1. For details of the Hückel theory, especially for early work in this field see: Hückel, E.: Grundzüge der Theorie ungesättigter und aromatischer Verbindungen. Berlin: Verlag Chemie 1940;
2. For details of the Hückel theory, especially for early work in this field see: Coulson, C. A., O'Leary, B., Mallion, R. B.: Hückel theory for organic chemists. London: Academic Press 1978
3. See for example: Effenberger, F., Fischer, P., Schoeller, W. W., Stohrer, W.-D.: Tetrahedron34, 2409 (1978); Bräuchle, C., Kabza, H., Voitländer, J.: Chem. Phys.48, 369 (1980)
4. See for example: Kollmar, H.: J. Am. Chem. Soc.101, 4832 (1979); Basilevsky, M. V., Weinberg, N. N.: Int. J. Quantum Chem.17, 399 (1980)
5. Woodward, R. B., Hoffmann, R.: The conservation of orbital symmetry. Weinheim: Verlag Chemie 1970