Field modulation effects induced by sample spinning: application to high-resolution magic angle spinning NMR

Area of Science:

• Nuclear Magnetic Resonance Spectroscopy
• Solid-State Chemistry
• Physical Chemistry

Background:

• High-resolution magic angle spinning (HRMAS) is a powerful technique for analyzing complex, heterogeneous systems.
• HRMAS utilizes sample spinning and liquid-state nuclear magnetic resonance (NMR) pulse sequences.
• While generally yielding predictable results, HRMAS can sometimes produce unexpected experimental outcomes.

Purpose of the Study:

• To investigate the origins of puzzling experimental results observed in HRMAS.
• To analyze the impact of time-dependent physical quantities induced by sample rotation.
• To model the effects of radiofrequency (B1) field inhomogeneities on specific HRMAS experiments.

Main Methods:

• Review of fundamental HRMAS hardware components.
• Analysis of time-dependent physical quantities influenced by sample rotation.
• Focus on B1 inhomogeneities in nutation, (90 degrees)+x-t-(90 degrees )-x, and MLEV16 experiments.
• Development of models for radiofrequency distribution in solenoidal coils based on geometrical considerations.
• NMR spin dynamics calculations to validate models against experimental data.
• Electromagnetic simulations to assess B1 field distribution.

Main Results:

• Experimental HRMAS results can be explained by the time-dependence of physical quantities due to sample rotation.
• Models of B1 field distribution, derived from geometrical considerations, accurately reproduce experimental NMR findings.
• The developed models are consistent with electromagnetic simulations of B1 fields within solenoidal coils.

Conclusions:

• The observed anomalies in HRMAS experiments stem from the inherent time-dependence of physical quantities affected by sample rotation.
• Radiofrequency field inhomogeneities play a critical role in generating these puzzling results.
• The proposed models provide a theoretical framework for understanding and predicting HRMAS behavior under B1 field variations.