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Transition-State Stabilization by n→π* Interactions Measured Using Molecular Rotors.

Erik C Vik1, Ping Li1, Perry J Pellechia1

  • 1Department of Chemistry and Biochemistry , University of South Carolina , Columbia , South Carolina 29208 , United States.

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Molecular rotors demonstrate that n→π* interactions significantly stabilize transition states (TSs) of bond rotation. This stabilization, up to 10 kcal/mol, was confirmed by computational studies and NBO analysis.

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Area of Science:

  • Organic Chemistry
  • Computational Chemistry
  • Supramolecular Chemistry

Background:

  • Understanding rotational barriers is crucial in molecular dynamics.
  • Non-covalent interactions play a key role in molecular structure and reactivity.
  • n→π* interactions are a type of non-covalent interaction with potential stabilizing effects.

Purpose of the Study:

  • To investigate the stabilizing effect of n→π* interactions on transition states (TSs) of bond rotation.
  • To quantify the stabilization energy provided by n→π* interactions.
  • To elucidate the role of specific functional groups in facilitating these interactions.

Main Methods:

  • Synthesis of 16 molecular rotors with varying electronic and steric properties.
  • Isolation of steric contributions using control rotors lacking n→π* interaction capability.
  • Kinetic studies to measure rates of rotation.
  • Computational modeling (e.g., DFT) to predict TS geometry and stabilization.
  • Natural Bond Orbital (NBO) analysis to confirm interaction types.

Main Results:

  • Rotors with strong acceptor π* orbitals (ketones, aldehydes) exhibited significantly increased rotation rates.
  • Observed TS stabilization reached approximately 10 kcal/mol.
  • n→π* interactions between imide carbonyl oxygens and ortho R groups in the planar TS were identified as the primary stabilizing factor.
  • Computational studies accurately predicted TS stabilization and geometry.

Conclusions:

  • n→π* interactions are effective in stabilizing transition states of bond rotation.
  • The strength of stabilization is influenced by the nature of the acceptor π* orbital.
  • Computational methods, including NBO analysis, are valuable tools for understanding and quantifying these non-covalent interactions in molecular rotors.