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Related Experiment Videos

Nitrogen inversion in cyclic amines and the bicyclic effect.

Anatoly M Belostotskii1, Hugo E Gottlieb, Michael Shokhen

  • 1Chemistry Department, Bar-Ilan University, Ramat-Gan 52900, Israel. belostot@mail.biu.ac.il

The Journal of Organic Chemistry
|December 21, 2002
PubMed
Summary
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The bicyclic effect causing high nitrogen inversion-rotation (NIR) barriers in bicyclic amines is explained by sigma-orbital energies. This study establishes a quantitative correlation between amine geometry and NIR barriers.

Area of Science:

  • Organic Chemistry
  • Computational Chemistry

Background:

  • The origin of high nitrogen inversion-rotation (NIR) barriers in 7-azabicyclo[2.2.1]heptanes, known as the bicyclic effect, remained unexplained.
  • Nitrogen inversion barriers are crucial for understanding amine reactivity and stereochemistry.

Purpose of the Study:

  • To elucidate the origin of the bicyclic effect on NIR barriers in bicyclic amines.
  • To establish quantitative correlations between amine geometry and NIR barriers.

Main Methods:

  • Utilized the natural bond orbital (NBO) approach for theoretical analysis.
  • Employed Density Nuclear Magnetic Resonance (DNMR) to experimentally determine NIR barriers for tropane.
  • Performed ab initio calculations at the MP2/6-31G level of theory.

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Main Results:

  • NIR barriers increase with decreasing ring size in nitrogen-bridged bicyclic systems.
  • Experimental NIR barriers were accurately reproduced by theoretical calculations.
  • NBO analysis revealed that sigma-orbital energies of C(alpha)-C(beta) bonds and the nitrogen lone pair primarily determine barrier heights.
  • The bicyclic effect is an extreme manifestation of a general geometry-NIR barrier dependence.

Conclusions:

  • The study provides a comprehensive explanation for the bicyclic effect in NIR barriers.
  • Established the first accurate quantitative correlations for NIR barriers in a wide range of amines, including monocyclic and bicyclic systems.
  • Identified the C(alpha)-N-C(alpha) tripyramid fragment as rate-determining for NIR.