顺式效应的起源: 双取代乙烯衍生物的密度泛函理论研究

一般情况下双取代乙烯衍生物的反式构象要比顺式构象稳定. 但是例外也存在, 例如1,2-二氟乙烯和1,2-二氯乙烯. 这种双取代乙烯衍生物顺式构象的超常稳定性被称之为顺式效应. 该效应的起源和本质目前仍没有定论. 本文以12 个体系(XHC=CHY (X, Y=F, Cl, Br, CN, CH₃, C₂H₆, OCH₃))为例, 对顺式效应的有效性、起源和本质进行系统的密度泛函理论研究, 其中9个体系存在顺式效应, 另外3个为正常体系没有该效应. 采用一系列泛函和基组研究其有效性, 并运用四种分析手段, 如自然键轨道(NBO)、能量分量分析(EDA)、密度泛函活性理论(DFRT)和非共价相互作用(NCI)分析, 剖析该效应的起源和本质. 发现在顺式构象的两个取代基之间存在一种微弱的非共价相互吸引作用. 能量分析表明, 静电效应、立体效应等对顺式效应的存在都起着重要作用, 但是它们均不能单独用来解释顺式效应的起源. 也就是说顺式效应没有一个简单的起源, 它是多种作用合力的结果. 本文采用双变量解释得到比较合理的相关回归系数R²=0.86-0.87,较好地解释了顺式效应的本质和起源.

Origin of the cis-Effect: a Density Functional Theory Study of Doubly Substituted Ethylenes

It is well known that the trans isomer of a doubly substituted ethylene is more stable than its cis counterpart because of the more favorable electrostatic and steric interactions in the trans conformer. Exceptions do exist nevertheless. 1,2-Difluoroethylene is such an example, so is 1,2-dichloroethylene. The unusual stability of the cis isomer of these doubly substituted ethylene compounds is referred to as the cis-effect, whose nature and origin are still not well understood. In this work, using 12 simple molecules, XHC=CHY (X, Y=F, Cl, Br, CN, CH₃, OCH₃, C₂H₆), as examples, we perform systematic studies to investigate the validity, nature, and origin of this effect. Among the systems studied, 9 of them exhibit the existence of the cis-effect and the remaining 3 systems are conventional systems used for the comparison purpose. We employ a large number of density functionals and basis sets to confirm its validity. We also use a few well-established analysis tools, such as natural bond orbital (NBO), energy decomposition analysis (EDA), density functional reactivity theory (DFRT), and non-covalent interaction (NCI) analysis, to pinpoint its nature and origin. We found that there exists a weak but attractive non-covalent interaction between the two substituting groups in the cis conformer. We also found that electrostatic, steric, and kinetic energies all play important roles for the validity of the cis-effect. Nevertheless, none of these quantities can be solely used as the single reason governing the general validity of the cis-effect, suggesting that the origin of the effect is complicated and its validity results from compound interactions from a number of interactions. In this work, we employ two-variable explanations to justify its validity through the electrostatic interaction plus steric effect or kinetic energy, with which reasonable fits with R²=0.86-0.87 were obtained.