期刊
ACTA PHYSICO-CHIMICA SINICA
卷 29, 期 1, 页码 43-54出版社
PEKING UNIV PRESS
DOI: 10.3866/PKU.WHXB201211121
关键词
Density functional theory; cis-Effect; Natural bond orbital analysis; Steric effect; Non-covalent interaction
资金
- 'XiaoXiang Scholar' Talents Foundation of Hunan Normal University, China [23040609]
- Hunan Provincial Innovation Foundation for Postgraduate, China [CX2012B223]
- Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, China
- Hundred Talents Foundation of Hunan Province
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 R-2=0.86-0.87 were obtained.
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