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| - Nitrogen Inversion in Cyclic Amines and the Bicyclic Effect
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| - The hitherto unsolved problem of the origin of the unusually high nitrogen inversion−rotation(NIR) barriers in 7-azabicyclo[2.2.1]heptanes (the bicyclic effect) was examined using the naturalbond orbital (NBO) approach. Reinvestigating the NIR barrier for tropane by DNMR, we foundthat NIR barriers increase smoothly on going from nitrogen-bridged bicyclic systems of a largerring size to the smaller ring homologous systems. The experimental NIR barriers are reproducedwith good accuracy using the MP2/6-31G* level of theory. The NBO analysis for these and otherazabicycles led to the conclusion that the height of these barriers is mostly determined by theenergy of the σ-orbitals of the Cα-Cβ bonds as well as the nitrogen lone pair. Thus, the bicycliceffect is actually an extreme case of a common Cα-N−Cα tripyramid geometry−NIR barrierdependence for N-bridged bicyclic amines. By establishing the rate-determining role of the Cα-N−Cα tripyramid fragment for NIR, we have derived the first sufficiently accurate quantitativecorrelations amine geometry−NIR barrier for monocyclic as well as bicyclic N-H and N-Me amines(i.e., for an amine set which also includes the bicyclic effect systems).
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