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A transition from solid effect to indirect cross effect with broadband microwave irradiation
1Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel.
Physical Chemistry Chemical Physics : PCCP
|March 9, 2022
Summary
This study extends dynamic nuclear polarization (DNP) theory to include broadband microwave irradiation, revealing its impact on signal enhancement in nuclear magnetic resonance (NMR). The findings show indirect cross effect (iCE) dominates DNP spectra with increasing microwave bandwidth.
Area of Science:
- Magnetic Resonance Spectroscopy
- Physical Chemistry
- Biophysics
Background:
- Dynamic nuclear polarization (DNP) significantly enhances nuclear magnetic resonance (NMR) signal sensitivity.
- High radical concentrations (15-40 mM) typically yield maximum DNP enhancement in static samples.
- The indirect cross effect (iCE), driven by electron-electron spectral diffusion, is the primary DNP mechanism for broad-line radicals at high concentrations.
Purpose of the Study:
- To rigorously integrate broadband microwave irradiation into the theory of the indirect cross effect (iCE) for DNP.
- To provide a theoretical framework for understanding DNP performance under broadband irradiation conditions.
- To quantitatively analyze the influence of excitation bandwidth on DNP mechanisms.
Main Methods:
- Extension of the iCE theory to explicitly incorporate broadband microwave irradiation.
- Quantitative fitting of DNP spectra lineshapes using experimental data acquired at 3.4 T and 7 T.
- Analysis of DNP mechanism shifts as a function of increasing excitation bandwidth.
Main Results:
- The developed theory successfully quantitatively fits DNP spectra lineshapes from diverse experimental datasets.
- A transition in the dominant DNP mechanism is observed with increasing excitation bandwidth.
- At high bandwidths, the iCE mechanism becomes completely dominant, a phenomenon not previously accounted for in existing theories.
Conclusions:
- The extended iCE theory accurately describes DNP under broadband microwave irradiation.
- Broadband irradiation shifts the DNP mechanism towards a complete dominance of iCE.
- This work provides crucial theoretical insights for optimizing DNP experiments for enhanced NMR signal.
