Bond-orbital analysis of rotation barriers. Polarization and delocalization effects in ethane

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Abstract
Ab initio calculations of the barrier to rotation in ethane using a one-configuration wavefunction of non-orthogonal bond orbitals are supplemented by small correction terms accounting for electron delocalization. These corrections are introduced through a second-order perturbative CI calculation in terms of non-orthogonal configurations constructed from all the single excitations to localized antibonding orbitals. Besides being very close to those obtained from the full MO wavefunction, the results allow a clear-cut analysis of the main factor contributing to the barrier. Similar calculations using different types of orthogonalization show much larger second-order contributions to the barrier because of the need to correct for the large errors in the one-configuration calculation resulting from the forced orthogonalization of the basis set.

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