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

We report a comprehensive variable-temperature solid-state (17)O NMR study of three (17)O-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 (17)O 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 (17)O NMR tensor parameters to be measured. Assignment of the observed (17)O 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 (17)O 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 (17)O 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 (17)O NMR in quantifying dynamic processes occurring in organic solids. Similar studies applied to selectively (17)O-labeled biomolecules would appear to be very feasible.