Solvent Polarity and pH Effects on the Magnetic Properties of Ionizable Nitroxide Radicals: A Combined Computational and Experimental Study of 2,2,5,5-Tetramethyl-3-carboxypyrrolidine and 2,2,6,6-Tetramethyl-4-carboxypiperidine Nitroxides

The modulation induced by different solvents and by pH on the nitrogen isotropic hyperfine coupling consts. (AN) of 2,2,5,5-tetramethyl-3-carboxypyrrolidine (3-carboxy-PROXYL,CP) and 2,2,6,6-tetramethyl-4-carboxypiperidine (4-carboxy-TEMPO,CT) nitroxides was studied through ab initio quantum mech. computations and by EPR spectroscopy. The geometry of the neutral and deprotonated forms and the corresponding dissocn. consts. are computed by using the hybrid PBE0 functional. With these structures, accurate AN values were obtained by an integrated post-Hartree-Fock/PBE0 approach. Specific solvent effects are modeled by including a few solvent mols. (strongly bound to the solute), whereas bulk contributions are taken into account by the polarizable continuum model (PCM). Both EPR spectra and quantum mech. computations point out that AN increases with increasing solvent polarity and, esp., H-bond ability. The AN values obtained by the authors' computational model both for the neutral and deprotonated forms in aq. soln. are in good agreement with EPR values. However, the computed pKa values(4.97 and 5.25 for CP and CT, resp.) are significantly larger than the value reported in the literature for CP (3.4). New potentiometric and spectroscopic measurements lead to values (4 and 4.30 for CP and CT, resp.) in better agreement with quantum mech. computations and chem. intuition. Also the calcd. magnetic titrn. curves in aq. soln. are in quite good agreement with the authors' new EPR curves.