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On a day like today, German physicist and chemist Erich Hückel died
February 16, 1980. Erich Armand Arthur Joseph Hückel (August 9, 1896, Berlin - February 16, 1980, Marburg) was a German physicist and physical chemist. Hückel is most famous for developing the Hückel method of approximate molecular orbital (MO) calculations on p electron systems, a simplified quantum-mechanical method to deal with planar unsaturated organic molecules. In 1930 he proposed a s/p separation theory to explain the restricted rotation of alkenes (compounds containing a C=C double bond). This model extended a 1929 interpretation of the bonding in triplet oxygen by Lennard-Jones. According to Hückel, only the ethene s bond is axially symmetric about the C-C axis, but the p bond is not; this restricts rotation. In 1931 he generalized his analysis by formulating both valence bond (VB) and molecular orbital (MO) descriptions of benzene and other cycloconjugated hydrocarbons. In 1936, Hückel developed the theory of p-conjugated biradicals (non-Kekulé molecules). The first example, known as the Schlenk–Brauns hydrocarbon, had been discovered in the same year. The credit for explaining such biradicals is usually given to Christopher Longuet-Higgins in 1950.


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