Bond-Function Analysis of Rotational Barriers: Ethane

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Abstract
The barrier potential to internal rotation in ethane is examined with bond‐orbital wavefunctions. It is found that reasonable values of the barrier height are obtained over a wide range of bond polarities if the wavefunction is constrained to satisfy the Pauli exclusion principle. By contrast, for a Hartree product of local nonorthogonal bond orbitals, the barrier is very sensitive to bond polarity. On integration of the Hellmann–Feynman forces from the determinantal bond‐orbital functions along a path that requires only force differences between staggered and eclipsed ethane, barrier values are calculated that closely parallel the corresponding total energy differences; use of an alternative path introduces a much larger error into the force calculation. The bond‐function results are utilized to examine the question of error cancellation in barrier calculations and for a comparison with other studies of the ethane barrier. It is concluded that the dominant contribution to the barrier is the overlap (exclusion‐principle) repulsion between closed‐shell, localized C–H bond orbitals and that the direct electrostatic and dispersion force interaction between these orbitals is relatively unimportant.