1. J. Bardeen, Phys. Rev. 49, 653 (1936)
2. W.A. Bingel, Z. Naturforsch. 18a, 1249 (1963)
3. W.A. Bingel, Theoret. Chim. Acta (Berl.) 8, 54 (1967)
4. C.A. Coulson, Valence (2nd edn., Clarendon, Oxford, 1961)
5. I. Fleming, Frontier Orbitals and Organic Chemical Reactions (Wiley, New York, 1976)