F-19/Si-29 distance determination in fluoride-containing octadecasil by Hartmann-Hahn cross-polarization under fast magic-angle spinning

F-19/Si-29 Hartmann-Hahn continuous wave cross-polarization (CP) has been a pplied under fast magic-angle spinning (MAS) to a powder sample of octadeca sil. Strong oscillations occur during CP on a sideband matching condition b etween the isolated Si-29-F-19 spin pairs formed by the silicons in the D4R units and the fluoride anions. The magnitude of the dipolar coupling const ant was deduced directly from the line-splitting between the intense singul arities of the Fake-Like patterns obtained by Fourier transformation of the oscillatory polarization transfer. The corresponding Si-F internuclear dis tance, r = 2.62 +/- 0.05 Angstrom, is found to be in very good agreement wi th the X-ray crystal structure and the value of 2.69 +/- 0.04 Angstrom rece ntly reported from rotational echo double resonance (REDOR) and transferred echo double resonance (TEDOR) nuclear magnetic resonance (NMR) experiments . Furthermore, the CP technique is still reliable under fast MAS where both REDOR and TEDOR sequences suffer from severe artefacts due to finite pulse lengths. In octadecasil, a spinning frequency of similar to 14 kHz is show n to be necessary for an effective suppression of F-19-F-19 spin diffusion. The influences of experimental missettings and radiofrequency (RF) field i nhomogeneity are taken into account. (C) 1999 Elsevier Science B.V. All rig hts reserved.