Rotor Synchronized MAS Two‐Dimensional Exchange NMR in Solids. Principles and Applications

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Abstract
The principles of the rotor synchronized magic angle spinning (MAS) two‐dimensional exchange NMR, first proposed by Veeman and coworkers are reviewed, with particular emphasis on situations where chemical exchange in solids proceeds in concert with molecular reorientation. Calculations of cross peak intensities as function of the ratio between the chemical shift anisotropy and the spinning rate are presented for several cases. These calculations emphasize the advantage of using slow spinning rates (ωR < ωLΔσ) in such experiments when detailed information about mechanistic pathways in solids is sought. Three applications of the method to solid systems using carbon‐13 NMR are described. These include: (a) Trimethylsulfoxonium iodide, in which the molecules undergo 120°‐jumps about the molecular C3 symmetry axis; (b) Tropolone, where the tautomeric hydrogen shift is found to be a consequence of the self diffusion within the crystal lattice, and in general accompanied by molecular reorientation. Here the two‐dimensional pattern is used to obtain information about the various mechanisms of the diffusion process; (c) Bullvalene, where a quantitative analysis of the cross peak intensities as function of the mixing time provides kinetic information on two independent processes, viz. symmetric threefold jumps and a concerted Cope rearrangement‐molecular reorientation reaction.