Our research is focused on understanding the systematics of packing in molecular crystals, a subject of great importance to the chemical and pharmaceutical industries. Our basic approach is to know the characteristics of "normal" crystal structures and to investigate the exceptions. A current project is the investigation of phases of [M(H₂O)₂(15-crown-5)](NO₃)₂, M = Mg, Mn, Fe, Co, Ni, Cu, and Zn. All M ions give at least two phases. All but one of the 18 structures known so far have more than one formula unit in the asymmetric unit; in three structures there are eight. The structures are all related, but the alternation patterns of the conformationally enantiomeric cations differ. In most cases the phases are related by reversible phase transitions during which crystals remain single. By next summer this project will be to the point of

resolving remaining questions. The analogous M = Cd complexes need to be studied because searches of the Cambridge Structural Database show that this is the only remaining divalent metal ion that can be expected to fit inside a 15-crown-5 ligand. The undergraduate student will, under careful supervision, synthesize the complex in water from Cd(NO₃)₂.4H₂O and 15-crown-5, will grow crystals, and will determine the crystal structure(s). DSC measurements will be made on powder samples in the range -100 to +50° C because phase transitions are expected. Crystallographic experiments will be done at temperatures above and/or below room temperature as suggested by the DSC traces. The student will also reinvestigate some structures of [M((H₂O)₂(15-crown-5)]²⁺ that have been reported to be disordered. We believe that many of these structures are ordered but in larger unit cells. Examples for which the syntheses are simple include [M((H₂O)(15-crown-5)]Cl₂.4H₂O, M = Mg and Co. Summer undergraduate students have worked in the X-Ray Lab in the past. Such students learn how structural results are obtained, interpreted, put in context, and reported in the literature, and gain experience with state-of-the-art instrumentation and software under the guidance of the professor, the staff crystallographer, and graduate students. Because the undergraduates spend lots of time using visualization software, they learn a great deal about intermolecular interactions (especially hydrogen bonding) and symmetry.

Further details of Dr. Brock's work are given on the Chemistry website.