Thermal effect on C-H stretching vibrations of the imidazolium ring in ionic liquids

We have studied temperature dependent IR spectra of the C-H stretching modes of the imidazolium ring in bmim]PF(6)], bmim]Tf(2)N], emim]Tf(2)N], hmim]Tf(2)N], and bmim]BF(4)]. Temperatures in this study are from 278 to 348 K at an interval of 10 K. Spectra of the C-H stretching modes have been deconvoluted using our previous computer program of the Voigt-lineshape function. Frequency shifts, Lorentzian spectral widths, and band absorbance were examined as a function of temperature. In order to interpret the observed behaviors, we have developed a simple mechanical model as well as a chemical equilibrium model. The model analyses suggest that enthalpy changes for the cluster and/or ion-pair breaking reactions in the liquid state are several kJ mol(-1) endothermic, and the degree of dissociations of ion pairs or hydrogen bonded clusters is in the range from 0.3 to 0.9 with different magnitudes for the five ionic liquids.