Crystal architecture: The beauties of supermolecular organizations

Alajos Kálmán

X-ray analysis of three 2-hydroxy-1-cyclopentane carboxylic acids and three analogous carboxamides revealed five basic forms of supermolecular close packing mediated by two intermolecular hydrogen bonds of types O-HO=C (HB1) and X-HO-H (X = O, NH) (HB2). These basic patterns, valid only for racemic crystals, led to an attempt to predict the missing forms of close packing expectable among similarly 1,2-disubstituted saturated carborings. From the combination of the four basic form of the hydrogen bonds (homochiral helix and tape and heterochiral meander and ring) further 12 patterns could be deduced. To substantiate these canonical forms of close packing analogous 2-hydroxy-1-cyclohexane- -cycloheptane- and -cyclooctane carboxylic acids were also synthesized, crystallized and studied by X-ray diffraction. The new structures demonstrated four novel patterns. The pattern with infinite rows of R22(12) dimers - joined alternatively by the HB1 and HB2 bonds - found in the crystals of trans-2-hydroxy-1-cyclooctane carboxylic acid (8T), suggested a new interpretation of the canonical forms. Indeed, a simple topological combination of the heterochiral OC and OH dimers and their homochiral associations (helices) induces 13 of the 17 patterns of close packing. At the moment, the revealed crystal structures (14) represent 9 canonical forms. Conclusion, these results are only the peak(s) of an iceberg.