Nonclassical Aryl Radicals: Intermediates or Transition States for the Hydrogen Shift Reactions?

Abstract
Electronic properties of aryl radicals obtained by removing single hydrogen atoms from the sterically congested regions of benzo[c]phenanthrene, biphenyl, triphenylene, phenanthrene, and perylene are studied at the UBLYP/6-311G** level of theory. Two structures are considered by each radical, the classical one involving a C−H···C arrangement of atoms and the nonclassical one possessing a three-center C−H−C linkage. The five nonclassical radicals under study are found to be transition states for degenerate 1,4- and 1,5-hydrogen shift reactions that interconvert the classical species. However, the results of the present calculations indicate that the nonclassical structures with the C−H distances in the C−H−C linkages shorter than 1.34 Å should be energy minima representing potentially observable chemical systems. The predicted energy barrier to the 1,5-hydrogen shift in the 1-benzo[c]phenanthrenyl radical is only 9.3 kcal/mol (6.1 kcal/mol with the zero-point energies included), making the hydrogen migration in this system facile at relatively low temperatures. Rigorous analysis of the computed electronic wave functions provides a clear-cut picture of bonding in both the classical and nonclassical aryl radicals.

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