We have utilized highly correlated, multireference quantum chemical methods to characterize the low lying electronic states of tetradehydronaphthalene and tetradehydrofulvalene tetraradicals formed by electrocyclization of tetraethynylethene. We find that that both forms of the tetraradical are stable and correspond to mimima on the potential energy surface. The tetradehydronaphthalene tetraradical lies approximately 70 kcal/mol lower in energy than the fulvalene tetraradical, and the barrier to a double 1,6 cyclization is substantially lower than the barrier to 1,5 double cyclization. Multireference coputations suggests a 1Ag ground state for the tetradehydronaphthalene tetraradical, in addition to a high-density cluster of six states near the ground state. The low spin ground state and its leading configurations imply that through bond coupling is the source of stabilization in the singlet, which outweighs aromatic stabilizing effects that dominate the quintet. Analysis of geometries also suggests that through bond coupling has a stabilizing effect, and it likely occurs between radical centers on the same hemisphere, in addition to centers across hemispheres through the central bond. Preliminary data suggests the tetradehydrofulvalene has similar electronic properties to tetradehydronaphthalene and a low spin ground state stabilized by through bond effects.

A highly correlated, multireference study of aromatic tetraradicals

ACS Fall 2021

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