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
journal contributionposted on 05.02.2009, 00:00 by Hao Dong, David A. Hrovat, Helmut Quast, Weston Thatcher Borden
CASSCF and CASPT2/6-31G(d) calculations have been performed on the low-lying electronic states of three, non-Kekulé, 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 1A1 and 1B2 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 1A1 the ground state of a nitrogen analogue of 4 is proposed.