Unraveling the Isotropic Hyperfine Coupling Constants of Nitroxide Radicals via Molecular Electrostatic Potential Analysis

Nitroxide radicals have wide and promising applications as organic magnetic materials. Modulating the isotropic hyperfine coupling constants (HFCCs) of these radicals through proper structural design is an effective strategy for their application as spin probes and spin labels. In the present work, density functional theory calculations were carried out to develop a robust descriptor based on the molecular electrostatic potential for nitrogen HFCCs of nitroxide radicals. Forty nitroxide radicals from five distinct classes, namely, derivatives of cyclic, acyclic, imino, nitronyl, and benzimidazole nitronyl nitroxides, were selected, and the molecular electrostatic potential (MESP) at the nitrogen atom (V_N) of the NO moiety was calculated. The V_N values efficiently capture the electronic changes associated with the steric and electronic nature of these systems. A significant correlation between V_N values and the experimental HFCCs of nitrogen nuclei demonstrates the applicability of V_N as a simple and efficient descriptor for monitoring HFCCs. Furthermore, a good correlation between V_N and experimental nitrogen HFCCs for each class of nitroxide radicals indicates the use of V_N in the evaluation of the magnetic nature of the nitroxide radicals. The findings in this work are expected to facilitate the design of novel nitroxide radicals with desirable magnetic properties based on MESP topology analysis.