M r= 220.27, cubic, Fm3m, a o= 8.134 (1)/~, V= 538.2 (2) A 3, Z = 4, D,, = 2.71 (1), D x = 2.719 (1) gcm -3, 2(MoKct)=0.71073A, p=19.9cm -1, F(000)=424, T = 295 K, R = 0.015 for 89 unique observed reflections. The Si--F bond length, 1.683 (2)/~,, is consistent with those of other hexafluorosilicates.

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An analysis of the structures of ®ve new cocrystals with saturated hydrogen bonding between amines and alcohols is presented. These ®ve are all cocrystals with p-phenylenediamine (PDA) and a mono-or diol. The cocrystals are. These crystals have two distinctly different supramolecular hydrogen-bond patterns, when considering only the hydroxyl and amine groups. There are, in addition, variations in the ways covalently bonded crosslinks connect these hydrogen-bond networks. The graph sets of the hydrogen-bond networks of these and other published saturated amine±alcohol crystals are compared and suggestions made on how to present the graph-set descriptions, especially those involving in®nite two-and three-dimensional networks of intersecting chains which are characteristic of most of these materials.

Crystallographic methods provide the details of molecular structure. Many chemistry texts show ball-and-stick pictures. electron densitv maps. and stereo diagrams based on crystsl structure det&minations. Most chemistry texts have sections devoted to crystals and the determination of their structures by diffraction methods. Many of these eo well beyond introducing the 14 Bravais lattices and the ~r a~~ law. By the time theycomplete their undergraduate training, most chemists have heard the statement: "This configuration has been proved by X-ray crystallographic analysis." Yet probably few have an idea of how that crystal structure determination is done. Though laboratory experience is often the hest way to learn. few crvstalloera~hic experiments are simple enoueh for most undkgrad;aies to perform. ~xperiments designed for the ohvsical cbemistw laboratorv are usuallv restricted to dem&trating the diffraction phenomenon for a cubic crystal using the powder method (1-4). Measurement of the diffraction angles, 28, and use of the Bragg law leads to the identification of the Bravais lattice and the determination of the single lattice constant needed to define the cubic unit cell. This may allow the determination of interatomic distances, since the atomic positions in most very simple crystals, such as NaC1, many pure metals, and even the somewhat more complex diamond, are fixed a t crystallographic special positions in the unit cell by symmetry constraints. Thus. unit cell dimensions alone mav be used to calculate interatomic distances. For example, "the 1.5445-A carboncarbon bond length in diamond is just (,/3)/4 times the 3.5669-A lattice constant. Most atom positions, however, are not fixed by the crystal's symmetry. The information needed to determine the atom positions and displacements caused by thermal motion comes from the relative intensities of the diffracted reflections.Several other experiments require specialized equipment, considerable crystallographic expertise on the partof the instructor, and frequently several weeks of class or lahoratorv time. A sinele crvstal camera can he utilized to determine the lattice coktants of an orthorhombic crystal and determine restrictions on nossible soace erouvs from the observed -.systematic extinctions (51, b A this requires both personnel and equipment available only in crystallographic laboratories. Several possibilities exist for advanced undergraduate students where a large block of time and the necessary specialiled equipment and/or computer programs are available (fi-8). They do closel\~ simulate real rrystal structure determinations; hut most-chemistry students will probably not take a course where these are used.This experiment attempts to overcome the shortcoming of the simpler experiments by using both diffraction-angle and diffraction-intensitv information to determine the lattice constant and a lattice independent molecular parameter, while still emolovine standard X-rav powder diffraction . " -. .techniques. The experiment requires only a little more st...