【摘要】 [1,n]σ迁移反应除了过渡态外整个反应过程不具对称性。同面过程的过渡态有C₁对称性,异面过程的有C₂对称性。n为偶数的禁阻[1,n]σ迁移中的偶然简并性曾被认为是对称性造成的。但按本文的交替性理论和简并微扰理论的探究,说明偶然简并的对称和反对称非键轨道须作线性组合成为没有对称性的成对轨道才能与前后状态保持衔按。因此,这种偶然简并性不能用对称性解释而应用交替性解释。由交替性理论容易导出[1,n]σ迁移的选择定则,而由对称性则不能导出。更多还原

【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 C₂ （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 alternan- ce and the degenerate perturbation theory, they should be linearly combined into MOs keeping continuity with the earlier and the later states, the resulting MOs be- ing 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 papers. 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.更多还原