Continuous diffusion model for concentration dependence of nitroxide EPR parameters in normal and supercooled water

Abstract

We measured electron paramagnetic resonance (EPR) spectra of 14N- and 15N-labeled perdeuterated TEMPONE radicals in normal and supercooled water at various radical concentrations. By fitting the EPR spectra to spectral shape functions based on the modified Bloch equations, we obtained concentration dependences of EPR parameters of radicals at each measured temperature. From concentration dependences of the EPR parameters quantifying spin dephasing, coherence transfer, and hyperfine splitting, we determined linear concentration coefficients, whose values depend on the relative motion of radicals due to modulation of the Heisenberg spin exchange (HSE) and dipole-dipole (DD) interactions between them. We applied the continuous diffusion model for relative motion of radicals and we evaluated the diffusion coefficients of radicals from the concentration coefficients using the standard relations and the relations derived from kinetic equations for the spin evolution of interacting radical pair. It was found that the latter equations lead to the better agreement between the diffusion coefficients calculated from different concentration coefficients. The calculated diffusion coefficients of 14N- and 15N-labeled radicals show similar values, which is an expected result that supports the presented method. Upon lowering the temperature into the supercooled state, the calculated diffusion coefficients decrease slower than is predicted by the Stokes-Einstein relation and slower than the rotational diffusion coefficient. Similar effects were detected in NMR studies of the rotational and translational motion of water molecules in supercooled water.