Theoretical study on the activity of hydrogen atom of imidazolium ring in ionic liquids

Abstract

The double-bond isomerization of 1-pentene catalyzed by [Emim]BF4 ionic liquid and the interaction relationship of the [Emim]BF4 ion pair have been studied by using density functional theory (DFT) at the B3LYP/6-31G(d,p) level. The obtained results reveal that the [Emim]+ and BF4− ions exist in the form of the ion pairs in the ionic liquid system, and only two stable structures are obtained. The isomerization process of 1-pentene on [Emim]BF4 ionic liquids proceeds via a concerted reaction pathway. The hydrogen atom transfer is accompanied by the migration of the intramolecular double bond. The ionic liquid acts as a proton donor, while it also favors the proton abstraction. The adsorption energies of 1-pentene on the 2-position and 4-position hydrogen atoms of the imidazolium ring are −11.32 and −7.42kJ/mol, respectively. The apparent activation energies of the double-bond isomerization for 2-position hydrogen atom and 4-position hydrogen atom are 196.91 and 214.64kJ/mol, respectively. Those indicate that two kinds of hydrogen atoms of the imidazolium ring have both catalytic activities, but the activity of 2-position hydrogen atom is much higher than the one of 4-position hydrogen atom.