Revealing intermolecular hydrogen bonding structure and dynamics in a deep eutectic pharmaceutical by magic-angle spinning NMR spectroscopy.

Liquid forms of pharmaceuticals (ionic liquids and deep eutectic solvents) offer a number of potential advantages over solid-state drugs; a key question is the role of intermolecular hydrogen bonding interactions in enabling membrane transport. Characterization is challenging since high sample viscosities, typical of liquid pharmaceutical formulations, hamper the use of conventional solution NMR at ambient temperature. Here, we report the application of magic-angle spinning (MAS) NMR spectroscopy to the deep eutectic pharmaceutical, lidocaine ibuprofen. Using variable temperature MAS NMR, the neat system, at a fixed molar ratio, can be studied over a wide range of temperatures, characterized by changing mobility, using a single experimental setup. Specific intermolecular hydrogen bonding interactions are identified by two-dimensional H-1-H-1 NOESY and ROESY MAS NMR experiments. Hydrogen-bonding dynamics are quantitatively determined by following the chemical exchange process between the labile protons by means of line-width analysis of variable temperature H-1 MAS NMR spectra.