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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

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journal contribution
posted 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 24. 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 24 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.