从头计算法研究取代锂卡宾正离子的分子轨道和稳定化能

用从头计算法计算了各种取代的锂卡宾正离子XCHLi⁺(X=OH,NH₂,CH₂CH,NC,C₆H₅)相对于锂卡宾正离子CH₂Li⁺的稳定化能,并讨论了稳定化能和它们分子轨道的关系。具有π给予功能的取代基,在其共平面分子构型中,由于和卡宾碳和锂原子空轨道形成二电子多中心共轭分子轨道,因而具有最大的稳定作用。相反,在相应的正交构型中,稳定作用则较小。同时具有π给予和σ接受效应的取代基,π给予起主导作用。仅有σ接受功能的取代基,则有去稳定作用。考察它们的LUMO特点,发现它们具有较高的亲电反应性能。

An Ab Initio Investigation of the Molecular Orbital and Stabilization Energy for Substituted Lithium Carbene Cations

Metal carbene cations, found in recent ten years, is a class of unstable intermediates with a high activity. In this paper, with CH₂Li⁺as a model compound, we first investigted the relative stabilization energies and molecular orbitals of substituted lithium carbene cations by ab initio method. The studies show that the substituents with π donation have the biggest stabilization because they can form a two electron-multicentre conjugated orbital with carbene carbon and lithium in their planar configurations. In contrast, their stabilization energies are less in perpendicular one. The π donation plays a leading role in the substituents with both π donation and σ induction. Then the substituents with only σ inductive effets have destabilization. The most of LUMO of lithium carbene cations are conjugated molecular orbitals formed by empty p^zof carbene carbon, lithium and substltuents, and the carbene carbon is the major consistent. Their energy levels all are negative values, so they can be considered with a high electrophilic activity and the acting centre should be on the carbene carbon.