Phase cycling schemes for finite-pulse-RFDR MAS solid state NMR experiments.

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
• Advanced pulse sequence development for enhanced spectral resolution and sensitivity.

Background:

• 2D homonuclear chemical shift correlation experiments under magic angle spinning (MAS) are crucial for structural elucidation.
• Finite-pulse radio frequency driven dipolar recoupling (fp-RFDR) is a key technique for magnetization transfer in these experiments.
• Optimizing fp-RFDR pulse sequences and phase cycles is essential for improving performance, especially under ultrafast MAS conditions.

Purpose of the Study:

• To comprehensively analyze the dipolar recoupling efficiencies of various fp-RFDR phase cycles (XY4, XY4(1)2, XY4(1)3, XY4(1)4, and XY8(1)4).
• To investigate the influence of spinning speeds (10-100 kHz), duty factor, and RF field inhomogeneity on recoupling efficiency.
• To compare theoretical calculations with experimental results for validation.

Main Methods:

• Theoretical analysis of the recoupled homonuclear dipolar coupling Hamiltonian for different phase cycles.
• Numerical simulations to analyze higher-order terms and cross-interactions.
• Experimental validation using ultrafast MAS NMR on glycine and l-alanine powder samples.

Main Results:

• The basic XY4 phase cycle introduces advantageous second-order terms for proton-proton magnetization transfer under ultrafast MAS.
• Recoupling efficiency depends on the duty factor and homonuclear dipolar coupling strength, increasing linearly with the duty factor.
• XY4(1)4 offers the best tolerance to chemical shift offset and RF inhomogeneity, while XY4 is most efficient at suppressing RF inhomogeneity under slow spinning.
• The performance differences between phase cycles diminish at higher MAS speeds, with ultrafast MAS (>60 kHz) enabling significant recoupling and fast magnetization exchange.

Conclusions:

• The XY4 phase cycle is particularly beneficial for fp-RFDR in 2D (1)H/(1)H correlation experiments under ultrafast MAS.
• Ultrafast MAS conditions significantly enhance the recoupling of homonuclear dipolar couplings, facilitating rapid magnetization exchange.
• The choice of phase cycle impacts performance concerning RF inhomogeneity and chemical shift offset, with XY4(1)4 and XY4 showing specific advantages under different conditions.