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E. Hückel and the aromaticity
At the beginning of the 18th century, chemists referred to such molecules as aromatics if they had a special "aromatic" smell, for example vanillin, which was isolated from the vanilla pod. Over the years, however, the chemical concept of aromaticity has changed, and so many “aromatic” smelling molecules, for example menthol, are not counted as part of the important class of aromatics, a class of substances that today plays a key role in the paint and pharmaceutical industries. Fragrance criterion ”has therefore been dropped and replaced by other aromaticity criteria. These new criteria were largely based on experimental findings which, for example, indicated a particular chemical stability of aromatics or related to typical “aromatic signals” in NMR (Nuclear Magnetic Resonance) spectroscopy. Since the rise of quantum chemistry in the first half of the 20th century, aromaticity criteria based on theoretical calculations have also been developed. Erich Hückel (1896-1980) found the famous rule named after him to be an important result of his theoretical work in the 1930s. In textbooks and school books, this is something like: Monocyclic planar molecules with (4n + 2) π electrons are aromatic. The π electrons are characterized by the fact that they are above and below, but not in the molecular plane of the planar aromatic. The experimentally observed special stability of benzene (empirical formula: ), the prototype of an aromatic compound, explained Hückel with the double occupancy of all available bonding orbitals with (4 · 1 + 2 = 6) π electrons. The figure shows benzene and the π molecular orbital with the lowest energy, which is above and below the Molecular plane extends. The different colors, red and blue, mark the areas positive and although Hückel made predictions with his theory that were later confirmed experimentally, the Hückel method, like the superordinate MO (Molecular Orbital) theory, led a shadowy existence in Germany for many years. In this context, Hückel made major reproaches to his colleague Linus Carl Pauling (1901-1994). The VB (Valence Bond) theory spread rapidly among chemists, especially through Pauling's very popular textbook at the time, who also preferred the VB theory to the MO theory for psychological reasons, because they believed they recognized the well-known valence structures in it. As Hückel emphasized, however, the VB theory is much less understandable than it may initially appear. Hückel also suffered personally from neglecting his method. As a pioneer in the then still young field of quantum chemistry, he stood between the subjects of physics and chemistry, and was denied full professorship for many years. It was not until 1961 that he received the title of “full professor” in Marburg, but he retired a year later.