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<p>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 <sup>18</sup>O-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.</p>

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Rotational observation of kinetically hindered neutral radical pairs

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年7月1日

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  • Yongtao Wang,
  • Wenqin Li,
  • Xiaolong Yi,
  • Xinyu Wang,
  • Meng Li,
  • Jens-Uwe Grabow,
  • Carlos Cabezas,
  • Ibon Alkorta,
  • Cristóbal Pérez,
  • Alberto Lesarri,
  • Haoran Li

摘要

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.