Computational and Experimental Confirmation of the Diradical Character of para-Quinonedimethide

The ground-state structure of the parent para-quinonedimethide (p-QDM) molecule is generally represented in its closed shell form, i.e., as a cyclic, nonaromatic, through-conjugated/cross-conjugated hybrid comprising four C═C bonds. Nonetheless, p-QDM has been theorized to contain a contribution from its open-shell aromatic singlet diradical form. VBSCF calculations identify an open-shell contribution of 29% to the structure, while CASPT2(16,16)/def2-TZVP and ωB97XD/aug-cc-pVTZ calculations predict that dimerization proceeds along an open-shell singlet diradical pathway with a low (77 kJ/mol) barrier toward dimerization, which occurs by way of C-C bond formation between the exocyclic methylene carbons. A similar low (98 kJ/mol) barrier exists toward the reaction between a p-QDM molecule and the radical trap TEMPO. These predictions are verified experimentally through the isolation of bis-TEMPO-trapped p-QDM, its C-C coupled dimer, and by demonstrating that a mixture of p-QDM and TEMPO can initiate the radical polymerization of n-butyl acrylate at ambient temperature. In contrast to p-QDM, tetracyanoquinone (TCNQ) neither dimerizes nor reacts with TEMPO, despite having a similar diradical character to p-QDM. This lack of reactivity is consistent with both a higher kinetic barrier and a thermodynamically unfavorable process, which is ascribed to destabilizing steric clashes and polar effects.