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Part 1: Bypassing the Multi-reference Character of Singlet Molecular Oxygen
  • +1
  • Malte Jespersen,
  • Solvejg Jørgensen,
  • Matthew Johnson,
  • Kurt Mikkelsen
Malte Jespersen
University of Copenhagen
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Solvejg Jørgensen
University of Copenhagen
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Matthew Johnson
University of Copenhagen
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Kurt Mikkelsen
University of Copenhagen
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Peer review status:POSTED

25 Jun 2020Submitted to International Journal of Quantum Chemistry
26 Jun 2020Assigned to Editor
26 Jun 2020Submission Checks Completed

Abstract

Ab initio calculations on systems involving singlet molecular oxygen (O2 (1g)) are challenging due to signicant multi-reference character arising from the degeneracy of the HOMO and LUMO orbitals in singlet oxygen. Here we investigate the stragegy of bypassing singlet oxygen’s multi-reference character by simply adding the experimen- tally determined singlet/triplet splitting (22.5 kcal/mol) to the triplet ground state of molecular oxygen. This method is tested by calculating rate constants for the reac- tions of singlet molecular oxygen with furan, 2-methylfuran, 2,5-dimethylfuran, pyrrole, 2-methylpyrrole, 2,5-dimethylpyrrole, and cyclopentadiene. The calculated rate con- stants are within a factor of 15 compared to experimentally determined rate constants. The results show that energy renement at the CCSD(T)-F12 level of theory is cru- cial to achieving accurate results. The reasonable agreement with experimental values validates the bypassing approach which can be used for other systems involving the 1,4-cyclo-addition of singlet oxygen. 2