Theoretical and experimental studies on proton transfer in acetate-based protic ionic liquids

Abstract

In this study, the proton transfer of three protic ionic liquids (PILs) pyrrolidinium acetate ([Pyrrol]OAc), diethylammonium acetate ([DEA]OAc) and bis-(2-methoxyethyl)-ammonium acetate ([BMOEA]OAc) were investigated. At first, the structures of the ion-pairs and molecular pairs of these PILs were optimized at B3LYP/6-311++G(d,p) level. The interaction energy between anions and cations was also obtained. The proton transfer processes were verified by intrinsic reaction coordinate (IRC) pathways tracing to the energy profiles connecting the transition state (TS) to the two desired minima, i.e. ion pair and molecular pair. The experimental attenuated total reflection (ATR) FTIR spectra of these PILs at room temperature were determined and compared with the results calculated at B3LYP/6-311++G(d,p) level. Vibrational mode analyses (VMA) for [Pyrrol]OAc found that δ(NH) has an imaginary frequency (−147.3cm−1), which is accounted for proton transfer from [NH2]+ to OAc−. Natural bond orbital (NBO) analyses pointed out that second order perturbation stabilization energy of (E(2)) of LP(N1)→σ*(O2-H5) was much larger than that of other orbitals, and should be the symmetrical matching with the maximum overlap and the minimum gap (0.73au). The hybridized index of N atom is varied from sp3.65 in ionic pair to sp4.49 in TS. The constituent of s orbital decreases 3.3% and the length of N1-H5 increases from 1.02Å in ionic pair to 1.65Å in TS, and the symmetric stretching vibration takes place the red shift. It could be explained that the N1-O2-H5 played an important role in the stabilization of molecular pair. The electron density ρ(r) and the Laplacian of the electron density ∇2ρ(r) derived from atoms in molecules (AIM) analyses were used to describe the intensity and characteristic of a bond. The results indicate that a very strong interaction of the hydrogen bonds exists in the ion-pair geometries and the bonds are the covalent bond.