Nucleophilic Addition to Singlet Diradicals: Homosymmetric Diradicals

Experiments have demonstrated that nucleophiles can attack singlet diradicals to generate bonded, closed-shell addition products. Here, we present a molecular orbital analysis for this reaction, focusing on the addition of nucleophiles to homosymmetric diradicals. We show that beginning with the Salem-Rowland molecular orbital description of homosymmetric diradicals, a continuous progression from open-shell diradical to closed-shell addition product occurs during the reaction via a gradual evolution of orbital and configuration interaction coefficients. This theoretical framework is supported by high-level multireference computations (CASPT2, EOM-SF-CCSD(dT)) using the addition of chloride to p-benzyne to generate a p-chlorophenyl anion as a case study. When using levels of theory that include dynamic correlation, the reaction is predicted to be barrierless. No abrupt switch from diradical to closed-shell species happens during the mechanism, but rather a gradual decrease in diradical character occurs as the nucleophile approaches the radical center before ultimately transforming into the closed-shell anion. The overarching conclusion from this work is that there are no electronic impediments of any kind, deriving from orbital symmetry or from any other source, that exist for the addition of nucleophiles to homosymmetric singlet diradicals.