Ionic Liquid–Solute Interactions Studied by 2D NOE NMR Spectroscopy

• Published in: The journal of physical chemistry. B 2015-07, Vol.119 (29), p.9225-9235
• Main Authors: Khatun, Sufia, Castner, Edward W
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp509861g

Intermolecular interactions between a Ru2+(bpy)3 solute and the anions and cations of four different ionic liquids (ILs) are investigated by 2D NMR nuclear Overhauser effect (NOE) techniques, including {1H–19F} HOESY and {1H–1H} ROESY. Four ILs are studied, each having the same bis­(trifluoromethylsulfonyl)­amide anion in common. Two of the ILs have aliphatic 1-alkyl-1-methylpyrrolidinium cations, while the other two ILs have aromatic 1-alkyl-3-methylimidazolium cations. ILs with both shorter (butyl) and longer (octyl or decyl) cationic alkyl substituents are studied. NOE NMR results suggest that the local environment of IL anions and cations near the Ru2+(bpy)3 solute is rather different from the bulk IL structure. The solute–anion and solute–cation interactions are significantly different both for ILs with short vs long alkyl tails and for ILs with aliphatic vs aromatic cation polar head groups. In particular, the solute–anion interactions are observed to be about 3 times stronger for the cations with shorter alkyl tails relative to the ILs with longer alkyl tails. The Ru2+(bpy)3 solute interacts with both the polar head and the nonpolar tail groups of the 1-butyl-1-methylpyrrolidinium cation but only with the nonpolar tail groups of the 1-decyl-1-methylpyrrolidinium cation.