The Aromatic Sextet

Abstract

The use of different spectra (UV, visible and NMR spectra) is demonstrated for the identification of polycyclic hydrocarbons. Soon after Kekule published his formula of benzene it became apparent that there should be 2 isomers (Fig. 1). This could not be found and Kekule assumed that the three double bonds change their place so fast that an isolation of these isomers would be impossible. Fig.I. o o Fig.2. Later a circle was used to symbolize aromaticity without a clear definition (Fig.2). Pauling assumed that the isomers did indeed not exist, but that the real state of benzene was the resul t of "resonanze" between the two isomers. E. Huckel used a mathematical method which gave molecular orbi-tals which had no double bonds at all. He subdivided the 6 IT electrons in groups of 4 IT electrons and 2 IT electrons. Both methods could be expressed by a circle (Fig.3) 49 D. Rondia et aL (eds.j, Mobile Source Emissions Including Polycyclic Organic Species, 49-58.