Cao Ye, Wong Ming Wah
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore.
J Mol Model. 2014 Apr;20(4):2185. doi: 10.1007/s00894-014-2185-9. Epub 2014 Mar 28.
Ab initio molecular orbital calculations were performed up to the CCSD(T)/CBS level to investigate the roles of the electrostatic interaction and dispersion in three basic types of intermolecular interaction, namely CH···CH, CH···π, and π···π interactions, in D(2d), C(2v), and C(i) ethylene dimers, respectively. SAPT energy decomposition revealed that the electrostatic interaction is more significant than expected, with its value being close to that of the net interaction energy. Dispersion is the largest stabilizing force and it plays the main role in balancing out exchange repulsion. This balance is related to the proposed concept of "contact." The roles of the σ and π electrons were distinguished in the electrostatic interaction by performing distributed multipole analysis and in dispersion by performing frozen-orbital SAPT (fo-SAPT) calculations. The electrostatic part of the interaction energy for each ethylene dimer can be understood as either a quadrupole-quadrupole attraction or the attraction between C and H atoms. (Electron pair)-(electron pair) contributions to the dispersion were calculated by the fo-SAPT method to shed light on the nature of dispersion. In these dimers, contributions to the dispersion can arise from π↔π, σ↔π, or σ↔σ electron-pair interactions. Surprisingly, σ↔π interactions dominate the dispersion in all three ethylene dimers. The π↔π contribution is very small, even in the displaced parallel structure (C(i)). The σ↔σ interaction contributes to intermolecular binding by helping the dispersion to balance out exchange repulsion, but this interaction is limited to the most stable D(2d) structure, which is characterized by four pairs of close dihydrogen contacts. The concept of an electron-pair "contact" was introduced to describe the exchange-dispersion balance. The D(2d) dimer is stabilized by a large number of such contacts.
进行了从头算分子轨道计算，直至CCSD(T)/CBS水平，以分别研究静电相互作用和色散在D(2d)、C(2v)和C(i)乙烯二聚体中三种基本类型分子间相互作用（即CH···CH、CH···π和π···π相互作用）中的作用。对称性适配微扰理论（SAPT）能量分解表明，静电相互作用比预期的更显著，其值接近净相互作用能的值。色散是最大的稳定力，它在平衡交换排斥中起主要作用。这种平衡与所提出的“接触”概念有关。通过进行分布式多极分析在静电相互作用中区分了σ电子和π电子的作用，并通过进行冻结轨道SAPT（fo-SAPT）计算在色散中区分了它们的作用。每个乙烯二聚体相互作用能的静电部分可以理解为四极-四极吸引或C与H原子之间的吸引。通过fo-SAPT方法计算了（电子对）-（电子对）对色散的贡献，以阐明色散的本质。在这些二聚体中，对色散的贡献可来自π↔π、σ↔π或σ↔σ电子对相互作用。令人惊讶的是，σ↔π相互作用在所有三种乙烯二聚体的色散中占主导地位。即使在错位平行结构（C(i)）中，π↔π贡献也非常小。σ↔σ相互作用通过帮助色散平衡交换排斥来促进分子间结合，但这种相互作用仅限于最稳定的D(2d)结构中，该结构以四对紧密的二氢接触为特征。引入了电子对“接触”的概念来描述交换-色散平衡。D(2d)二聚体通过大量这样的接触而稳定。
