1. A superb presentation of this subject can be found in the book: Woodward, R. B., Hoffmann, R.: The conservation of orbital symmetry. Weinheim: Verlag Chemie 1970.
2. Woodward, R. B., Hoffmann, R.: J. Am. Chem. Soc. 87, 395 (1965).
3. Longuet-Higgins, H. C., Abrahamson, E. W.: J. Am. Chem. Soc. 87, 2045 (1965).
4. Hoffmann, R., Woodward, R. B.: J. Am. Chem. Soc. 87, 2046 (1965).
5. Ref. 1A superb presentation of this subject can be found in the book:, page 65.