Conformation of bis-nitroxide polarizing agents by multi-frequency EPR spectroscopy

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy
• Electron Paramagnetic Resonance (EPR) spectroscopy
• Chemical physics

Background:

• The efficiency of dynamic nuclear polarization (DNP) is critically dependent on the chemical structure of polarizing agents.
• For cross-effect DNP, bis-nitroxide biradicals are preferred, requiring optimal interaction and relative orientation of their unpaired electrons.
• The structure of biradicals in frozen glassy DNP matrices may differ from crystal structures, influencing their performance.

Purpose of the Study:

• To determine the conformations of six bis-nitroxide polarizing agents in their DNP matrix.
• To correlate the determined structures with their observed DNP performance.
• To investigate the influence of molecular structure on electron spin interactions and orientations.

Main Methods:

• Utilized Electron Paramagnetic Resonance (EPR) spectroscopy at multiple frequencies (9.7 GHz, 140 GHz, and 275 GHz).
• Employed an advanced fitting routine to analyze EPR data and extract structural parameters.
• Compared the conformations of six bis-nitroxides, including AMUPol, in a frozen glassy matrix.

Main Results:

• Successfully determined the conformations and relative orientations of the nitroxide moieties for six bis-nitroxide polarizing agents.
• Established a correlation between the molecular structures in the DNP matrix and the DNP performance at 500 MHz/330 GHz.
• Identified key structural features influencing the efficiency of polarizing agents.

Conclusions:

• The molecular structure of biradicals in the DNP matrix is crucial for optimizing DNP efficiency.
• Multi-frequency EPR spectroscopy is a powerful tool for characterizing biradical structures in situ.
• Understanding and controlling biradical conformation is essential for developing improved DNP polarizing agents.