Optical rotation of tartaric acid crystals finally determined

(Editors note: After hearing Bart Kahr’s presentation at the XIth Symposium on Organic Crystal Chemistry, I asked him to contribute a brief description of the determination of optical rotation of tartaric acid.)

Crystals of tartaric acid and its salts recur in the story of the development of the sciences of stereochemistry and molecular chirality. Pasteur, in 1848, used the sodium ammonium salt to establish the first link between macroscopic chirality, the shapes of hemihedral crystals, and microscopic chirality, the sign of the optical rotation of molecules in solution.¹ Swapping ammonium for rubidium, Bijvoet established the absolute configuration of a molecule for the first time by exploiting the breakdown of Friedel’s Law in the presence of anomalous dispersion. ² Most recently, in 1997, Mucha, Stadnicka, Kaminsky and Glazer reported the optical rotation tensor of tartaric acid crystals, a measurement that is essential for the interpretation of the optical rotatory power in terms of structure.³ Appearing nearly a century and a half after Pasteur’s experiments on the optical rotation of dissolved tartrate crystals, this last paper, by its tardiness, tells a story in its own right, the unhappy marriage of crystals and chiroptics.

Despite the fact that the first measurement of optical rotation was made on a section of quartz by Arago,⁴ Pasteur had no hope of measuring the optical rotation of his tartrate crystals because the large linear refractive index anisotropy has a much greater influence on the state of polarization of light passing through the sample than does chirality. As the science of crystal optics progressed throughout the nineteenth century, researchers identified the two optic axes in tartaric acid, those very special directions where the refractive index anisotropy disappears. When optical rotation was measured for light passed along these axes, researchers were struck with the most confounding result; the optical rotation for dextrorotatory tartaric acid in solution was along both optic axes levorotatory.⁵ Where were the dextrorotatory directions? Did the sign of rotation change upon crystallization? These questions remained unanswered until Mucha and Stadnicka from the Jagiellonian U. brought this problem to the attention of Kaminsky and Glazer, scientists at Oxford U. Building upon advances in chiroptics first introduced in 1983 by Kobayahsi and Uesu⁶ at Waseda U. the researchers from Oxford U. introduced refinements^7,8 that ultimately produced a general, robust strategy for measuring optical rotation in low symmetry crystals. Application of these techniques finally revealed the orientational dependence of optical rotation in tartaric acid crystals as shown in Figure 1.

If the Science Citation Index is any indication, the work of Mucha et al. has scarcely received the attention it deserves. Few measurements in science are so difficult as to require 150 years. In the case of the optical rotation of tartaric acid crystals, so much time had elapsed since the work of Pasteur that we forgot what we did not know, principally how to measure the orientational dependence of optical rotation in molecules and crystals. Perhaps this note will bring a fine, historic study to the attention of a wider audience.

Bart Kahr, U. of Washington
Seattle, WA USA

References

[1] L. Pasteur, Comptes Rendus, 1849, 29, 297.

[2] J. M. Bijvoet, A. F. Peerdeman, A. J. van Bommel, Nature (London), 1951, 168, 271.

[3] D. Mucha, K. Stadnicka, W. Kaminsky, A. M. Glazer, Journal of Physics, Condensed Matter, 9, 10829.

[4] D.-F. Arago, Memoire de le Institut Paris, 1811, Part I, 93.

[5] A. V. Shubnikov, Principles of Optical Crystallography, Consultants Bureau, New York, 1960, p 141.

[6] J. Kobayashi, Y. Uesu, Journal of Applied Crystallography, 1983, 16, 204.

[7] W. Kaminsky, S. Haussuehl, Z. Kristallogr., 1993, 203, 79-91.

[8] W. Kaminsky, A.M. Glazer, Ferroelectrics, 1996, 183, 133-141.