rial in the other. The greater shift (than in (b)) appears related to the ring-oxygen since it has been detected for the axial and equatorial hydrogens of the methylene group of /3-D-xylopyranose tetraacetate.

No abstract

Gels containing a poly(boraziny1 amine) and tetrahydrofuran were processed by C 0 2 supercritical drying techniques followed by pyrolysis. The resulting BN ceramic aerogels are highly porous, and the microstructure, porosity, and surface area characteristics have been examined. The aerogels show excellent thermal stability exhibiting surface areas in excess of 350 m'/g and porosities greater than 0.8 even when heated in argon at 1500°C for 8 h. By removing solvent via evaporation before supercritical drying, the mean pore radius can be varied between 3.6 and 10 nm.

1,3 5-Trimethylcycloborazane [CH3N(H)BH2]3 is obtained as a mixture of eee and eea isomers in good yield from the reaction of BH3'THF and methylamine. The isomers are separated by fractional crystallization from benzene and characterized by mass, infrared, and , 13C, and nB NMR spectroscopies. The eee isomer crystallizes in the monoclinic space group vD2]/c, with a = 8.514(2) A, = 13.577 (4) A, c = 8.927 (2) A, ß= 112.09 (2)°, Z = 4, K= 956.1 (4) A1 23, and p = 0.89 g cm-3. Least-squares refinement gave RF = 8.3% and = 6.1%. The compound displays a six-membered ring structure with alternating boron and nitrogen atoms in a chair conformation. The methyl groups on the nitrogen atoms are all in equatorial positions. The average B-N bond distance is 1.565 A. The eea isomer crystallizes in the monoclinic space group Cc, with a = 9.481 (3) A, b = 12.477 (4) A, c = 8.329 (2) A, ß = 114.05 (2)°, Z = 4, V = 900.0 (5) A3, and p = 0.95 g cm"3. Least-squares refinement gave RF = 4.8% and = 4.5%. This isomer also has a six-membered ring structure with alternating boron and nitrogen atoms in a chair conformation. Two methyl groups are found in equatorial positions, and one methyl group is found in an axial position. The average B-N distance is 1.578 A.

Atoms in 3), Table VII (Atomic Coordinates and Isotropic Thermal Parameters for Hydrogen Atoms in 3), Table VIII (Bond Lengths and Bond Angles in 3), and spectral/analytical data for compounds 1, 2, 4, 5, and 6 (23 pages); structure factor amplitude tables for 2 and 3(18 pages). Ordering information is given on any current masthead page.