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Variable-Temperature 17O NMR Studies Allow Quantitative Evaluation of Molecular Dynamics in Organic Solids

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posted on 05.09.2012, 00:00 by Xianqi Kong, Luke A. O’Dell, Victor Terskikh, Eric Ye, Ruiyao Wang, Gang Wu
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, NH3+–R–SO3, in which the SO3 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 SO3 groups in these compounds undergo a 3-fold rotational jump mechanism but also extracted the corresponding jump rates (102–105 s–1) and the associated activation energies (Ea) for this process (Ea = 48 ± 7, 42 ± 3, and 45 ± 1 kJ mol–1 for T, HT, and ABSA, respectively). This is the first time that SO3 rotational dynamics have been directly probed by solid-state 17O NMR. Using the experimental activation energies for SO3 rotation, we were able to evaluate quantitatively the total hydrogen bond energy that each SO3 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.

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