有机化学基本理论问题新的研究方法、新的观念和新 的思维模式

有机化学基本理论研究的总结和回顾。15年来,在国家自然科学基金委员会的支持下,为了认识电子离域的本质,在量子化学领域,我们建立和发展了新的作用能分解方法和大型计算程序,发展和完善了轨道定域化程序。我们的方法可以为任何一个共轭分子(无论是平面的还是非平面的,是含共轭双键的还是含累积双键的),提供一个π与σ体系彻底分离的片断分子轨道基组。这个轨道基组不仅满足分子特殊的对称性,而且还具有确切的电子占据数。与Hückel理论完全不同,我们强调：π电子的离域除了对它原先的定域π体系有强烈的失稳定作用外,它还可通过π-σ空间作用,对σ构架产生强烈的稳定作用。据此,我们提出了芳环化合物新的分类准则,揭示了芳香环流起因的必要条件,定义环的刚度为芳香性一个新判据。发现,分子内基团间的局部作用(CT和EX)同它们对分子整体性能的影响是完全相反的。就构象而言,稳定的CT作用是相斥的,失稳定的EX作用是相吸的;就电子转移而言,大的EX作用是电荷转移的助动力。其助动性在于,它能降低因CT作用而产生的给体自身对电荷转移的阻力。论证了,在二苯乙烯类分子中,π-π共轭,π-σ超共轭和σ-σ非键轨道作用都是失稳定的。与σ-σ和π-σ作用相比,π-π作用对于分子构像的影响是非常微弱的。与经典的思维模式相反,有机分子总是倾向于较小的失稳定,而不是较大的稳定。为了维持尽可能最稳定的电子总能量,在σ-σ作用的驱动下,共轭基团应该尽量地偏离共平面。阻止分子扭曲的(非电子作用力)是核排斥力。因此,一个空间拥挤的构象可以是能量有利的构象。在我们的研究中,经典有机结构理论的整体因果关系已经全面地被颠倒。

A novel energy partition for gaining new insight into the fundamental principles of organic chemistry

Our researches, supported by the National Natural Science Foundatino of China, on the fundamental theories of organic chemistry are reviewed. A new method for ab initio energy partition is being developed in order to gain new insight into the nature of electron delocalization, and it is able to provide any molecule with a fragment MO basis set in which the π and σ systems have been separated out no matter whether molecule is planar or nonplanar as well as whether the π system is conjugated or cumulated. Contrary to the HMO theory, the π electron delocalization has a great influence upon the original π systems themselves as well as upon the σ framework due to the π-σ space interaction. Accordingly, aromatic compounds can be divided into three groups and a delocalized π system itself is always destabilized, the dependence of aromatic ring current upon the CT was reveled, and ring rigidity has been defined as a new aromatic index. Based on the distinction between the CT and EX delocalizations, it was found that the stabilizing CT interaction is practically destabilization and the destabilizing EX interaction is stabilization as far as their effects on the behaviors of whole molecule are concemed. Consequently, the conclusions that the π-π, π-σ and non-bonded σ-σ interaction are always destabilization in the case of stilbene-like species are reasonable. Out of the classical expectation, the π-π interaction has only a slight effect on molecular behaviors. Particularly, it is the σ-σ destabilization interaction, rather than nuclear repulsion as classicaly thought, to distort molecule away from its planar geometry, and stilbene-like species may exist preferentially in a "crowded" geometry. The roles of the π and σ electrons in determining molecular behaviors should be reevaluated.