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

Shree Sowndarya S V,Peter C St John,Robert S Paton

doi:10.1039/d1sc02770k

Shree Sowndarya S V, Peter C St John + Show 1 more

Open Access

https://doi.org/10.1039/d1sc02770k

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Abstract

Long-lived organic radicals are promising candidates for the development of high-performance energy solutions such as organic redox batteries, transistors, and light-emitting diodes. However, “stable” organic radicals that remain unreactive for an extended time and that can be stored and handled under ambient conditions are rare. A necessary but not sufficient condition for organic radical stability is the presence of thermodynamic stabilization, such as conjugation with an adjacent π-bond or lone-pair, or hyperconjugation with a σ-bond. 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. We describe a quantitative approach to map organic radical stability, using molecular descriptors intended to capture thermodynamic and kinetic considerations. The comparison of an extensive dataset of quantum chemical calculations of organic radicals with experimentally-known stable radical species reveals a region of this feature space where long-lived radicals are located. These descriptors, based upon maximum spin density and buried volume, are combined into a single metric, the radical stability score, that outperforms thermodynamic scales based on bond dissociation enthalpies in identifying remarkably long-lived radicals. This provides an objective and accessible metric for use in future molecular design and optimization campaigns. We demonstrate this approach in identifying Pareto-optimal candidates for stable organic radicals.

Highlights

From the initial discovery of free radicals to their becoming textbook chemistry, it has been emphasized that a molecule containing an unpaired electron is likely very reactive
Inspired by thermodynamic and kinetic considerations, we propose two key computational descriptors for quantifying radical stability, the maximum spin density and the buried volume around the atom where this spin is located
Stable radicals appear in a distinct region of this feature space associated with thermodynamic stability and kinetic persistence

Summary

Introduction

From the initial discovery of free radicals to their becoming textbook chemistry, it has been emphasized that a molecule containing an unpaired electron (i.e., a free radical) is likely very reactive. N-, O- and S-centered radicals are disproportionately represented among known stable radical structures.[27] This observation may appear counterintuitive from a thermodynamic standpoint since the BDE values of X–H bonds are generally larger than C–H bonds.

Results

Conclusion