Hydrogen Bonding in Ionic Liquids Probed by Linear and Nonlinear   Vibrational Spectroscopy
  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. 
C. Roth, S. Chatzipapadopoulos, D. Kerl\'e, F. Friedriszik, M.   L\"utgens, S. Lochbrunner, O. K\"uhn and R. Ludwig
physics.chem-ph