Roles of electrostatic interaction and dispersion in CH···CH, CH···π, and π···π ethylene dimers

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.