有机化合物电子光谱中的助色基及其作用机理探讨(Ⅱ)

Study on the Auxochrome of the Electronic Spectrum of Organic Compounds and Its Effect Mechanism (Ⅱ )

本文就电子光谱中助色基的作用进行了讨论,认为不能一概地认为助色基能使与之相连的生色基最大吸收波长统统发生红移,而应具体问题具体分析:对于n→π*来说,一方面助色基的p轨道和生色基的最低π*反键空轨道相互作用,使π*反键轨道能量上升,另一方面助色基的p轨道和生色基的n轨道正交,n轨道能量基本保持不变,因此n→π*跃迁能增加,吸收波长发生蓝移.对于π→π*跃迁来说,助色基的p轨道和生色基的最高π成键轨道相互作用生成的新的最高π成键轨道,能量虽然有所升高,但升高的程度与π*反键轨道升高的程度相比,在一些化合物中π轨道升高程度较大,因而π→π*跃迁能减小,吸收波长发生红移;在另一些化合物中π轨道升高程度较小,因而π→π*跃迁能增加,吸收波长发生蓝移.

The interaction between the p orbit of the auxochrome, the maximum bonding orbit (π) and the minimum antibonding orbit ( π* ) of the chromophore forms three new molecular orbit, among which the antibonding orbit ( π *') has the highest energy that is higher than those of the original π* . The maximum occupied orbit ( π ') will have energy lower or higher than those of π according to different auxochrome and chromophore, and the energy of n orbit will remain steady, because the n orbit of the original chromophore is perpendicular to the p orbit of the auxochrome and the interaction is negligeable. As a result, the absorption wavelength of transition nπ*→ will shift towards higher photon energy, but for transition π→π* , the absorption wavelength will shift towards either lower or higher photon energy, depending on the species of auxochrome and chromophore.