1H relaxation dispersion in solutions of nitroxide radicals: influence of electron spin relaxation

The work presents a theory of nuclear ((1)H) spin-lattice relaxation dispersion for solutions of (15)N and (14)N radicals, including electron spin relaxation effects. The theory is a generalization of the approach presented by Kruk et al. [J. Chem. Phys. 137, 044512 (2012)]. The electron spin relaxation is attributed to the anisotropic part of the electron spin-nitrogen spin hyperfine interaction modulated by rotational dynamics of the paramagnetic molecule, and described by means of Redfield relaxation theory. The (1)H relaxation is caused by electron spin-proton spin dipole-dipole interactions which are modulated by relative translational motion of the solvent and solute molecules. The spectral density characterizing the translational dynamics is described by the force-free-hard-sphere model. The electronic relaxation influences the (1)H relaxation by contributing to the fluctuations of the inter-molecular dipolar interactions. The developed theory is tested against (1)H spin-lattice relaxation dispersion data for glycerol solutions of 4-oxo-TEMPO-d16-(15)N and 4-oxo-TEMPO-d16-(14)N covering the frequency range of 10 kHz-20 MHz. The studies are carried out as a function of temperature starting at 328 K and going down to 290 K. The theory gives a consistent overall interpretation of the experimental data for both (14)N and (15)N systems and explains the features of (1)H relaxation dispersion resulting from the electron spin relaxation.