C. Sabelli

A new borate polyanion in the structure of N8[B12O20(OH)4]

Na20.3B20 a. H~O is a new phase synthesized at 523 K under hydrothermal conditions. Crystal data are: a = 8.709 (3), b = 11.917 (5), c = 9.468 (3) ,/k, fl = 96.02 (2) °, space group P2t/c and Z = 4. The atomic positions, including those of the H atoms, were deter- mined by direct methods and difference Fourier syntheses, and refined by least-squares calculations to an R of 0.030 for 2077 reflexions measured on a computer-controlled automatic diffractometer. The basic structural unit is the isolated polyanion (B12020(OH)4 )8- with point symmetry i. The poly- anion, which is new in the crystal chemistry of borates, is built up by the linkage of three tetrahedra and three triangles, doubled by the symmetry centre. It can be described as a loop of six six-membered B-O rings. Of the three independent oxygens linked to only one triangular boron, only two exist as hydroxyl groups. Na-O polyhedra form a tight network by sharing corners, edges and faces. All O atoms linked to Na atoms belong at the same time to the B-O polyanions. The structural formula is Nas(B 12020(OH)4).

Alunogen: Its structure and twinning

The crystal structure of alunogen, space group\(P\bar 1\),a=7.425(2),b=26.975(2),c=6.0608(5) A, α=90.03(1), β=97.66(2), γ=91.94(1)°, was determined by means of photographic X-ray data collected from a twinned crystal. The structure was solved by direct methods and refined by least-squares to anR index of 0.064. The dominant structural feature is the three-dimensional hydrogen bonding system which connects the discrete Al(H2O)6 octahedra with the SO4 tetrahedra (two and three respectively in the asymmetric unit). As to the water molecules, twelve behave as ligands coordinated to the Al atoms; four lie in a channel running parallel to thec axis; a fifth one with partial occupancy also lies in the channel and forms still weaker bonds. The crystal chemical formula of the investigated sample is therefore [Al(H2O)6]2(SO4)3·4.4H2O. An explanation of twinning, with (010) as the twin plane, is given.

Crystal and molecular structure of an octamer of acetonitrile oxide

Crystals of (CH3CNO)8 are monoclinic, space group C2/c, with Z= 8 in a unit cell of dimensions a= 23.915(7), b=17.467(6), c= 11.169(4)A, β= 100.71(6)°. Diffractometer data yielded 2169 observed reflections. The structure was solved by direct methods and refined by a least-squares procedure to R 0.071. The molecule is a cyclic octamer of acetonitrile oxide with anti-CH3 configuration at all double bonds.