Dynamic angle hopping

M.A. Eastman, P.J. Grandinetti, Y.K. Lee, A. Pines, Double-mned hopping-coil probe for dynamic-angle-spinning NMR, J. Magn. Reson. 98 (1992) 333-341. 

When the H-atom becomes dynamically disordered in the hydrogen bond, the electronic structures of the hydrogen-bonded atoms are switched over in the rhythm of the H-hopping. The electronic structure of the H-donor and acceptor atoms controls the O-H and H- O lengths as well as the R-O-H and H-O-R angles. When the H-atom becomes disordered between the H-donor and acceptor groups, the only way to preserve these dimensions is to bend the... 

The temperature-dependent spectra were interpreted in terms of a two-site hop model, in which the deuterons undergo jumps through a dihedral angle of 103°. This type of motion is consistent with gauche-trans conformational transitions. At -88"C these motions appear static on the time scale of the deuterium NMR experiment, and at +85 °C the motions are in the fast exchange limit. The rate constants for these motions were obtained from the calculated spectra. An Arrhenius plot of these data show that the apparent activation energy is 5.8 kcal/mol. (Dynamic mechanical data (20 Hz) fall on the Arrhenius plot.) The transitions have an intermediate rate on the deuterium NMR time scale at 20 °C, with the correlation time for the motion being 7 x 10 6 s at this temperature. 

Figure 7 Two-dimensional exchange spectrum of deuterated dimethylsulfoxide. The strong diagonal ridge reflects molecules that have not reoriented during the mixing time, while the pattern of ellipses off the diagonal shows those that have. The ellipses arise due to the orientational dependence of the quadrupole interaction, the dominant anisotropy here. The distribution of jump angles is shown on the right, and sharply peaked at zero (static molecules) and 72°, the included angle of the C-D bonds as the entire molecule executes hops about its symmetry axis. With this type of experiment, slow to moderate dynamics of molecules and polymers can be followed in atomic level detail. (Reproduced with permission from Schmidt-Rohr K and Spiess HW (1994) Multidimensional Solid-Stale NMR and Polymers. London Academic Press.)...

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