Quantitative analysis of ¹⁴N quadrupolar coupling using ¹H detected ¹⁴N solid-state NMR

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy
• Materials science
• Biophysics

Background:

• Magic-angle spinning solid-state NMR is a powerful technique for studying molecular structure and dynamics.
• Nitrogen-14 (14N) nucleus, a spin-1 nucleus, offers valuable insights but is challenging to study directly.
• Indirect detection using sensitive nuclei like proton (1H) can enhance characterization of less sensitive nuclei.

Purpose of the Study:

• To develop and validate a method for indirectly characterizing nitrogen-14 (14N) sites in molecules using proton (1H) detection.
• To leverage moderate radiofrequency (rf) fields and magic-angle spinning (MAS) for efficient coherence transfer.
• To enable quantitative analysis of 14N quadrupolar interactions for structural and dynamic information.

Main Methods:

• Utilized magic-angle spinning solid-state NMR with indirect detection via proton (1H) observation.
• Employed moderate radiofrequency (rf) fields to generate coherence between 1H and 14N nuclei.
• Developed efficient numerical simulations for quantitative lineshape analysis of 14N signals.

Main Results:

• Successfully characterized 14N sites indirectly by exploiting proton (1H) detection sensitivity.
• Quantitatively determined the size and asymmetry of the quadrupolar interaction for 14N sites.
• Demonstrated the utility of the method at moderate and fast magic-angle spinning frequencies.

Conclusions:

• The developed method allows for sensitive, indirect characterization of 14N sites using naturally abundant isotopes.
• This approach avoids the need for isotopic labeling, simplifying material analysis.
• Provides additional structural and dynamic insights through the characterization of the quadrupolar interaction.