Iain C. Paul
Emeritus Professor of Chemistry
Professor Iain C. Paul attended Glasgow University for his undergraduate and graduate work and received a B.S. in 1959 and his Ph.D. in 1962. He joined the faculty at Illinois in 1964. His main research interests are in organic and structural chemistry.
Research
Our research involved the use of X-ray crystallography to determine the crystal and molecular structures of a number of classes of organic compounds. The major portion of this work was directed toward a study of the crystalline environment in which several types of reactions of organic compounds occur.
Proton Transfer in the Solid State. Many hydroxyl-containing derivatives of aromatic compounds have been shown to exhibit proton transfer in the solid state. Among systems being studied are the polymorphs of naphthazarin, quinizarin and derivatives, and the quinhydrones, complexes of quinones with hydroquinones. Naphthazarin C has been shown to undergo intramolecular proton transfer in the crystal at room temperature by 13C NMR "magic-angle" studies. As a result of this fast exchange process, the hydroxyl hydrogen atoms in the molecule appear disordered in the diffraction experiment. When the crystal is cooled to 110K, a second order phase transition occurs and the molecules become ordered with a freezing of the proton positions. The space group of the crystal changes from P21/c to Pc. Other related systems that have been studied include the quinizarins and the complexes of quinones and hydroquinones, and quinhydrones.
Considerable effort has been devoted towards studies of the influences of polar axes in organic crystals. The Kundt-Burker powder test has been performed on many polar crystals under varying conditions, to try to establish its range of applicability. The results of these tests have been correlated with the crystal structure and with the results of semi-empirical dipole moment calculations. In favorable instances, such methods can lead to the determination of the absolute configuration of a crystal without having recourse to the measurement of Bijvoet pairs.
A search for structural features that favor crystals with polar axes led to a detailed examination of molecules containing both oxime and carboxylic acid groups. We have found this combination of functional groups invariably leads to the formation of chains of molecules formed by carboxyl-oxime hydrogen bonds, rather than centrosymmetric dimers carboxyl-carboxyl or oxime-oxime. Other such combinations of functional groups that give rise to polar chains are being sought.
Other Solid State Processes. Several types of gas-solid reactions, intramolecular rearrangements, and crystals that serve as the medium for chemical reactions are being studied. The photograph on the right shows the experimental difference electron density map of a diacetylene monomer crystal which has undergone approximately 10% polymerization. The solid lines show the monomer structure, while the contours and crosses show portions occupied by atoms of the polymer (ref. 5). (Much of this work is being done in collaboration with Professor D. Y. Curtin.)
Publications
Curtin, D. Y., and Paul, I. C., "Chemical Consequences of the Polar Axis in Organic Solid State Chemistry," Chem. Rev. 81, 525 (1981).
Curtin, D. Y. and Paul, I. C., Gas-Solid Reactions and Polar Crystals, Ed. by G. R. Desiraju, Elsevier, Amsterdam, 331-370 (1987).
Herbstein, F. H., Kapon, M., Reisner, G. M., Lehmann, M. S., Kress, R. B., Wilson, R. B., Shiau, W.-I., Duesler, E. N., Paul, I. C., and Curtin, D. Y., "Polymorphism of Naphthazarin and Its Relation to Solid State Proton Transfer. Neutron X-Ray Diffraction Studies on Naphthazarin C," Proc. Roy. Soc. (London), A399, 295-319 (1985).
Pennington, W. T, Chakraborty, S., Paul, I. C., and Curtin, D. Y., "Crystal Structures of D-(+) and Meso-Dydrobenzoin. Absolute Direction of the Dipole Moment of D- and L-Hydrobenzoin in the Crystal and the Correlation with Crystal Morphology, Pyroelectric Effect, and Absolute Configuration," J. Am. Chem. Soc., 110, 6498-6504 (1988).
