The concepts of aromaticity and antiaromaticity play central roles in organic chemistry. Although somewhat elusive concepts, a combination of experimental and calculational tools may be used to assess aromacity and antiaromaticity through analysis of magnetic, structural, and energetic criteria. Indenylidene dications allow the assessment of magnetic properties through the paratropicity of protons on both the five- and six-membered rings, allowing evaluation of the antiaromaticity suggested by the nucleus independent chemical shift values. Because the cyclopentadienyl ring of the indenylidene cation can respond to increased antiaromaticity through deformation, structural properties can be assessed. Energetic properties can be assessed by electrochemical oxidation and comparison of redox potentials with those of non-antiaromatic model systems. By playing the various criteria against each other in aromatic and antiaromatic systems, the criteria that most effectively define aromatic/antiaromatic systems will be exposed.
Ever since the initial realization that benzene and related "aromatic" hydrocarbons display unusually high stability, the factors responsible for this stability have been of great interest. "Aromaticity" and its counterpoint, antiaromaticity, in which compounds display lower than anticipated stability, play central roles in defining organic chemical structure and reactivity. With the support of a Research at Undergraduate Institutions (RUI) grant from the Organic and Macromolecular Chemistry Program, Professor Nancy S. Mills, of the Department of Chemistry at Trinity University, is exploring new molecular systems expected to display antiaromaticity. Through the rational design and synthesis of appropriate organic molecules and analysis of their magnetic, structural, and energetic properties, Professor Mills is defining the criteria that most effectively define aromaticity and antiaromaticity. A large number of undergraduate research assistants will participate in these studies, providing them with valuable research experience and education in a variety of experimental, instrumental, and theoretical techniques.
