  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.
