Imidazoline-N-oxyl: a DFT study of its protonation reaction.
Imidazoline-based nitroxides are developed as pH probes. Their pKa 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 PBE1PBE 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 pKa. The studies show that the decrease of seven pKa 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 NO2 group with delocalization of the lone pair on the amino N atom of the pi system of the vinyl linker, the pKa of 4.5 of 8 was increased by around three units to 7.8 for 1/2.
Université de Provence, UMR-6264 Laboratoire Chimie Provence case 521, Avenue Escadrille Normandie, Niemen, 13397 Marseille Cedex 20, France.
Sandrine
S
Chenesseau
Nicolas
N
Ferré
Sylvain R A
SR
Marque
Didier
D
Siri
D000465:Algorithms; D006860:Hydrogen Bonding; D048288:Imidazolines / Q000737:chemistry*; D007700:Kinetics; D011522:Protons; D011759:Pyrrolidines / Q000737:chemistry; D013816:Thermodynamics
D048288:Imidazolines; D011522:Protons; D011759:Pyrrolidines; C032519:pyrrolidine