Keep Your TEMPO Up: Nitroxide Radicals as Sensors of Intermolecular Interactions

This study examines experimental data on the influence of the surrounding medium and non-covalent interactions on the isotropic hyperfine coupling constant, A_iso(¹⁴N), of the stable nitroxide radical 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) in solution. The data were used to identify a density functional theory functional/basis set combination that accurately reproduces the experimental A_iso(¹⁴N) values. The variations in A_iso(¹⁴N) due to external factors are two orders of magnitude greater than the accuracy of its experimental measurements, making A_iso(¹⁴N) a highly sensitive experimental probe for quantifying these effects. Additionally, it was found that the proton-accepting ability of the N-O^• moiety in TEMPO resembles that of the P=O moiety, enabling the simultaneous formation of two equally strong hydrogen bonds.