Theoretical and spectroscopic studies of 1-butyl-3-methylimidazolium iodide room temperature ionic liquid: Its differences with chloride and bromide derivatives

1-Butyl-3-methylimidazolium iodide (bmimI) a room temperature ionic liquid (RTIL), widely used as nonvolatile electrolyte solution in dye-sensitized photoelectrochemical cells has been synthesised and its UV–Visible and Infrared (IR) spectroscopic studies were performed. The spectral features were characterized by Density Functional Theory (DFT) calculation at B3LYP level using DGDZVP basis sets. Time-Dependent-DFT (TD-DFT) calculation on this RTIL was done at the same level of computation, which helped us to assign the reason for the pale yellow colour of bmimI. In addition, the observation that bmimI is liquid, while its analogous ILs bmimCl and bmimBr are solids at room temperature has been addressed. Further, comparison of hydrogen bonding and ionic interactions present in these three ILs reveal that C2–H stretching band frequency shifts to higher wavenumber (blue shift) with change to a less electronegative anion , was ascribed to the formation of weaker hydrogen bonds. In addition, a scale based on ‘stabilization energy factor’ has been proposed for possible prediction of a salt to be a liquid or a solid at room temperature.