同面异面环加成中分子轨道的对称性和简并性

本文论述了[2_s+n_a]有C_2对称性, 而m>2的[m_s+n_a]由HMO模型得到的分子轨道的特殊对称性是过分简化造成的。这种简化可认为是把m_s组分的碳骨架看成直线, 因而可安排成C_2对称性。按照分子轨道的特殊对称性容易构成相关图。本文还论证了[m_s+m_a]全部分子轨道的二重简并性。

SYMMETRY AND DEGENERACY OF MOs IN SUPRA-ANTARA CYCLOADDITIONS

The [2_s+n_a] system can have C_2 symmetry (Fig.1) rather than local C_2 and local σ as described in Ref.4. For the [m_s +n_a] cycloaddition with m>2, the reaction system is no doubt unsymmetrical, the special symmetry of the MOs obtained by the HMO model is due to oversimplification. Bond angles are not concerned in the HMO method. Thus, if the m_s component is regarded as a straight carbon skeleton, the system will be of C_2 symmetry, which is shown in Fig.1 with the m_s in place of the ethylene. Correlation diagrams of [m_s+n_a] canbe constructed from the MO energies and special symmetries of the initial and the final states. For [m_s+m_a] reaction, the eigenvalue problem is equivalent to that of two identical systems with straight carbon chains (Fig.3). Therefore, double degeneracy holds for all MOs throughout the reaction (Fig.4).