Seeing Molecular Configuration in Twisted Crystal Form

Structural chemistry began with Louis Pasteur, who first saw the configuration of molecules manifest in the shapes of crystals. Here is reviewed forgotten, century-old efforts to make molecular configuration vivid in polycrystalline ensembles. Resorcinol, and a variety of other simple molecular crystals, when grown from the melt in the presence of resolved chiral additives, such as tartaric acids, form spectacular bull's-eye patterns evident in the petrographic microscope. Concentric, rhythmic optical bands in so-called banded spherulites are tell-tale signs of the twisting of helicoidal crystalline lamellae. The sense of twisting was assayed by correlating displacements of extinction bands on rotation with additive configurations. Thus, the arrangements of atoms in space were made manifest in crystal form – not in the appearance of hemihedral facets, but rather, in the chirality of nonclassical morphologies with Gaussian curvature. The far reaching consequences of these experiments for molecular crystal growth and form, long ago foreseen, are evaluated with the hindsight of an extra century of experience and sophistication. The narrative aims to draw a continuous thread through the work of Pasteur, Grigorii Vyroubov, Frédéric Wallerant, John Monteath Robertson, Hélène Metzger, and Jack Dunitz.