Investigating Solvent Effects on the Stability of the Diethylnitroxyl Radical via Electron Paramagnetic Resonance.

Abstract

Operando electron paramagnetic resonance is used to monitor the light-initiated generation of the diethylnitroxyl radical from diethylhydroxylamine (DEHA) and its decay kinetics, thereby unveiling solvent effects on both the electronic structure and stability of the nitroxyl radical. The observed trends in hyperfine coupling constants (A_N and A_H) across different solvents align with previously reported values of the 4-amino-2,2,6,6-tetramethylpiperidoxyl radical (ATEMPO). Regarding the stability of the DEHA radical in various solvents, the obtained decay-kinetic constants (k) correlated more strongly with A_N than with the permittivity of the solvents. Moreover, distinguishing between protic and aprotic solvents leads to a better relationship between A_N and k, and a positive and constant correction of the nitroxyl radical's A_N supports the hydrogen-bonding effect in protic solvents. These findings indicate that mere hydrogen-bonding interaction does not enhance the stability of radicals. A_N of ATEMPO may serve as a superior metric for assessing the impact of solvent effects on both the electronic structure and stability of nitroxyl radicals. The data presented in this study will offer substantiation for the judicious selection of suitable solvents in reactions involving radicals.