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Quadricyclane radical cation isomerization reactions: A theoretical study

Authors

  • Per-Erik Larsson,

    1. Department of Quantum Chemistry, Uppsala University, Box 518, S-751 20 Uppsala, Sweden
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  • Nessima Salhi-Benachenhou,

    Corresponding author
    1. Department of Quantum Chemistry, Uppsala University, Box 518, S-751 20 Uppsala, Sweden
    • Department of Quantum Chemistry, Uppsala University, Box 518, S-751 20 Uppsala, Sweden
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  • Xicheng Dong,

    1. Department of Quantum Chemistry, Uppsala University, Box 518, S-751 20 Uppsala, Sweden
    Current affiliation:
    1. Laboratory of Computer Chemistry, Chinese Academy of Sciences, Shanghai Institute of Organic Chemistry, Shanghai 200032, People's Republic of China
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  • Sten Lunell

    1. Department of Quantum Chemistry, Uppsala University, Box 518, S-751 20 Uppsala, Sweden
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Abstract

On the C7H8·+ potential energy surface studied in this work, the electrocyclic isomerization of the quadricyclane (Q·+) to the norbornadiene (N·+) radical cations has attracted much attention. Our interest, however, concerns alternative rearrangement reactions on this surface. Using quantum chemical methods, B3LYP/6-311+G(d,p), MP2/6-31G(d,p), and single-point CCSD(T)/6-311+G(d,p), a skeletal rearrangement path leading from Q·+ to the bicyclo[3,2,0]hepta-2,6-diene radical cation (BHD·+) is found. The barrier for this path is 4.9 kcal/mol higher at the CCSD(T)/6-311+G(d,p)//B3LYP/6-311+G(d,p) level than the Q·+ to N·+ barrier. Moreover, this rearrangement is found to proceed through an intermediate, bicyclo[2.2.1]hepta-2-ene cation (BHE·+), with a conversion barrier to BHD·+ of 8.1 kcal/mol while the barrier for conversion back to Q·+ is 12.8 kcal/mol, computed with CCSD(T)/6-311+G(d,p)//B3LYP/6-311+G(d,p). Interestingly, the 1H hyperfine coupling constants computed for this intermediate structure show an excellent agreement with the experiment by Williams and coworkers. This ESR spectrum, observed after irradiation of N and Q loaded CF3CCl3 matrices, was assigned to BHD·+; our calculations, however, show that it is BHE·+ that gives rise to the observed spectrum. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

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