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Bypassing the Multi-reference Character of Singlet Molecular Oxygen. Part 2: Ene-reaction
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  • Kurt Mikkelsen,
  • Malte Jespersen,
  • Matthew Johnson,
  • Solvejg Jørgensen,
  • Emma Petersen-Sonn
Kurt Mikkelsen
University of Copenhagen
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Malte Jespersen
University of Copenhagen
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Matthew Johnson
University of Copenhagen
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Solvejg Jørgensen
University of Copenhagen
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Emma Petersen-Sonn
University of Copenhagen
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Abstract

Theoretical calculations involving singlet molecular oxygen (O2(1g)) are challeng- ing due to their inherent multi-reference character. We have tested the quality of re- stricted and unrestricted DFT geometries obtained for the reaction between singlet oxy- gen and a series of alkenes (propene, 2-methylpropene, trans-butene, 2-methylbutene and 2,3-dimethylbutene) which are able to follow the ene-reaction. The electronic en- ergy of the obtained geometries are rened using 3 dierent methods which account for the multi-reference character of singlet oxygen. The results show that the mechanism for the ene-reaction is qualitatively dierent when either one or two allylic-hydrogen groups are available for the reaction. When one allylic-hydrogen group is available the UDFT calculations predict a stepwise addition forming a biradical intermediate, while, the RDFT calculations predict a concerted reaction where both hydrogen abstrac- tion and oxygen addition occur simultaneously. When two allylic-hydrogen groups are available for the reaction then UDFT and RDFT predict the same reaction mechanism, namely that the reaction occurs as a stepwise addition without a stable intermediate between the two transition states. The calculated rate constants are in reasonable agreement with experimental data, except for trans-butene where the calculated rate constant is three orders of magnitude lower than the experimental one. In conclusion we nd that the simple bypassing scheme tested in this paper is a robust approach for calculations of reaction involving singlet oxygen in the limit that the transition state processes low multi-reference character. 2

Peer review status:IN REVISION

23 Dec 2020Submitted to International Journal of Quantum Chemistry
02 Apr 2021Assigned to Editor
02 Apr 2021Submission Checks Completed
12 Apr 2021Reviewer(s) Assigned
12 Apr 2021Review(s) Completed, Editorial Evaluation Pending
12 Apr 2021Editorial Decision: Revise Minor