Determination of accurate F chemical shift tensors with R-symmetry recoupling at high MAS frequencies (60-100 kHz).

Fluorination is a versatile and valuable modification for numerous systems, and F NMR spectroscopy is the premier method for their structural characterization. F chemical shift anisotropy is a sensitive probe of structure and dynamics, even though F chemical shift tensors have been reported for only a handful of systems to date. Here, we explore γ-encoded R-symmetry based recoupling sequences for the determination of F chemical shift tensors in fully protonated organic solids at high, 60-100 kHz MAS frequencies. We show that the performance of F-RNCSA experiments improves with increasing MAS frequencies, and that H decoupling is required to determine accurate chemical shift tensor parameters. In addition, these sequences are tolerant to B-field inhomogeneity making them suitable for a wide range of systems and experimental conditions.