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, ¹⁷ O and ² H NMR analyses of crystalline ibuprofen and terephthalic acid are reported. First, improved ¹⁷ O-labelling protocols of these molecules are described using mechanochemistry. Then, dynamics occurring around the carboxylic groups of ibuprofen are studied considering variable temperature ¹⁷ O and ² H 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 ¹⁷ O resonances at high temperatures cannot be explained by the sole presence of one of these motions. Lastly, preliminary experiments were performed with a ² H-¹⁷ O diplexer connected to the probe. Such configurations can allow, among others, ² H and ¹⁷ O 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 ¹⁷ O and ² H NMR.