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

✍ Scribed by Philippe Bertani; Jésus Raya; Pierre Reinheimer; Régis Gougeon; Luc Delmotte; Jérôme Hirschinger

• Publisher: Elsevier Science
• Year: 1999
• Tongue: English
• Weight: 228 KB
• Volume: 13
• Category: Article
• ISSN: 0926-2040
• DOI: 10.1016/s0926-2040(98)00093-9

✦ Synopsis

19 29

Ž . Fr Si Hartmann-Hahn continuous wave cross-polarization CP has been applied under fast magic-angle spinning Ž . MAS to a powder sample of octadecasil. Strong oscillations occur during CP on a sideband matching condition between the isolated 29 Si-19 F spin pairs formed by the silicons in the D4R units and the fluoride anions. The magnitude of the dipolar coupling constant was deduced directly from the line-splitting between the intense singularities of the Pake-like patterns obtained by Fourier transformation of the oscillatory polarization transfer. The corresponding Si-F internuclear distance, r s 2.62 " 0.05 A, is found to be in very good agreement with the X-ray crystal structure and the value of ˚Ž . 2.69 " 0.04 A recently reported from rotational echo double resonance REDOR and transferred echo double resonance Ž . Ž . 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 ; 14 kHz is shown to be necessary for an effective suppression of 19 F-19 F spin diffusion. The Ž . influences of experimental missettings and radiofrequency RF field inhomogeneity are taken into account.