Structure of a high-pressure phase of vanadium pentoxide, beta-V2O5.

A high-pressure phase of vanadium pentoxide, denoted beta-V2O5, has been prepared at P = 6.0 GPa and T = 1073 K. The crystal structure of beta-V2O5 has been studied by X-ray and neutron powder diffraction, and high-resolution transmission electron microscopy. The V atoms are six-coordinated within distorted VO6 octahedra. The structure is built up of quadruple units of edge-sharing VO6 octahedra linked by sharing edges along [010] and mutually connected by sharing corners along [001]. This arrangement forms layers of V4O10 composition in planes parallel to (100). The layers are mutually held together by weak forces. beta-V2O5 is metastable and transforms to alpha-V2O5 at 643-653 K under ambient pressure. Structural relationships between beta- and alpha-V2O5, and between beta-V2O5 and B-Ta2O5-type structures are discussed. The high-pressure beta-V2O5 layer structure can be considered as the parent of a new series of vanadium oxide bronzes with cations intercalated between the layers.

Structures and phase transitions of B-Ta2O5 and Z-Ta2O5: two high-pressure forms of Ta2O5.

A sample of Ta2O5, ditantalum pentaoxide, heat-treated in a "toroid"-type high-pressure chamber at P = 8 GPa and T = 1470 K, was studied by X-ray powder diffraction and high-resolution transmission electron microscopy (HRTEM). Two high-pressure modifications of Ta2O5, isostructural with B-Nb2O5 and Z-Nb2O5, were identified from the X-ray powder pattern. Both structures were refined by the Rietveld method from the X-ray diffraction data: B-Ta2O5, a = 12.7853 (4), b = 4.8537 (1), c = 5.5276 (2) A, beta = 104.264 (2)degrees, V = 332.45 A3, Z = 4, space group C2/c; Z-Ta2O5, a = 5.2252 (1), b = 4.6991 (1), c = 5.8534 (1) A, beta = 108.200 (2)degrees, V = 136.53 A3, Z = 2, space group C2. The Z-Ta2O5 modification is new. The Ta atoms are six-coordinated in B-Ta2O5 and seven-coordinated in Z-Ta2O5. The two structures are closely related, which makes an intergrowth and a transformation between them possible. An idealized model of the intergrowth structure has been given. The HRTEM study showed defect-rich B-Ta2O5 crystals, which could be interpreted as an intergrowth between the B-Ta2O5 and Z-Ta2O5 phases.

Resolution and absolute stereochemistry of 6,7-dimethoxy-4-phenyl- 1,2,3,4-tetrahydroisoquinoline. The crystal structure of the R-hydrochloride salt form.

Stereospecific multistep synthesis and resolution of 6,7-dimethoxy-4- phenyl-1,2,3,4-tetrahydroisoquinoline (3) has been achieved from its racemic base. The absolute configurations of the optical antipodes converted into their hydrochloride salt forms have been determined by X-ray diffractometric analysis, thus permitting assignment of the antipodes as the (+)-(4R)-3 and (-)-(4S)-3 enantiomers. The crystal structures of the two enantiomers are related as mirror images and only the (4R)-3.HCl form has been fully determined by three-dimensional X-ray diffraction. In the solid state, the carbon atoms of the two methoxy groups deviate slightly from the benzene-ring plane and the chirally oriented phenyl substituent is almost perpendicularly tilted out of conjugation with the isoquinoline system. Examination of the enantiomers in several biochemical tests for 5-HT, NE and DA uptake inhibition-activity revealed an exclusive preference for the (4S)-enantiomer. These results are in accord with previous suggestions that the S-configurational state of the 4-phenyl substituent is important for biological activity.