<b>“</b>Stable” organic radicals are those that are unreactive for an extended period of time and can be stored and handled under ambient conditions. This stability results in unique properties that make them promising candidates for the development of high-performance energy solutions such as organic redox batteries, transistors and light emitting diodes. A necessary, but not sufficient condition for the formation of stable organic radicals is the presence of thermodynamic stabilization, such as conjugation with an adjacent pi-bond or lone-pair or hyperconjugation with a sigma-bond. The extent of thermodynamic stabilization exerted by a radical’s substituents is typically quantified in terms of the radical stabilization energy (RSE), based on differences in R-H bond dissociation energies. However, thermodynamic factors alone do not result in radicals with extended lifetimes: many resonance-stabilized radicals are transient species that exist for less than a millisecond. Kinetic stabilization is also necessary for persistence, such as steric effects that inhibit radical dimerization or reaction with solvent molecules. Unlike RSE, a quantitative scale for kinetic persistence has proven more elusive to define. While qualitative analyses are able to relate structural differences between organic radicals to their relative stabilities, a quantitative measure capable of identifying stable, persistent organic radicals has yet to emerge. Leveraging a large database of quantum chemical calculations for organic radicals and new calculations for experimentally known stable radical species, we put forth a quantitative metric for identifying these stable organic radicals. Motivated by thermodynamic and kinetic considerations this metric is built using two parameters associated with radical stability: spin delocalization and buried volume around the radical center. Unlike conventional thermodynamic metrics, our developed metric is able to capture the stability of experimentally known stable organic radicals like TEMPO. As an additional application, the metric is used to determine the stability of different radicals obtained in an organic cascade reaction. Overall, we show that the metric is able to explain and quantify the stability of new organic radicals.

A quantitative metric for organic radical stability and persistence using thermodynamic and kinetic features

ACS Fall 2021

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