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 constants (A(N)) 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 is studied through ab initio quantum mechanical computations and by EPR spectroscopy. The geometry of the neutral and deprotonated forms and the corresponding dissociation constants are computed by using the hybrid PBE0 functional. With these structures, accurate A(N) values are obtained by an integrated post-Hartree-Fock/PBE0 approach. Specific solvent effects are modeled by including a few solvent molecules (strongly bound to the solute), whereas bulk contributions are taken into account by the polarizable continuum model (PCM). Both EPR spectra and quantum mechanical computations point out that A(N) increases with increasing solvent polarity and, especially, H-bond ability. The A(N) values obtained by our computational model both for the neutral and deprotonated forms in aqueous solution are in good agreement with EPR values. On the other hand, the computed pK(a) values (4.97 and 5.25 for CP and CT, respectively) 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, respectively) in better agreement with quantum mechanical computations and chemical intuition. Also the calculated magnetic titration curves in aqueous solution are in quite good agreement with our new EPR curves.