Looking into the dynamics of molecular crystals of ibuprofen and terephthalic acid using 17O and 2H nuclear magnetic resonance analyses

Abstract

Oxygen-17 and deuterium are two quadrupolar nuclei that are of interest for studying the structure and dynamics of materials by solid-state nuclear magnetic resonance (NMR). Here, 17O and 2H NMR analyses of crystalline ibuprofen and terephthalic acid are reported. First, improved 17O-labelling protocols of these molecules are described using mechanochemistry. Then, dynamics occurring around the carboxylic groups of ibuprofen are studied considering variable temperature 17O and 2H NMR data, as well as computational modelling (including molecular dynamics simulations). More specifically, motions related to the concerted double proton jump and the 180° flip of the H-bonded (–COOH)2 unit in the crystal structure were looked into, and it was found that the merging of the C=O and C–OH 17O resonances at high temperatures cannot be explained by the sole presence of one of these motions. Lastly, preliminary experiments were performed with a 2H–17O diplexer connected to the probe. Such configurations can allow, among others, 2H and 17O NMR spectra to be recorded at different temperatures without needing to tune or to change probe configurations. Overall, this work offers a few leads which could be of use in future studies of other materials using 17O and 2H NMR.