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Natural steric analysis of internal rotation barriers

✍ Scribed by J. K. Badenhoop; F. Weinhold

Publisher: John Wiley and Sons
Year: 1999
Tongue: English
Weight: 301 KB
Volume: 72
Category: Article
ISSN: 0020-7608
DOI: 10.1002/(sici)1097-461x(1999)72:4<269::aid-qua9>3.0.co;2-8

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✦ Synopsis

We apply natural bond orbital NBO steric analysis introduced in a . previous article to obtain the steric exchange contribution to the internal rotation barriers Ž . of butane, ethane, and other related molecules CH NH , CH OH, NH OH . The 3 2 3 2

expected exchange repulsion between the two methyl group C-H bonds within van der Waals contact in butane is shown to be the major contributor to the syn barrier and provides a method for calculating allowed ranges of torsional angles in macromolecules. However, the exchange-energy difference between the staggered and eclipsed forms predict a counterintuitive eclipsed minimum for ethane, methylamine, and methanol. We show that the full SCF barrier in such apolar molecules can be reasonably approximated as a sum of this steric term plus the hyperconjugative terms as previously evaluated.

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