Abstract
The energy change on each Occupied Molecular Orbital as a function of rotation about the C-C bond in ethane was studied using the B3LYP, mPWB95 functional and MP2 methods with different basis sets. Also, the effect of the ZPE on rotational barrier was analyzed. We have found that σ and π energies contribution stabilize a staggered conformation. The σs molecular orbital stabilizes the staggered conformation while the stabilizes the eclipsed conformation and destabilize the staggered conformation. The πz and molecular orbitals stabilize both the eclipsed and staggered conformations, which are destabilized by the πv and molecular orbitals. The results show that the method of calculation has the effect of changing the behavior of the energy change in each Occupied Molecular Orbital energy as a function of the angle of rotation about the C–C bond in ethane. Finally, we found that if the molecular orbital energy contribution is deleted from the rotational energy, an inversion in conformational preference occurs.
Highlights
The existence of a rotational barrier of 2.875 kcal·mol−1 about the C-C bond in ethane has been known for many years [1,2,3,4,5,6,7,8,9]
In order to contribute to the understanding of the conformational driving force in ethane, we propose an alternative point of view based on a systematic analysis of its Molecular Orbitals (MOs), the most basic concept in conformation, to assign the different MOs to each of the preferred conformations and estimate the overall net effect by subtracting the molecular orbital energy from the total energy during the rotation about the C–C bond in the ethane
The Kohn-Sham total energy rotation (Erot) for conversion of ethane from staggered to eclipsed conformation was calculated in the gas phase and the geometries were fully optimized
Summary
The existence of a rotational barrier of 2.875 kcal·mol−1 about the C-C bond in ethane has been known for many years [1,2,3,4,5,6,7,8,9]. The steric effect has its origin in the fact that atoms in molecules occupy a certain amount of space, resulting in changes in shape, energy, and reactivity. Shubin Liu recently proposed an energy partition scheme under the framework of Density Functional Theory (DFT) [23,24]. In this scheme the total energy density functional is decomposed into three independent contributions from steric, electrostatic, and quantum effects. This scheme was used to explore the internal rotation barrier of various molecules [14,25,26,27]. Weisskopf [28] attributed it to the “kinetic energy pressure” in atoms and molecules, whereas others [9,10,12,13,29] employed the quantum contribution from the Pauli Exclusion
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