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

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...

Multi-objective goal-directed optimization of de novo stable organic radicals for aqueous redox flow batteries

“…Further, we estimate radical stability using a recently developed metric that incorporates both thermodynamic and kinetic stabilization of the radical center using 3D structural features and electron spin density obtained via DFT. 23 Highly-delocalized and sterically protected radicals are prioritized by this approach. Synthesizability is considered by constraining the synthetic accessibility score (SAscore) of the closed-shell R-H molecule to be less than 4.0 35,49 and by ensuring that the R-H bond is relatively weak, with a homolytic bond dissociation enthalpy (BDE) of 60-80 kcal/mol.…”

“…Using the same chemical connectivity inputs, we trained a second surrogate GNN model on a recently published database of radical stability scores. 23,57 In this dataset, radical stability is correlated with two quantum chemical descriptors: the delocalization of the radical electron's spin, and the buried volume at the location of maximum spin (calculated as the fractional occupancy of a 3.5 Å radius sphere surrounding a target atom). 58 This GNN is trained to predict local aspects of the optimized 3D-geometry along with the quantum mechanical electron density (more precisely, the density difference between aand b-spin electrons) at each atomic position.…”

“…Stability scores range from near zero for highly unstable radicals (i.e., the methyl radical) to 75 or higher for radicals known to be stable experimentally. 23 Second, for the redox potential score, a maximum of 100 extra points were awarded for meeting each of four separate criteria (25 points each): (i) a reduction potential between -0.5 V and +0.2 V, (ii) an oxidation potential between +0.5 V and +1.2 V, (iii) a total voltage difference of at least 1 V, and (iv) an R-H BDE between 60-80 kcal/mol. BDEs were predicted for the hydrogen-terminated radical using a recently developed ML model.…”

“…Recent work has demonstrated that the stability of organic radicals, viewed in terms of thermodynamic stabilization and kinetic persistence, can be estimated using density functional theory (DFT). 23 In addition to stability, the electron exchange half-reactions of RFB candidates must have a precise redox potential for optimal performance. For organic radicals, their single-electron half-reaction potentials are determined by their adiabatic electron affinity and ionization energy, which can be reliably estimated via DFT.…”

t BuOLi-promoted terminal alkyne functionalizations by aliphatic thiols and alcohols

“…12 Interestingly, controlling the stability of radicals is one of the important tasks to afford any selectivity in a chemical reaction. 13 Concepts like radical hydrogen bonding 14 have been explored as well in organic synthesis. 7…”