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 ( 19 F/ 29 Si) are reported. 19 F -> 29 Si 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 19 F- 29 Si distance of 2.53 ± 0.04 Å 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 19 F/ 29 Si REDOR and TEDOR distance measurements. The advantages and limits of the different dipolar-based NMR techniques for heteronuclear distance determinations are examined and discussed.Key words: cross polarization (CP), NMR distance measurements, octadecasil, 19 F- 29 Si distances, REDOR, TEDOR.