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

The NMR chemical shifts of the aromatic hydrogen atoms of 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI+BF4-) and 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI+PF6-) have been determined in nine deuterated solvents. The aromatic hydrogen atoms of BMI+PF6- are more sensitive to solvation effects than the hydrogen atoms of BMI+BF4-. The interaction of the hydrogen atoms of the imidazolium cation with the relatively small and basic BF4- anion is more intimate than the interaction with the fairly large, polarizable and less basic PF6- 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 BMI+BF4-, its aromatic hydrogen atoms are less sensitive to solvation effects than the hydrogen atoms of BMI+PF6-. Copyright (C) 2001 John Wiley Sons, Ltd.