FLEXING ANALYSIS OF STERIC EXCHANGE REPULSION ACCOMPANYING ETHANE INTERNAL-ROTATION

Relaxation calculations using natural bond orbitals are carried out fo r the Puali exchange steric repulsion changes associated with ethane i nternal rotation using sufficiently extended Dunning correlation-consi stent and Gaussian orbitals,so that interpretations are not basis set dependent. These calculations show that the total exchange repulsion i s strongly dependent on the C-C bond lengthening that accompanies rota tion. The effect of this relaxation is that the total exchange repulsi on favors the eclipsed conformer by similar to 6 kcal/mol, far greater than previous estimates obtained without skeletal relaxation. The sou rce of the skeletal flexing dependence stems from the strong dependenc e of the orthogonalization-induced-energy-shift of the sigma(CH) NBO a nd carbon core orbitals on RC-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-for ming one to gauche repulsions. These senses are opposite to the barrie r-forming anti/syn CH pairwise interactions found in previous ethane s teric considerations. (C) 1998 American Institute of Physics. [S0021-9 606(98)30325-6].