Epitaxial NbC thin films were grown by a chemical solution technique, polymer assisted deposition. High quality epitaxial NbC film showed a transition temperature of 10 K and a hardness of 19.54 GPa.
Solid-state NMR (SSNMR) and X-ray powder diffraction (XRPD) allow a study of a novel and stable
polymorph of paclitaxel (Taxol) with two molecules per asymmetric unit (Z‘) in the P212121 space group. The
asymmetric unit volume is 2167 Å3, about four times larger than that previously characterized in combined XRPD/SSNMR studies. The method, employing SSNMR constraints, allows the XRPD Rietveld analysis to establish many
of the lattice details that otherwise would be unavailable. NMR structural constraints are provided by isotropic
shifts and three-dimensional (3D) chemical shift tensors (CST), which are determined by ab initio quantum mechanical
calculations. CST data give highly sensitive information on short-range structural features such as intra-atomic
distances (particularly for proton positions that are undetermined with XRPD methods) and short-range valence
angles that exhibit relatively poor sensitivity in reasonably large microcrystalline powders. Conversely, space group
symmetry, unit cell volumes, long-range cell dimensions, and dihedral angles of extended chains are estimated
with XRPD measurements. Corroboration of many structural parameters by combined quantum mechanical, SSNMR,
and XRPD results indicate the efficacy of these combined approaches in relatively sizable microcrystalline powders.
The population of the asymmetric unit, Z‘ = 2 is clearly observed even in the one-dimensional isotropic 13C spectra,
which also confirmed the stability of the polymorph over a three-year period. This structural determination depends
specifically on the agreement between previous SSNMR CSTs and single crystal results for baccatin, the rigid part
of paclitaxel. Hence, CST data provide a reasonable initial model for the early iterative steps of a Rietveld analysis
of XRPD data for a new polymorph of Taxol.
A simple model is used to show that, in principle, random close packing of equal disks on a plane should be stable when the packing fraction is 0.813, the average number of contacts per disk is 3.42, and the connectivity of the simplicial net is 4. The assembly is unstable with respect to shear stresses, which will be a consequence of compressive stresses applied to the randomly packed assembly. In practice, the packing fraction of the assembly will increase until it reaches the value associated with the triangulated simplicial net, the regularly packed disk assembly. ͓S1063-651X͑98͒07906-9͔
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