The mechanism of spin polarization in aromatic free radicals

The spin-polarization mechanism in aromatic systems is analyzed with reference to the prototypical phenoxyl. cyclohexadienyl and benzyl radicals. In particular, a decomposition into first-order'' and second-order contributions is proposed, which helps to rationalize the different nature of the spin density for atoms in alpha or in beta positions with respect to the radical center. The different weights of the two contributions are discussed on the basis of Hartree-Fock and density functional computations.