Direct determination of the rotational barrier in ethane using perturbation theory

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Abstract
A transition state perturbation method is derived which permits the direct determination of the energy barrier to hindered rotation in ethane as opposed to the usual subtraction of two large numbers. The method is implemented by using the coupled Hartree–Fock perturbation theory (CPHF). The value obtained for the rotational barrier of ethane through third order is 2.60 kcal/mole, while the infinite‐order result gives 3.17 kcal/mole. A generalized derivation of the energy formulas in CPHF is given which provides expressions for the energy to any order. This also establishes the CPHF analog of the theorem in ordinary perturbation theory that the energy correct to order 2n+1 can be obtained from perturbed orbitals of order n.