HYDRATES AND HYDROGEN BONDING: CRYSTAL CHEMISTRY OF AND WITH SOME SELECTED ACIDS AND BASES Dietrich Mootz, Institut fr Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universitt, 40225 Dsseldorf, Germany.

Work in this laboratory over the last few years on hydrated and other hydrogen-bonded systems and crystal structures, in part already published, will be surveyed. The emphasis is on some twenty low-melting hydrates of Brnsted-type small-molecule acids and bases, weak and strong, of inorganic and organic chemistry. Their formation has been studied using thermal analysis and temperature-dependent X-ray powder diffraction. Crystals for structure determination have typically been grown in situ on a low-temperature diffractometer by miniature zone melting [1].

The bases selected are charged as well as uncharged, like cesium and tetramethylammonium hydroxide [2] on one hand and certain new phosphazenes, aliphatic and aromatic amines [3], and the prototype synthetic macrocyclic polyether 18-crown-6 [4] on the other. The acids are mainly halogenoacetic ones [5].

In contrast to the charged-base hydrates, ionic a priori, all hydrates of the uncharged bases, even of the highly basic phosphazenes, are found to be molecular. The acid hydrates are partly also molecular and partly H5O2+ salts, including the unusual case of either type realized, with a lower and a higher hydrate, by the same acid. Some of the higher hydrates are new representatives of the polyhedral clathrate or semi-clathrate hydrates. Others contain characteristic 2D water layers. These display various patterns of condensed four-, five- and/or six-membered rings and in some cases can clearly be attributed to their low temperature of formation.

Particular features of hydrogen bonding will be discussed also for the anhydrous acids and some interacid adducts. They embrace a new 1D chain conformer for a substituted acetic acid and the reversal of all OH...O bonds in the otherwise unaltered chain of formic acid when this is cocrystallized with hydrogen fluoride.

[1] Brodalla et al.: J. Appl. Crystallogr. 18 (1985) 316.

[2] Mootz & Stben: J. Am. Chem. Soc. 116 (1994) 4141.

[3] Mootz & Born: Z. Naturforsch. 49b (1994) 243;

Born et al.: Z. Naturforsch. 50b (1995) 101;

Stben & Mootz: J. Incl. Phenom. 22 (1995) 145.

[4] Mootz et al.: J. Am. Chem. Soc. 116 (1994) 12045.

[5] Schilling et al.: J. Fluorine Chem. 73 (1995) 225;

Schilling & Mootz: J. Fluorine Chem. 74 (1995) 255.