ScienceWeek

ON THE HISTORY OF CRYSTAL ENGINEERING

In general, in this context, "goniometry" involves the measurement of interfacial angles for the comparison of crystals of different development. William Wollaston (1766-1828) developed in 1809 a reflecting (optical) goniometer for use with small crystals: a fixed mirror is illuminated from a collimator so that part of the parallel beam falls on the crystal, which is fixed on an axis parallel to the mirror and a short distance above it, and is so adjusted that the edge of the facial angle to be measured is parallel to the axis. All the interfacial angles in a given zone can be found by rotation of the crystal.

The following points are made by Mark D. Hollingsworth (Science 2002 295:2410):

1) Legend has it that modern crystallography owes its roots to an accidental discovery reported in 1781 by the French physicist Rene Just Hauey (1743-1822) (1). While admiring a friend's mineral collection, Hauey dropped a particularly large crystal of Iceland spar (calcite), which cleaved into equivalent fragments. With keen insight, Hauey recognized that internal structure was related to external form, and after spending the next years smashing his mineral collection and those of his friends, he reckoned that all crystals were composed of a limited number of building blocks that were stacked together in simple ways. With the subsequent development of optical goniometry, polarized light microscopy, and other physical techniques, 19th-century chemists and crystallographers focused on macroscopic properties of crystals such as birefringence, optical activity, pyroelectricity (electric polarization caused by temperature change), and, later, piezoelectricity (electric polarization under external stress), which was discovered by Pierre and Jacques Curie in 1880. These efforts culminated in Paul Groth's Chemische Krystallographie (2), which documents in five volumes what was known about the external form and physical properties of more than 7000 organic and inorganic crystals that had been characterized by the beginning of the 20th century.

2) Groth's treatise and Hauey's deconstruction of macroscopic crystalline objects provide instructive contrasts with the modus operandi of modern-day solid state organic chemists and "crystal engineers", who have embraced the notion of the supramolecular "synthon" (3) as the critical design element for generating new materials. In its renaissance, as inaugurated by G.M.J. Schmidt and coworkers in the 1960s (4), solid state organic chemistry has focused on the molecular building blocks and their connections with the anticipation that reliable functional group interactions can be used to assemble a variety of useful molecular materials.

3) The synthesis of organic molecules relies on the strength of covalent bonds and on the relative rates of bond-forming processes to lead in a rational and step-wise process to the final product. It is therefore no surprise that many organic chemists have until recently shied away from crystal synthesis. For the supramolecular synthetic chemist, the specific goal is a macroscopic property, and the final product is often a moving target that changes each time the crystal synthesis yields something different from that predicted by the imperfect models we use. The fundamental difficulty for this field is that molecular crystals are held together by a multitude of weak interactions, and a huge number of free energy minima (polymorphs) exist within a few kilojoules/mol of the global minimum. The process of crystal engineering is therefore an iterative one that involves synthesis, crystallography, crystal structure analysis, and computational methods.(5)

References (abridged):

1. J. G. Burke, Origins of the Science of Crystals (Univ. of California Press, Berkeley, CA, 1966), pp. 83-85.

2. P. A. Groth, Chemische Krystallographie (Verlag von Wilhelm Engelmann, Leipzig, 1906-1919), vol. I-V.

3. G. R. Desiraju, Angew. Chem. Int. Ed. 34, 2311 (1995)

4. G. M. J. Schmidt, Pure Appl. Chem. 27, 647 (1971)

5. K. D. M. Harris, M. Tremayne, P. Lightfoot, P. G. Bruce, J. Am. Chem. Soc. 116, 3543 (1994)