W Chad McKee

University of Georgia, Атина, Georgia, United States

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Publications (4)22.22 Total impact

  • William Chad McKee · Paul von Rague' Schleyer
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    ABSTRACT: The "alkane branching effect" denotes the fact that simple alkanes with more highly branched carbon skeletons, e.g. isobutane and neopentane, are more stable than their normal isomers, e.g. n-butane and n-pentane. Although n-alkanes have no branches, the "kinks" (or "protobranches") in their chains (defined as the composite of 1,3 alkyl-alkyl interactions - including methine, methylene, and methyl groups as alkyl entities - present in most linear, cyclic and branched alkanes, but not methane or ethane) also are associated with lower energies. Branching and protobranching stabilization energies are evaluated by isodesmic comparisons of protobranched alkanes with ethane. Accurate ab initio characterization of branching and protobranching stability requires post-SCF treatments, which account for medium range (~1.5-3.0Å) electron correlation. LMO-MP2 partitioning of the correlation energies of simple alkanes into localized contributions indicates that correlation effects between electrons in 1,3 alkyl groups are largely responsible for the enhanced correlation energies and general stabilities of branched and protobranched alkanes.
    No preview · Article · Aug 2013 · Journal of the American Chemical Society
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    ABSTRACT: Hydrocarbon radical stabilization energy (RSE) estimates based on the differences in R-H vs CH(3)-H bond dissociation energies have inherent advantages over RSEs based on R-CH(3) vs CH(3)-CH(3), as well as R-R vs CH(3)-CH(3) comparisons, since the R-CH(3) and R-R reference systems are prone to unbalanced contaminating intramolecular interactions involving the R groups. When the effects of steric crowding, branching, protobranching, conjugation, and hyperconjugation are taken into account, R-CH(3) and R-R based RSE values are nearly identical to R-H RSEs. Corrections for electronegativity differences between H and R are not needed to achieve agreement.
    No preview · Article · Mar 2011 · The Journal of Organic Chemistry
  • Paul von Ragué Schleyer · W Chad McKee

    No preview · Article · Feb 2010 · The Journal of Physical Chemistry A
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    ABSTRACT: The protonated acetylene cation, C2H3+, (also known as the vinyl cation) and the proton-bound acetylene dimer cation (C4H5+) are produced by a pulsed supersonic nozzle/pulsed electrical discharge cluster source. The parent ions are also generated with weakly attached argon "tag" atoms, e.g., C2H3+Ar and C4H5+Ar. These ions are mass selected in a specially designed reflectron time-of-flight mass spectrometer and studied with infrared laser photodissociation spectroscopy in the 800-3600 cm-1 region. Vibrational resonances are detected for both ions in the C-H stretching region. C2H3+ has a strong vibrational resonance near 2200 cm-1 assigned to the bridged proton stretch of the nonclassical ion, while C4H5+ has no such free-proton vibration. Instead, C4H5+ has resonances near 1300 cm-1, consistent with a symmetrically shared proton in a di-bridged structure. Although the shared proton structure is not the lowest energy isomer of C4H5+, this species is apparently stabilized under the supersonic beam conditions. Larger clusters containing additional acetylene units are also investigated via the elimination of acetylene. These species have new IR bands indicating that rearrangement reactions have taken place to produce core C4H5+ ions with the methyl cyclopropane cation structure and/or the protonated cyclobutadiene isomer. Ab initio (MP2) calculations provide structures and predicted spectra consistent with all of these experiments.
    No preview · Article · Apr 2008 · The Journal of Physical Chemistry A