Pushing the limit of MQMAS for low-γ quadrupolar nuclei in pharmaceutical hydrochlorides.

Solid-state NMR of quadrupolar nuclei such as 35Cl has become a useful tool to characterize polymorphism in pharmaceutical hydrochlorides. The two-dimensional multiple-quantum magic-angle spinning (MQMAS) experiment can achieve isotropic resolution, and separate quadrupolar line shapes for samples with multiple sites but the pulse sequence efficiency is often low, limiting applications due to the intrinsically low NMR signals and rf field from the low gyromagnetic ratios γ. The use of cosine low-power MQMAS pulse sequences and high magnetic fields is presented to push the limit of MQMAS for insensitive low-γ quadrupolar nuclei. The improved efficiency and fields up to 35.2 T enable the acquisition of MQMAS spectra for pharmaceutical samples with multiple 35Cl sites, large quadrupolar couplings and/or in diluted dosage forms.