Electronic effect of ionic-pair substituents

For conventional organic substituents, an almost linear relationship can be established between the para Hammett constants in aqueous solutions and chemical shifts or natural population analysis charges (NPA charges). Based on these correlations, the Hammett constants of six synthesized ionic-pair substituents were estimated via the chemical shifts, which were well in agreement with the results calculated by NPA charges. The para Hammett constants of 89 different ionic-pair substituents (77 anion-cationic and 12 cation-anionic) were therefore calculated in the gas phase based on the density functional theory method. The results show that both the anion-cationic and cation-anionic substituents are electron-withdrawing groups and the different cation–anion combinations could tune the Hammett constants of the ionic-pair substituents in a range from 0.03 to 0.77. Copyright © 2013 John Wiley & Sons, Ltd.