Magnetically aligned nanodiscs enable direct measurement of ¹⁷O residual quadrupolar coupling for small molecules

The use of ¹⁷O in NMR spectroscopy for structural studies has been limited due to its low natural abundance, low gyromagnetic ratio, and quadrupolar relaxation. Previous solution ¹⁷O work has primarily focused on studies of liquids where the ¹⁷O quadrupolar coupling is averaged to zero by isotropic molecular tumbling, and therefore has ignored the structural information contained in this parameter. Here, we use magnetically aligned polymer nanodiscs as an alignment medium to measure residual quadrupolar couplings (RQCs) for ¹⁷O-labelled benzoic acid in the aqueous phase. We show that increasing the magnetic field strength improves spectral sensitivity and resolution and that each satellite peak of the expected pentet pattern resolves clearly at 18.8 T. We observed no significant dependence of the RQC magnitudes on the magnetic field strength. However, changing the orientation of the alignment medium alters the RQC by a consistent factor, suggesting that ¹⁷O RQCs measured in this way can provide reliable orientational information for elucidations of molecular structures.