Translational diffusion in paramagnetic liquids by 1H NMR relaxometry: nitroxide radicals in solution

Area of Science:

• Physical Chemistry
• Chemical Physics
• Materials Science

Background:

• Nuclear spin-lattice relaxation rates in solutions containing nitroxide radicals exhibit distinct frequency dependencies.
• Existing theories describe proton spin-lattice relaxation in nitroxide radical solutions.
• Translational dynamics significantly influence spectral density functions.

Purpose of the Study:

• To derive relationships between relaxation rate changes and molecular diffusion coefficients in spin-labeled systems.
• To establish a straightforward method for determining diffusion coefficients using proton NMR relaxometry.
• To investigate the applicability of this method for both (14)N and (15)N spin probes.

Main Methods:

• Combining a theory of nuclear spin-lattice relaxation with spectral density functions of translational dynamics.
• Utilizing (1)H nuclear magnetic resonance (NMR) relaxometry, specifically field cycling spectroscopy.
• Performing experiments across a wide range of frequencies (10 kHz-20 MHz) and temperatures.

Main Results:

• Linear relationships were derived between relaxation rate changes and the relative translational diffusion coefficient.
• The method was validated using extensive experimental data for various solvents and spin probes.
• NMR relaxometry in spin-labeled systems provides access to faster diffusion processes than in diamagnetic systems.

Conclusions:

• Proton NMR relaxometry offers a simple and effective way to determine diffusion coefficients in systems with nitroxide radicals.
• The study highlights the potential of this technique for investigating fast molecular dynamics.
• Limitations related to translational dynamics timescale and electron spin relaxation were discussed.