Hydrogen Bonding in Ionic Liquids Probed by Linear and Nonlinear Vibrational Spectroscopy

TL;DR

This study investigates hydrogen bonding in imidazolium-based ionic liquids using vibrational spectroscopy and computational modeling, revealing weak and stronger hydrogen bonds involving specific hydrogen atoms in the ionic liquid structure.

Contribution

It combines experimental vibrational spectroscopy with anharmonic vibrational calculations to elucidate hydrogen bonding patterns in ionic liquids with varying alkyl chain lengths.

Findings

Weak hydrogen bonding involving C(4)-H and C(5)-H groups.

Stronger hydrogen bonds involving C(2)-H groups.

Spectroscopic evidence supports the hydrogen bonding hypothesis.

Abstract

Three imidazolium-based ionic liquids of the type [Cnmim][NTf2] with varying alkyl chain lengths (n = 1, 2 and 8) at the 1 position of the imidazolium ring were studied applying IR, linear Raman, and multiplex CARS spectroscopy. The focus has been on the CH-stretching region of the imidazolium ring, which is supposed to carry information about a possible hydrogen bonding network in the ionic liquid. The measurements are compared to calculations of the corresponding anharmonic vibrational spectra for a cluster of [C2mim][NTf2] consisting of four ion pairs. The results support the hypothesis of weak hydrogen bonding involving the C(4)-H and C(5)-H groups and somewhat stronger hydrogen bonds of the C(2)-H groups.