A STUDY OF DISORDER IN THE SIO2 HOST LATTICE OF DODECASIL 1H USING SYNCHROTRON-RADIATION

Single-crystal diffraction data of dodecasil 1H (D1H), a clathrasil (S iO2) with 1-aminoadamantane and NH3 as guest molecules, were collected from a small crystal (V = 2.5 x 10(5) mu m3) with synchrotron radiati on (SR) (lambda = 0.78914 angstrom) at the five-circle diffractometer of HASYLAB (DESY, Hamburg, Germany). The space group is P6/mmm with a = 13.826 (3), c = 11.186 (3) angstrom, V = 1852 (1) angstrom3, the con tent of the unit cell is 34SiO2(C10H15NH2)5NH3, F(000) = 1154, D(x) = 2.043 g cm-3, M(r) = 2278.4 g mol-1. The structure refinement is based on 784 unique reflections (R = 2.62%, wR = 1.76%). The results are co mpared with studies using a conventional X-ray source. All O-atom site s were found to be split (average O-O distance 0.5 angstrom) indicatin g a disorder of the SiO2 host framework. The mean Si-O distance, Si-O- Si angle and U(eq) (oxygen) from anisotropic refinement without splitt ing were 1.568 angstrom, 170.7-degrees, 0.065 angstrom2, respectively, changed to 1.596 (11) angstrom, 155 (3)degrees, 0.038 (6) angstrom2. Simulations based on distance least-squares calculations qualitatively match the results of the refinement. Additional data collected on a c onventional diffractometer with monochromatized Mo Kalpha radiation re vealed only limited evidence for split O-atom positions. Anisotropic r efinement of non-split O atoms with data collected at 155 K [a = 13.74 8 (3), c = 11. 128 (3) angstrom] with Mo Kalpha radiation gave only a minor decrease of the large O-atom r.m.s. amplitudes perpendicular to the direction of the Si-O bond. Population parameters > 1.0 for the O atoms obtained from refinement are shown to be an artefact inherent in the least-squares algorithm in the case of split sites. The 1-aminoad amantane guest molecule does not accommodate the symmetry of the host cage. Its threefold axis is inclined about 20-degrees with respect to the sixfold axis of the cage.