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-H•••O=C
(HB1) and X-H•••O-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.