Frontier orbital interactions dominating atom transfer reactions. Radical-molecule atom transfers can usually be modeled as three electron problems. Initially, the molecule (RH, colored orbitals) and radical (X, open orbitals) are far apart (upper left). Three configurations are of interest: (1) the ground state, G, with two electrons in the RH HOMO and one in the X SOMO, (2) an ionic state, I, with an electron transfered from RH to X, and a 'triplet' state, T, with an electron trasfered to the RH LUMO. All configurations are doublets (S=1/2). At the transition state (lower center), two configurations dominate: (1) the ground state, G, and a promoted state, P. Each configuration has two electrons in a fully delocalized MO spanning the entier TS macromolecule. The third electron is forced by the Pauli principle into a higher energy orbital, and the energies of the two states, G and P, cross as the atom is transfered. In many cases the promoted TS configuration corresponds to the ionic molecular configuration.