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

Abstract

We have studied temperature dependent IR spectra of the C–H stretching modes of the imidazolium ring in [bmim][PF₆], [bmim][Tf₂N], [emim][Tf₂N], [hmim][Tf₂N], and [bmim][BF₄]. 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.