F-19 SI-29 DISTANCE DETERMINATIONS IN FLUORIDE-CONTAINING OCTADECASILFROM SOLID-STATE NMR MEASUREMENTS/

In the present work we report results from CP, REDOR, and TEDOR NMR ex periments carried out to locate the fluoride anions within the three-d imensional framework of the silicate octadecasil. Accurate Si-F distan ces were obtained-through measurements of the F-19/Si-29 dipolar coupl ings from these experiments. Weighted, nonlinear least-squares fitting s of the REDOR and TEDOR data for the silicons in the D4R units gave a n average value of D = 1150 +/- 50 Hz from which the Si-F interatomic distance was calculated to be 2.69 +/- 0.04 Angstrom. This distance is in excellent agreement with the value of 2.63 Angstrom 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 = 120 /- 20 Hz when the specific geometrical (tetrahedral) arrangement of th e multiple equivalent fluoride anions surrounding the Si nuclei in the se T-2 sites was taken into account. This yields a Si-F distance of 5. 7 +/- 0.4 Angstrom, compared to the value 5.69 Angstrom obtained from the diffraction measurements. Factors influencing the selection of the optimum experiments to perform in different situations and the depend ence of the results on experimental variables are discussed. These stu dies clearly establish the viability and reliability of F-19/Si-29 RED OR and TEDOR NMR experiments to obtain distance information which can be used to determine the precise location of fluorine-containing speci es within such framework structures or in suitable cases to assist in the determination of the framework structure itself where it is unknow n.