1. Jug, K.: J. Am. Chem. Soc.99, 7800 (1977)
2. Jug, K.: J. Am. Chem. Soc., submitted
3. Streitwieser, Jr., A.: Molecular orbital theory for organic chemists, p. 168. New York: Wiley 1961
4. Coulson, C. A.: Valence, p. 271. Oxford: University Press 1961
5. Coulson, C. A.: Proc. Roy. Soc. (London)169A, 419 (1939)