L. Goodman et Hb. Gu, FLEXING ANALYSIS OF STERIC EXCHANGE REPULSION ACCOMPANYING ETHANE INTERNAL-ROTATION, The Journal of chemical physics, 109(1), 1998, pp. 72-78
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].