Variable-temperature ¹⁷O NMR studies allow quantitative evaluation of molecular dynamics in organic solids

We report a comprehensive variable-temperature solid-state ¹⁷O NMR study of three ¹⁷O-labeled crystalline sulfonic acids: 2-aminoethane-1-sulfonic acid (taurine, T), 3-aminopropane-1-sulfonic acid (homotaurine, HT), and 4-aminobutane-1-sulfonic acid (ABSA). In the solid state, all three compounds exist as zwitterionic structures, NH ₃ ⁺-R-SO ₃ ^-, in which the SO ₃ ^- group is involved in various degrees of O•••H-N hydrogen bonding. High-quality ¹⁷O NMR spectra have been obtained for all three compounds under both static and magic angle spinning (MAS) conditions at 21.1 T, allowing the complete set of ¹⁷O NMR tensor parameters to be measured. Assignment of the observed ¹⁷O NMR parameters to the correct oxygen sites in the crystal lattice was achieved with the aid of DFT calculations. By modeling the temperature dependence of ¹⁷O NMR powder line shapes, we have not only confirmed that the SO ₃ ^- groups in these compounds undergo a 3-fold rotational jump mechanism but also extracted the corresponding jump rates (10 ²-10 ⁵ s ^-1) and the associated activation energies (E _a) for this process (E _a = 48 ± 7, 42 ± 3, and 45 ± 1 kJ mol ^-1 for T, HT, and ABSA, respectively). This is the first time that SO ₃ ^- rotational dynamics have been directly probed by solid-state ¹⁷O NMR. Using the experimental activation energies for SO ₃ ^- rotation, we were able to evaluate quantitatively the total hydrogen bond energy that each SO ₃ ^- group is involved in within the crystal lattice. The activation energies also correlate with calculated rotational energy barriers. This work provides a clear illustration of the utility of solid-state ¹⁷O NMR in quantifying dynamic processes occurring in organic solids. Similar studies applied to selectively ¹⁷O-labeled biomolecules would appear to be very feasible.