Natural steric analysis of internal rotation barriers

We apply natural bond orbital (NBO) steric analysis (introduced in a previo us article) to obtain the steric exchange contribution to the internal rota tion barriers of butane, ethane, and other related molecules (CH3NH2, CH3OH , NH2OH). The expected exchange repulsion between the two methyl group C-H bonds within van der Waals contact in butane is shown to be the major contr ibutor to the syn barrier and provides a method for calculating allowed ran ges of torsional angles in macromolecules. However, the exchange-energy dif ference between the staggered and eclipsed forms predict a counterintuitive eclipsed minimum for ethane, methylamine, and methanol. We show that the f ull SCF barrier jin such apolar molecules can be reasonably approximated as a sum of this steric term plus the hyperconjugative terms as previously eva luated. (C) 1999 John Wiley & Sons, Inc.