Lord of the Rings

Professor Amnon Stanger of the Schulich Faculty of Chemistry delves into the mysteries of aromatic rings, which continue to tantalize researchers decades after their discovery

Prof. Amnon Stanger. Photo: Yoav Bechar

By Noam Bercovitz

In a feature article published in the scientific journal Chemical Communications at the beginning of 2009, Prof. Amnon Stanger of the Faculty of Chemistry summarized nearly 80 years of research on the group of organic ring molecules known as aromatic molecules. Despite the many years that have passed since the phenomenon of aromatic rings was first described, Stanger reached the surprising conclusion that science has still not attained a conclusive and approved definition of what aromaticity exactly is and there is still no metric to measure it. According to Stanger, the acceptable explanations describe most of the phenomena related to aromaticity but some exceptions raise questions and uncertainty regarding the validity of all the theories. Parenthetically, their name notwithstanding, aromatics have no connection to odors.

The Energy Perspective

The inquiry into aromatic molecules began about the time they were discovered in the middle of the 19^th century. Chemists noted that certain ring molecules such as benzene are very stable despite the fact that they are highly unsaturated; that is, between their carbon atoms are double bonds that normally tend to undergo annexation reactions by non-aromatic substances.

In a double bond there are two pairs of electrons shared by the carbon atoms, delineated as C=C. In a single bond, there is a single pair of common electrons, which is noted as C-C. If we try to draw these bonds within the benzene molecule with six carbon atoms, the normal description is not apparently enough because either of the following two situations is possible: the double bond may be on the right of the carbon, which is at twelve o'clock in the picture below or to its left:

Moreover, by measuring we find that the benzene molecule presents a perfect six-sided structure with all the bonds between the carbon atoms having a uniform length, whereas double bonds are shorter and single bonds are longer.

At the beginning of the 1930s Erich Hükel, using quantum mechanics calculations, showed that under the assumption that benzene has a basic framework of single bonds (also called sigma bonds) between all six carbons, the three remaining pairs of electrons are spread in a joint cloud above and below the ring plane in a p type molecular orbital system. The electrons in this system are delocalized. Hükel claimed that the p system is responsible for the relative stability of benzene and other aromatic molecules. The system appears in the figure below as a ring:

Hükel's determination regarding the system's stability stood unchallenged for more than 50 years. In the mid 1980s some groups used new research techniques that showed that the basic framework itself is the source of the stability of aromatic structures and that the p system has a marginal role only.

The Geometric Perspective

Among those who behind this claim was Prof. Stanger. As a computational chemist specializing in organic chemistry, Stanger attacked the problem by creating a computational model in which he could differentiate between the contribution of the sigma skeleton and that of the p system to the geometric stability. To this end, he calculated what would happen to the ring's molecule should tension that would change the angle of the basic framework be applied to it. The theoretical calculations are done by solving Schrödinger's equation at different proximities and the result should predict exactly the molecule's behavior in reality.

The results showed that the p system has no significance in the geometric determination of the molecule and the latter tries as much as it can to arrange itself according to the geometry dictated to it by the sigma skeleton. The paper was published in 1992 and was received at first with a certain reservation because it demolished several ‘sacred cows.' Prof. Stanger relates that over time, confirmation of the determinations and predictions appearing in the paper piled up and it turned into one of the most frequently cited papers in the field.

Stanger continues to substantiate his findings by preparing a strained aromatic molecule (while developing a new synthetic method), with which he will be able to prove the computational results experimentally. The aromatic ring in an experimental system is strained by connecting small rings on the aromatic skeleton.

The Magnetic Perspective

The development of new research methods has allowed Stanger to investigate additional perspectives of aromatic compounds, one of which is the magnetic perspective. The delocalized electrons on the p plane create an induced magnetic field perpendicular to the molecule's plane in reaction to the activation of the external magnetic field. This induced field can be calculated.

Stanger developed a new computational model, based on a chemical model, in which he negates the magnetic contribution of the molecule's sigma's electrons by saturating all its bonds. This step enables the calculation of the size of the induced magnetic field by the p system only, at different distances from the molecular plane.

The results he arrived at are in line with mathematical models supposed to give similar results; however, Stanger claims that his method is more reliable given that it includes an internal measure of the exactness of the result whereas the mathematical methods do not have a similar evaluation and consequently, may lead to mistaken conclusions when the proximities are not appropriate for the system under study. Likewise, Stanger's system is more general and not limited to planar systems.

On the basis of the system that he developed for measuring the induced magnetic field size, at the same time neutralizing the effect of the molecule's sigma plane, Stanger wants to establish a model that will eventually determine a quantitative, agreed-upon measure for determining aromaticity, and thereby, once and for all determine the characteristics of this phenomenon, which will be an exact description not just for most aromatic molecules but for all, including extreme cases.

The Practical Perspective?

Prof. Stanger does not like the question of what are the practical results of his research and does not make an effort to hide his displeasure with the question. According to him, fundamental research does not need to justify itself. Practical knowledge in any case expands due to fundamental research and there is no reason to direct it ahead of time to any product whatsoever.

After stating his position in principle, Stanger is ready to reveal that his research also has practical aspects. The same synthetic method he developed in the 1990s in order to prove his claims about the effect of strain on aromatic rings experimentally, is suitable for developing a whole family of conductive polymers, which may turn out to be superconductors at high temperature, and which in contrast to metal conductors, conduct electricity on only one plane and insulate on other planes. Taking into account these practical implications, the Technion has taken out a patent on the discovery.