Imidazoline‐N‐oxyl: A DFT Study of Its Protonation Reaction

Imidazoline-based nitroxides are developed as pH probes. Their pK a values vary over a wide range (from 1 to 11), depending on the substituents attached to the five-membered cyclic nitroxide. Density functional calculations using the PBE1 PBE method at the 6-31 +G(d,p) level, combined with natural bond orbital (NBO), frontier molecular orbital (FMO), geometry, Mulliken charge, and thermodynamic analyses, are carried out to disclose the effects involved in the changes in pK a . The studies show that the decrease of seven pK a units from pyrrolidine (11) to imidazoline-N-oxyl 8 is due to the inductive electron-withdrawing capacity of the nitroxyl group. On the other hand, by combining both the inductive and mesomeric electron-withdrawing capacities of the NO 2 group with delocalization of the lone pair on the amino N atom of the π system of the vinyl linker, the pK a of 4.5 of 8 was increased by around three units to 7.8 for 1/2.