  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.
