Using an effective spin-dependent Heisenberg Hamiltonian, we have modeled the interaction between closedshell aromatic molecules and the free radical methyl. Physically, the model is based on a proportionality relationship, suggested by McConnell (McConnell, H. J. Chem. Phys. 1963, 41, 1910), between the product of the spin densities on each molecular fragment and the interaction energy of the system. The spin polarization, initially on the radical, is partly transferred to the molecule as the two fragments approach each other determining an effective spin interaction. The parameters of the effective Heisenberg-type Hamiltonian are determined through ab initio calculations of the electronic structure of the complex at the UMP2 level. Our calculations confirm the validity of a cage model recently proposed by Mujica et al. (Mujica, V.; Nieto, P.; Puerta, L.; Acevedo, S. Energy Fuels 2000, 14, 632) for spin trapping in fragments of asphaltenes while providing a consistent semiempirical approach to the interaction and stabilization problems.