Natural Bond Orbital Analysis of Internal Rotation Barriers and Related Phenomena

Abstract The origin of barriers to internal rotation in ethane‐like molecules has been investigated with ab initio molecular orbital theory and natural bond orbital (NBO) analysis. The “hyperconjugative” (vicinal σ → σ*, n → σ*) nature of these interactions, as previously suggested on the basis of semiempirical INDO studies, is confirmed at higher 4–31G, 6–31G* levels of theory. Wave functions lacking such hyperconjugative interactions, but satisfying the Pauli principle, are shown to generally exhibit unphysical barrier behavior (i.e., wrong sign) , contrary to the implication of the “Pauli‐repulsions” model of Sovers et al. Each trans vicinal σ → σ* interaction is found to contribute about 0.7 kcal/mol to stabilization of the staggered conformer. Geminal interactions and Rydberg orbital contributions are found to be relatively unimportant. The consistency of NBO analysis is exhibited for the methyl series CH 3 ‐X (X = CH 3 , NH 2 , OH, F) and the family of fluoromethanes, to demonstrate the essential continuity that relates internal rotation barrier forces to static and kinetic anomeric effects, gauche effects, anionic hyperconjugation, bond strengthening in polyhalogenated methanes, (e, 2e) spectroscopic evidence of lone pair delocalization, and other stereoelectronic phenomena.