Continuous Diffusion Model for Concentration Dependence of Nitroxide EPR Parameters in Normal and Supercooled Water

Area of Science:

• Physical Chemistry
• Chemical Physics
• Spectroscopy

Background:

• Electron paramagnetic resonance (EPR) spectra are sensitive to radical motion in solution.
• Radical diffusion influences Heisenberg spin exchange and dipole-dipole interactions, affecting EPR spectral parameters.
• Accurate determination of radical diffusion from EPR requires reliable spectral fitting and valid theoretical models.

Purpose of the Study:

• To develop and validate a method for calculating radical diffusion coefficients from EPR spectra.
• To investigate the concentration and temperature dependence of radical diffusion in water.
• To compare experimental results with theoretical predictions like the Stokes-Einstein relation.

Main Methods:

• Measuring EPR spectra of 14N- and 15N-labeled perdeuterated TEMPONE radicals in water across various concentrations and temperatures.
• Fitting EPR spectra using modified Bloch equations to determine concentration coefficients for spin dephasing, coherence transfer, and hyperfine splitting.
• Calculating diffusion coefficients from concentration coefficients using standard and kinetically derived relations.

Main Results:

• Reliable concentration coefficients for EPR parameters were obtained through spectral fitting.
• Diffusion coefficients calculated using kinetically derived relations showed better agreement across different parameters.
• Radical diffusion coefficients were similar for both isotopically labeled radicals.
• Observed diffusion coefficients decreased with decreasing temperature slower than predicted by the Stokes-Einstein relation and rotational diffusion.

Conclusions:

• The presented method accurately determines radical diffusion coefficients from EPR spectra.
• The temperature dependence of radical diffusion in supercooled water deviates from classical predictions.
• The findings align with the anomalous diffusion behavior observed in water molecules under similar conditions.