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

The work presents a theory of nuclear (H-1) spin-lattice relaxation dispersion for solutions of N-15 and N-14 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 H-1 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 H-1 relaxation by contributing to the fluctuations of the inter-molecular dipolar interactions. The developed theory is tested against H-1 spin-lattice relaxation dispersion data for glycerol solutions of 4-oxo-TEMPO-d(16)-N-15 and 4-oxo-TEMPO-d(16)-N-14 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 N-14 and N-15 systems and explains the features of H-1 relaxation dispersion resulting from the electron spin relaxation. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4795006]