The structure of the dihydrate of the nonreducing disaccharide a,a-trehalose (a-D-glucopyranosyl a-D-glucopyranoside) has been determined independently by direct methods in three different laboratories. There are four units C~2H22Ol~.2H20 in an orthorhombic cell (P212a2a) with a= 12"230 (2), Comparison of coordinates suggests that their standard errors may have been underestimated by a factor of ~2. The a,a-trehalose molecule has approximate C2 symmetry, but there are some significant structural differences between the two chemically equivalent halves. The angles at the ring oxygen atoms are 114-0 and 114-2°; the angle at the glycosidic oxygen is 115.8 °. The two rings have Cl conformations; the conformations about the linking C-O bonds are stabilized by the conformational 6anomeric effect' and also by a complex system of hydrogen bonds involving all of the hydroxyl groups, both water molecules, and one ring oxygen. By hydrogen bonding each glucopyranosy] moiety is linked to the other in the same molecule indirectly through two different water molecules. The helical arrangement of hydrogen bonds about the screw axes parallel to e suggests the possibility of observing ferroelectric behavior.

The crystal structure of sodium 5,5-diethylbarbiturate NaC8N203Hlx (sodium barbital) has been determined from X-ray diffraction data measured on an automatic diffractometer. R = 0.025. The crystal data are: P212121, a--6.724 (1), b= 11.950 (2), c= 12.129 (2)/~, Z=4, Din= 1"408 g.cm-3. The conformation of the barbital ion is similar to that of the barbital molecule. Deprotonization of the pyrimidine ring at N(3) decreases the internal bond angle at N(3) by 7 ° and increases those at the neighbouring atoms C(2) and C(4) by 5 °. The greater part of the formal negative charge seems to be equally distributed between the two oxygen atoms 0(2) and 0(4). The molecules are hydrogen bonded (NH... O = 2.84/~) to form ribbons along the a axis. The sodium atom is tetrahedrally coordinated by barbital oxygen atoms, with tetrahedra sharing corners to form columns parallel to a.

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The crystal structures of potassium tetrametaphosphimate tetrahydrate [K40aO2NH)4.4H20; P21[c; a--6.786, b= 10.371, c= 14.335 A; fl=57.91°; Z=2] and caesium tetrametaphosphimate hexahydrate [Cs4(PO2NH)4.6H20; P42/nmc; a=9'693, c=11"306/~; Z=2] were determined from three-dimensional X-ray diffraction intensities measured on an automatic diffractometer. The refinement yielded R values of 0"040 for the potassium salt and 0.055 for the caesium salt (1517 and 489 observed reflexions, respectively). The most interesting feature of the structures is the conformational isomerism of the anionic eight-membered P-N ring derived earlier from crystal vibrational spectra by Steger & Lunkwitz. In good agreement with their conclusions the ring in the potassium salt displays a chair form with almost 2/m symmetry and two opposite phosphorus atoms on the pseudo mirror plane. On the other hand the ring symmetry in the caesium salt is ~2m (mirror planes through nitrogen atoms) and thus higher than ~ as derived from the spectra. The conformation is a saddle form.