Calculations of the relative energies of the low-lying electronic states of 2-methylenedihydrophenalene-1,3-diyl: effects of a 1,8-naphtho bridging group on trimethylenemethane and of a vinylidene bridging group on 1,8-naphthoquinodimethane.

CASSCF and CASPT2/6-31G(d) calculations have been performed on the low-lying electronic states of three, non-Kekule, hydrocarbon diradicals: 2-methylenedihydrophenalene-1,3-diyl (2), trimethylenemethane (3), and 1,8-naphthoquinodimethane (4). The computational results reveal how addition of ferromagnetic coupling groups (1,8-naphtho to 3 and vinylidene to 4) modulates the energy differences between the three lowest electronic states of 2-4. The most dramatic effect is the 30.4 kcal/mol change in the relative energies of the (1)A(1) and (1)B(2) states on addition of a vinylidene bridging group to 4 to form 2. The relative energies of the electronic states of 2-4 are discussed in terms of the topologies of the pair of nonbonding MOs for each state of each diradical, and a strategy for making (1)A(1) the ground state of a nitrogen analogue of 4 is proposed.