14N overtone NMR spectra under magic angle spinning: experiments and numerically exact simulations

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Quantum Chemical Simulations
• Materials Characterization

Background:

• Previous theoretical treatments were inconsistent with preliminary experimental results of (14)N overtone NMR under magic angle spinning (MAS).
• Observed phenomena included narrowed powder pattern widths, MAS rate-dependent frequency shifts, and an apparent absence of spinning sidebands.

Purpose of the Study:

• To reproduce and explain the experimental observations of (14)N overtone NMR under MAS conditions using numerical simulations.
• To validate the theoretical framework for (14)N overtone NMR spectroscopy.
• To demonstrate the utility of (14)N overtone MAS NMR for determining quadrupolar parameters and chemical shifts.

Main Methods:

• Numerically exact simulations of (14)N overtone NMR spectra were performed, incorporating the full nuclear spin Hamiltonian.
• Simulations considered the effect of sample spinning under magic angle conditions.
• The (14)N overtone MAS spectrum of L-histidine was acquired and analyzed.

Main Results:

• Simulations successfully reproduced the experimental effects, including the splitting of the (14)N overtone signal into five sidebands.
• The dominant +2ω(r) sideband under MAS was identified, with other sidebands showing lower intensities.
• Characteristic quadrupolar lineshapes were obtained, allowing for the determination of (14)N quadrupolar parameters and isotropic chemical shifts.

Conclusions:

• The study provides a theoretical basis for interpreting (14)N overtone MAS NMR spectra.
• MAS NMR effectively averages dipolar couplings and shielding anisotropy, enhancing spectral resolution.
• The presented method allows for clear resolution of multiple nitrogen sites, as demonstrated with L-histidine.