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Fast-MAS solid-state NMR of efavirenz: Insights into spectral assignments and molecular asymmetry
Ziqian Li1, Dina Wang1, Hailu Zhang1
1Interdisciplinary Institute of NMR and Molecular Sciences, Academy of Advanced Interdisciplinary Research, Key Laboratory of Hubei Province for Coal Conversion and New Carbon Materials, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan, 430081, China.
Abstract:
Solid-state nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for characterizing pharmaceutical solids, yet inconsistencies in spectral assignment can hinder accurate structural interpretation. In this study, we revisit the solid-state NMR of efavirenz (EFV), a widely used anti-HIV drug, by employing fast magic-angle spinning (MAS) at 37 kHz. Through a combination of one-dimensional (1H, 19F, 13C) and two-dimensional correlation (13C-1H and 13C-19F) experiments, we resolve previously disputed assignments. Critically, we correct the misassignment of the aromatic C3 and C4 resonances, which is unambiguously confirmed by a 2D 13C-1H correlation spectrum using the key HN-C4 proximity. The 19F spectrum reveals two distinct peaks with an integral ratio of 1:2, directly reflecting the three crystallographically independent molecules in the unit cell and offering a facile diagnostic for molecular asymmetry. 19F-detected 13C-19F correlations further differentiate carbon environments. This work not only rectifies prior assignment errors but also establishes a robust multinuclear approach, integrating 1H, 13C, and 19F NMR, for detailed analysis of fluorinated pharmaceuticals.
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