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Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
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Kinetic isotope effects calculated with the instanton method.

Jan Meisner1, Judith B Rommel, Johannes Kästner

  • 1Computational Biochemistry Group, Institute of Theoretical Chemistry, University of Stuttgart, Germany.

Journal of Computational Chemistry
|September 8, 2011
PubMed
Summary

Heavy-atom tunneling drives cyclopropylcarbinyl radical reactions at low temperatures. Instanton theory accurately predicts kinetic isotope effects, including inverse effects, offering a computationally efficient method.

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Area of Science:

  • Quantum chemistry
  • Chemical kinetics
  • Computational chemistry

Background:

  • The ring-opening of cyclopropylcarbinyl radicals is a key reaction in organic chemistry.
  • Understanding reaction mechanisms at low temperatures often requires considering quantum effects like tunneling.

Purpose of the Study:

  • To investigate the role of heavy-atom tunneling in the ring-opening reaction of the cyclopropylcarbinyl radical.
  • To develop and validate a computationally efficient method for calculating kinetic isotope effects (KIEs) using instanton theory.

Main Methods:

  • Utilized instanton theory combined with density functional theory (B3LYP) for on-the-fly energy calculations.
  • Verified the accuracy of the instanton method with explicitly correlated coupled-cluster calculations (UCCSD(T)-F12).
  • Proposed a simplified instanton method assuming mass-independent paths for efficient KIE calculations.

Main Results:

  • Calculated significant protium/deuterium KIEs up to 13 and inverse KIEs down to 0.2 at cryogenic temperatures.
  • Observed good agreement between the simplified instanton method and more rigorous calculations for KIEs.
  • Successfully applied the method to both the cyclopropylcarbinyl radical reaction and an intramolecular tautomerization.

Conclusions:

  • Heavy-atom tunneling is a crucial factor in the low-temperature ring-opening of cyclopropylcarbinyl radicals.
  • The proposed simplified instanton method provides accurate and computationally efficient KIE predictions.
  • The instanton method demonstrates significant capabilities for studying quantum effects in chemical reactions.