Hidden diradical character plays a critical yet often underappreciated role in controlling chemical reactivity and reaction mechanisms. Although any covalent bond may, in principle, access diradical configurations through homolysis, quantitative assessments of diradical character are highly method dependent and frequently difficult to interpret. To address this challenge, this work develops an integrated framework that combines valence bond (VB) theory, molecular orbital theory, and density functional theory, leveraging their complementary strengths to achieve a unified description of diradicaloid reactivity.
This approach is first applied to para- and ortho-quinodimethane (p‑QDM and o‑QDM), which exhibit similar (~30%) diradical character by VB analysis but markedly different reactivity. p‑QDM dimerizes through an open-shell singlet (OSS) transition state, whereas o‑QDM predominantly follows a concerted [4+2] cycloaddition, with radical pathways accessible only at elevated temperatures. These mechanistic distinctions are validated by high-level multireference calculations and supported experimentally by TEMPO addition studies, revealing unexpected radical-like reactivity.
The framework is then extended to topochemical polymerization of para-aza-quinodimethane and to acid-catalyzed self-initiation of vinyl monomers, where electrostatic fields lower initiation temperatures to ~70 °C, offering a greener alternative to conventional initiators. Finally, a novel trisulfide metathesis mechanism involving an OSS diradical transition state is identified, demonstrating the broad relevance of hidden diradical character beyond conjugated hydrocarbons. Overall, this work establishes hidden diradical character as a general and chemically consequential phenomenon, providing new opportunities for reaction design, polymer synthesis, and materials discovery.