Flexing analysis of steric exchange repulsion accompanying ethane internal rotation

Relaxation calculations using natural bond orbitals are carried out for the Puali exchange steric repulsion changes associated with ethane internal rotation using sufficiently extended Dunning correlation-consistent and Gaussian orbitals, so that interpretations are not basis set dependent. These calculations show that the total exchange repulsion is strongly dependent on the C–C bond lengthening that accompanies rotation. The effect of this relaxation is that the total exchange repulsion favors the eclipsed conformer by ∼6 kcal/mol, far greater than previous estimates obtained without skeletal relaxation. The source of the skeletal flexing dependence stems from the strong dependence of the orthogonalization-induced-energy-shift of the ${\sigma }_{\mathrm{CH}}$ NBO and carbon core orbitals on $R_{\mathrm{C–C}}.$ Strong basis set dependence is found for the pairwise repulsions. Extended basis set calculations attribute an antibarrier energy change to anti/syn repulsions and a barrier-forming one to gauche repulsions. These senses are opposite to the barrier-forming anti/syn CH pairwise interactions found in previous ethane steric considerations.