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

The vibrational frequencies of the C=O + C=C band of diphenylcyclopropenone and the NH(2) stretching band of p-aminobenzonitrile were determined in various room temperature ionic liquids (RTILs). The vibrational frequency shifts of the C=O + C=C stretching mode were compared with Kamlet α values, and frequency shifts of the NH(2) 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 NH(2) 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.