Ion Pairing versus Solvation of Dinitrobenzene Anions in Room-Temperature Ionic Liquids (RTILs): Vibrational Signatures of RTIL-Substrate Interactions.

The mechanism of solvation of ions by ionic liquids is more complex than solvation in most molecular solvents as the ionic liquid itself provides the counter ion. Solvation and ion pairing of anionic substrates in room-temperature ionic liquids (RTILs) were investigated using resonance Raman spectroscopy and DFT calculations. The purpose of this study was to differentiate between the formation of discrete cation/anion structures and a double-layer cloud of counter ions without specific atomic interactions between the ionic species. In acetonitrile/RTIL mixtures, the radical anion and dianion of dinitrobenzene (DNB) are stabilized by RTILs through solvation and ion pairing. The formation of the lowest-energy ion pair led to the largest shifts in the Raman band in DNB-·, while significantly smaller shifts were predicted for general solvation. The effect of general solvation and ion pair formation was studied using DFT with the implicit solvation model. Identification of the bands most sensitive to tight ion pairing allowed for the interpretation of the observed vibrational changes. The formation of tight ion pairs between the anionic solutes depends on both cation-solute and RTIL cation-anion interactions. Tight ion pairs were observed in RTILs, but general solvation was also important. This work establishes the advantageous use of vibrational spectroscopy to provide detailed structural information not accessible from voltammetry alone.