NMR characterization of substituent effects in cation–π interactions

Substituent effects in C6H5X…M+ (M+ = Li+, Na+, K+, and NH 4 + ) systems have been characterized using isotropic nuclear magnetic shielding constants (σ). Good linear relationship between σ and the Hammett substituent constant σp is established which suggests that the substituent effects can be quantified by NMR experiments. Irrespective of the electronic nature of X, meta carbons, and all the hydrogens are always deshielded and this property is useful for detecting cation–π interaction. The σ-scan plots revealed that the deshielding effect of the cation on the aromatic ring is significant even at large cation–π distance (∼4.5 Å).