A Homonuclear Rotational Echo Double-Resonance Method for Measuring Site-Resolved Distance Distributions in I = 1/2 Spin Pairs, Clusters, and Multispin Systems

摘要

The unique power and potential of high-resolution solid-state NMR spectroscopy to provide structure information at the atomic level for complex, disordered, and molecular solids have been amply demonstrated. In particular, the analysis of intermiclear dipole-dipole couplings can provide information about distances, bond connectivities, and spatial spin distributions. For heteronuclear spin systems, one of the most powerful methods to accomplish this in a site-resolved fashion has been the rotational echo double-resonance (REDOR) technique, where a difference signal is measured of signal amplitudes S0 with the interaction absent (owing to magic-angle spinning, MAS) and reduced signal amplitudes S' with the interaction recoupled (through application of inversion pulses during the rotor period). REDOR has been widely used for measuring intermiclear distances and their distributions in biological systems and inorganic materials alike and has been adapted in many different versions and variants. In contrast, an analogous homonuclear difference strategy of similar versatility has up to now not yet been realized for multispin systems. Although a large variety of homonuclear reeoupling approaches have been pub-lished, their calibration to yield information on dipolar couplings on the basis of simulations or model compound work has remained a difficult issue. While promising progress was recently reported using improved double-quantum excitation strategies, important remaining issues include the limited efficiency of double-quantum coherence excitation, T2 relaxation, and dipolar truncation effects.The only homonuclear difference method known so far involves an excitation of zero-quantum coherences using the "transverse- echo simple excitation for the dephasing of rotational-echo amplitude (t-SEDRA)" method, where the dephasing of transverse magnetization I_(x1)+I_(x2) is compared with that under the influence of an effective Hamiltonian [Eq. (1)]. While transverse-echo SEDRA does succeed in ehminating the influence of T2 relaxation upon the experimental data, applications to systems beyond isolated two-spin pairs have not been reported so far.