Acta Phys. -Chim. Sin. ›› 1987, Vol. 3 ›› Issue (01): 4-10.

• ARTICLE •

### ACCIDENTAL DEGENERACY AND SELECTION RULE IN [1, n] SIGMATROPIC REARRANGEMENTS

Chen Zhixing; Zhang Yala

1. Chemistry Department; Zhongshan University; Guangzhou
• Received:1985-08-15 Revised:1900-01-01 Published:1987-02-15

Abstract: There is no symmetry throughout the reaction process of [1, n] sigmatropic rearrangements except in the transition state, in which the suprafacial process is of C symmetry, while the antarafacial one is C_2 (Fig.1).The accidental degeneracy at the crossing of the FMOs in the forbidden [1, n] sigmatropic rearrangenents, where n is an odd number, has been explained by two different concepts, i.e., by symmetry and by alternance. There are indeed a symmetrical NBMO and an antisymmetrical NBMO in the transition state. According to the theory of alternance and the degenerate perturbation theory, they shoule be linearly combined into MOs keeping continuity with the earlier and the later states, the resulting MOs being thus unsymmetrical (cf. Table 1). Therefore, the accidental degeneracy cannot be explained by the concept of symmetry, and the correct explanation should be concerned with the alternance as in the previous paper~[5]. The selection rules for [1, n] sigmatropic rearrangements with an odd number of n can readily be deduced by the theory of alternance, but not by symmetry.