Rotational observation of kinetically hindered neutral radical pairs.

Probing the molecular structure of radical pairs has remained challenging because of their short lifetimes. Here we use the persistent nitroxyl radical 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) as model system, reporting the rotational characterization of its radical pairs in a supersonic molecular jet. The experiment employs isotopic-sensitive high-resolution microwave spectroscopy, revealing two distinct neutral pairing patterns avoiding electron transfer. In each radical pair the nitroxyl molecules establish non-covalent van der Waals interactions with N···O distances of 3.38 and 4.93 Å, respectively, as confirmed by 18O-labeling experiments. The absence of electron spin hyperfine effects is consistent with quantum mechanical density functional predictions favoring an unexpected open-shell singlet. These results starkly contrast with predictions for the dimethyl nitroxyl radical dimer, which undergoes a spontaneous hydrogen transfer to closed-shell species and confirms the kinetically hindered or frustrated character of the TEMPO dimer. The experiment thus provides a direct molecular-level picture of neutral radical pairs in isolation and open avenues for exploring the quantum structure and reactivity of correlated radical species.