Steady-state free precession NMR in solids undergoing magic angle spinning.

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
• Quantum spin dynamics and relaxation phenomena.

Background:

• Nuclear magnetic resonance (NMR) is crucial for studying solids but faces sensitivity and resolution limitations.
• Magic-angle spinning (MAS) enhances NMR sensitivity and resolution in solids.
• Steady-State Free Precession (SSFP) is a pulse sequence that can improve sensitivity, especially for wide-line solids NMR.

Purpose of the Study:

• To investigate the compatibility and interplay between SSFP and MAS in solid-state NMR.
• To understand the spin dynamics of SSFP under MAS conditions for isolated spins.
• To determine the conditions under which SSFP and MAS can be effectively combined.

Main Methods:

• Theoretical re-examination of SSFP spin dynamics under MAS for isolated spins.
• Analysis of spin ensembles subjected to time-dependent second-rank interactions.
• Comparison of rotor-synchronized and non-rotor-synchronized conditions.
• Experimental validation of theoretical predictions.

Main Results:

• Rotor synchronization of SSFP interpulse time with MAS rate (ωR) yields solution-like NMR responses.
• Without rotor synchronization, SSFP-MAS exhibits non-trivial magnetization behavior.
• For single crystals with spin anisotropies >> ωR, no steady state is achieved without synchronization.
• Powdered samples under similar desynchronized conditions can achieve a steady state.
• The steady state shows non-monotonic dependence on anisotropy (oscillations) and mis-synchronization (resonant dips).

Conclusions:

• SSFP and MAS can coexist in solid-state NMR experiments.
• Rotor synchronization is critical for predictable SSFP-MAS behavior, mimicking solution NMR.
• Desynchronization leads to complex phenomena, including the possibility of achieving a steady state in powdered samples, with specific dependencies on anisotropy and rotor timing.