A comparison of NMR distance determinations in the solid state by cross polarization, REDOR, and TEDOR techniques

Solid-state NMR distance determinations using Hartmann-Hahn cross polarization between spin-1/2 nuclei (F-19/Si-29) are reported. F-19 –> Si-29 polarization transfer for the T-1 silicon site in the clathrasil octadecasil shows an oscillatory behaviour as a function of the contact time. These oscillations were observed for non-spinning powder samples and also for different spinning speeds at the various MAS sideband matching conditions. Three analytical functions that allow efficient nonlinear least-square regression analyses of the experimental data to determine the internuclear distances for non-spinning powder samples as well as at the +/-1 and +/-2 MAS sideband matching conditions are reported. Using these functions, a F-19-Si-29 distance of 2.53 +/- 0.04 Angstrom was determined for the T-1 silicon in octadecasil from fitting of the oscillatory behaviour. This distance is in good agreement with that known from the X-ray structure and the previous F-19/Si-29 REDOR and TEDOR distance measurements. The advantages and limits of the different dipolar-based NMR techniques for heteronuclear distance determinations are examined and discussed.