What is the origin of the contrathermodynamic behavior in methyl radical addition to alkynes versus alkenes?

Rodolfo Gómez-Balderas, Michelle L. Coote*, David J. Henry, Hanns Fischer, Leo Radom

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    40 Citations (Scopus)

    Abstract

    High-level ab initio calculations of the barriers, enthalpies, and rate constants have been performed for methyl radical addition to ethyne, propyne, ethene, and propene. We find that addition to alkenes is kinetically favored over addition to alkynes, despite the larger exothermicity of the alkyne addition reactions. The results have been rationalized using the curve-crossing model. To this end, the singlet-triplet gaps and charge-transfer energies in the reactants, and the extent of charge separation in the transition structures, have been calculated. It is concluded that the greater barrier for addition to alkynes is primarily the result of the larger singlet-triplet gap in the substrate. This barrier-raising effect dominates the barrier-lowering effect of the reaction exothermicity.

    Original languageEnglish
    Pages (from-to)6082-6090
    Number of pages9
    JournalJournal of Physical Chemistry A
    Volume107
    Issue number31
    DOIs
    Publication statusPublished - 7 Aug 2003

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