5-Dehydro-1,3-quinodimethane: a hydrocarbon with an open-shell doublet ground state.

electrons, that is, an “open-shell doublet.” Although low-spin, open-shell states occur in molecules containing transition metals, the ground states of organic molecules are rarely of this type. Organic biradicals can have open-shell singlet ground states, depending on the orbital structure, but DMX is the first example of an organic triradical with an open-shell doublet ground state. Qualitatively, the preference for the open-shell doublet ground state in DMX can be understood by considering the electronic structures in m-xylylene (MX) and a,3-dehydrotoluene (DHT). The MX biradical has a triplet ground state with two ferromagnetically coupled electrons in two p molecular orbitals (Figure 1). DHT, also shown in Figure 1, is a sp biradical with singlet-coupled unpaired electrons, a result of spin polarization that favors antiferromagnetic coupling. The features responsible for the MX and DHT ground state multiplicities are also present in DMX. The p system is similar to that of MX, such that it should contain two ferromagnetically coupled electrons. Likewise, the interaction between the s and p systems resembles that in DHT, and should therefore have a preference for antiferromagnetic coupling. The net result is a hydrocarbon with a spp, openshell doublet, ground state configuration (Figure 1). More rigorously, the electronic structure of triradicals is described by the three-electrons-in-three-orbitals model. Different occupations of three nearly degenerate triradical orbitals result in one quartet and eight doublet low-lying states. The eight lowest energy electronic states of DMX are shown in Figure 2. Orbital near-degeneracies in the triradical