This technique reveals limitations in applying Hückel’s rule to polycycles, indicating necessary pedagogical changes in chemical education.
A semicircular mnemonic is described which permits one to anticipate the Hückel eigenvalue patterns for symmetrical contiguous nonfused bicycles (e.g., biphenyl). The mnemonic constitutes a new technique that is complementary to the existing Frost circle mnemonic for hydrocarbon monocycles. Pedagogically, the development of this semicircular mnemonic in class will serve to establish that the widespread practice of applying Hückel’s rule to polycycles is unsound.
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Richard Francis Langler (1998) studied this question.
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