19F/29Si Distance Determinations in Fluoride-Containing Octadecasil from Solid-State NMR Measurements

In the present work we report results from CP, REDOR, and TEDOR NMR experiments carried out to locate the fluoride anions within the three-dimensional framework of the silicate octadecasil. Accurate Si−F distances were obtained through measurements of the 19F/29Si dipolar couplings from these experiments. Weighted, nonlinear least-squares fittings of the REDOR and TEDOR data for the silicons in the D4R units gave an average value of D = 1150 ± 50 Hz from which the Si−F interatomic distance was calculated to be 2.69 ± 0.04 Å. This distance is in excellent agreement with the value of 2.63 Å reported from the X-ray determined single crystal structure and confirms that the F- anions are located inside the D4R units. Calculations and fittings of the REDOR and TEDOR data for the silicons not in the D4R gave D = 120 ± 20 Hz when the specific geometrical (tetrahedral) arrangement of the multiple equivalent fluoride anions surrounding the Si nuclei in these T-2 sites was taken into account. This yields a Si−F distance of 5.7 ± 0.4 Å, compared to the value 5.69 Å obtained from the diffraction measurements. Factors influencing the selection of the optimum experiments to perform in different situations and the dependence of the results on experimental variables are discussed. These studies clearly establish the viability and reliability of 19F/29Si REDOR and TEDOR NMR experiments to obtain distance information which can be used to determine the precise location of fluorine-containing species within such framework structures or in suitable cases to assist in the determination of the framework structure itself where it is unknown.