Solute–solvent hydrogen-bonding in room temperature ionic liquids studied by Raman spectroscopy

Abstract

The vibrational frequencies of the CO + CC band of diphenylcyclopropenone and the NH₂ stretching band of p-aminobenzonitrile were determined in various room temperature ionic liquids (RTILs). The vibrational frequency shifts of the CO + CC stretching mode were compared with Kamlet α values, and frequency shifts of the NH₂ stretching mode were compared with Kamlet β values. A nearly linear relationship was obtained for both parameters, although the solvatochromic parameters were more sensitive to changes of the cation species. Vibrational frequency calculations of a 1 : 1 cluster of p-aminobenzonitrile with the RTIL anions using DFT theory reproduced the observed frequency shifts of the NH₂ stretching mode fairly well. The frequency shifts of the CN stretching mode were well reproduced by the linear combination of dipolarity parameters, the hydrogen-bond donating and accepting parameters determined by the Raman shift of the solute molecule.