The effect of the anion on the chemical shifts of the aromatic hydrogen atoms of liquid 1‐butyl‐3‐methylimidazolium salts

Abstract

Abstract The NMR chemical shifts of the aromatic hydrogen atoms of 1‐ n ‐butyl‐3‐methylimidazolium tetrafluoroborate $(\hbox{BMI}^{+}\hbox{BF}_{4}^{-})$ and 1‐ n ‐butyl‐3‐methylimidazolium hexafluorophosphate $(\hbox{BMI}^{+}\hbox{PF}_{6}^{-})$ have been determined in nine deuterated solvents. The aromatic hydrogen atoms of $\hbox{BMI}^{+}\hbox{PF}_{6}^{-}$ are more sensitive to solvation effects than the hydrogen atoms of $\hbox{BMI}^{+}\hbox{BF}_{4}^{-}$ . The interaction of the hydrogen atoms of the imidazolium cation with the relatively small and basic $\hbox{BF}_{4}^{-}$ anion is more intimate than the interaction with the fairly large, polarizable and less basic $\hbox{PF}_{6}^{-}$ anion. For both salts, the interaction involving the anion and the hydrogen atoms of the imidazolium cation is more intimate for H2, compared with H4 or H5. Owing to the intimate interaction of the ions of $\hbox{BMI}^{+}\hbox{BF}_{4}^{-}$ , its aromatic hydrogen atoms are less sensitive to solvation effects than the hydrogen atoms of $\hbox{BMI}^{+}\hbox{PF}_{6}^{-}$ . Copyright © 2001 John Wiley & Sons, Ltd.