Stabilities and Spin Distributions of Benzannulated Benzyl Radicals

The effects have been studied of mono- and dibenzannulation of a benzyl radical with hybrid density functional theory (B3LYP) and quadratic configuration interaction theory (QCISD). Bond dissociation energies and enthalpies are reported that were determined at the common level QCISD/6-311G**//B3LYP/6-31G* for the benzylic C-H bonds of toluene 1H, the monobenzannulated polycyclic aromatic hydrocarbons (PAH) 1- and 2-methylnaphthalene 2H and 3H, the dibenzannulated PAHs 9-methylanthracene 4H and 9-methylphenanthrene 5H, and the model hydrocarbons 1-phenylpropene 6H and propene 7H. The conformational preferences and the symmetries of 1H-7H and of their corresponding radicals 1-7 have been determined. The analysis of the electron and spin density distributions of radicals 1-7 at the QCI level are reported, and these high-level data are discussed in comparison to results obtained with density functional theory and with an awareness of a general perception shaped by Hückel molecular orbital theory. The results show in a compelling fashion that electron and spin delocalization onto an annulated arene is not the decisive principle for stabilization of the benzyl radicals formed by homolysis of the methylated PAHs C10H7-CH3 and C14H9-CH3, and instead, the analysis of QCI spin density distributions suggests that spin delocalization onto annulated arenes is avoided as much as possible while spin polarization does occur to a significant extent.