Computational and Experimental Confirmation of the Diradical Character of <i>para</i>-Quinonedimethide
Zhipeng Pei, Nicholas L. Magann, Madison J. Sowden, Rhys B. Murphy, Michael G. Gardiner, Michael S. Sherburn, Michelle L. Coote
Abstract
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.