Bacterial copolymers from Alcaligenes eutrophus containing ß-hydroxybutyrate and ß-hydroxyvalerate monomer units were characterized by X-ray diffraction, differential scanning calorimetry, solution NMR, and solid-state 13C NMR. The copolyesters were shown to be statistically random and of high crystallinity (>60%) throughout a range of compositions varying from 0 to 47 mol % ß-hydroxyvalerate units. A minimum in the melting point vs. composition curve was found at approximately 30 mol % ß-hydroxyvalerate. Only two crystal forms were detected: the ß-hydroxybutyrate crystalline phase on one side of the melting point minimum and the ß-hydroxyvalerate crystalline phase on the other. Solid-state 13C NMR spectra support the conclusion that the poly(d-hydroxybutyrate-co-d-hydroxyvalerate) system is isodimorphic with each crystalline phase accommodating the repeating unit of the other monomer as part of its organized structure.
The location of the fluoride ion in tetrapropylammonium fluoride silicalite-1 ([TPA]-F-[Si-MFI]), an as-synthesized siliceous zeolite with the MFI topology, has been unambiguously determined using solidstate NMR experiments alone. With the 1 Hf 29 Si CP-INADEQUATE experiment, the 12 peaks in the highly resolved 29 Si MAS NMR spectrum of [TPA]-F-[Si-MFI] were assigned. Using these peak assignments it was possible to perform 1 H/ 19 F/ 29 Si triple resonance CP, REDOR, and TEDOR experiments to measure F-Si distances and thus locate the fluoride ion. It is covalently bonded to Si-9 in the [4 1 5 2 6 2 ] cage of the zeolite framework and exchanges between two "mirror-related" Si-9 sites, making them equivalent on the NMR time scale. The importance of this result and the general applicability of the approach are discussed.
A new 29Si solid-state MAS NMR experiment is described for investigating the framework structures of pure silica zeolites. The symmetry-based homonuclear dipolar recoupling sequence SR26411 has been incorporated into a two-dimensional NMR experiment to probe the Si-O-Si bonding connectivities and long-range Si-Si distances in zeolite frameworks. This dipolar recoupling sequence is shown to have a number of advantages over the J-coupling-based INADEQUATE experiment. For the clathrasil Sigma-2, it is demonstrated that there is excellent agreement between experimental double-quantum build-up curves obtained from a series of two-dimensional double-quantum correlation spectra and simulated curves which consider all Si-Si distances out to 8 A. This result suggests that this experiment could be used to solve zeolite frameworks with unknown structures.
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