying the inner hydrogen bond structures or inducing charge transfer. This provides a theoretical basis for increasing acid strength of ionic liquid and screening acidic catalyst in ionic liqudid.