화학공학소재연구정보센터
Journal of Physical Chemistry B, Vol.110, No.51, 25751-25760, 2006
Structure and dynamics of perfluoroalkane/beta-cyclodextrin inclusion compounds as studied by solid-state F-19 MAS and H-1 -> F-19 CP/MAS NMR spectroscopy
The molecular structure and dynamics of novel inclusion compounds (ICs) consisting of n-perfluoroalkane (PFA) guests and beta-cyclodextrin (beta-CD) host (PFA/beta-CD) have been investigated using F-19 magic angle spinning (MAS) and H-1 -> F-19 cross polarization (CP)/MAS NMR spectroscopy with the aid of thermal analyses, FT-IR spectroscopy, and X-ray diffraction method. The ICs of C9F20/beta-CD and C20F42/beta-CD were successfully obtained as precipitates from mixtures of respective PFAs and saturated aqueous solution of beta-CD. The wide-angle X-ray diffraction (WAXD) revealed that C9F20/beta-CD forms a channel-type crystallite, while C20F42/beta-CD is nearly amorphous at room temperature. The structural orders in both ICs increase at elevated temperatures. The F-19 NMR signals obtained by the direct polarization (DP) method for PFA/beta-CD are resonated at higher frequencies than those for original PFA. This can be ascribed to the lower dielectric environment of the beta-CD cavity. Above 80 degrees C, H-1 -> F-19 CP/MAS NMR technique revealed that C9F20 molecules undergo vigorous molecular motion and partly come out of the beta-CD channel. However, the guests hardly degrade or evaporate unless the host is pyrolytically decomposed above ca. 300 degrees C. The spin-lattice relaxation times in the laboratory frame for F-19 (T-1(F)) are almost identical for all the fluorines in PFA/beta-CD at each temperature, while significantly different values were observed for fluorines in neat PFA. This indicates that effective intramolecular spin diffusion occurs within a PFA molecule included in beta-CD.