The hydrogen bond plays a key role in weak directional intermolecular interactions. It is operative in determining molecular conformation and aggregation, and controls the function of many chemical systems, ranging from inorganic, organic to biological molecules. Although an enormous amount of spectroscopic information has been collected about hydrogen-bond formation between molecules with closed-shell electronic configuration, the details of such interactions between open-shell radicals and closed-shell molecules are still rare. Here we report on an investigation of hydrogen-bonded complexes between pyrroline-type as well as piperidine-type neutral nitroxide radicals and an alcohol, i.e., 2-propanol. These nitroxide radicals are commonly used as EPR spin labels and probes. To obtain information on the geometry of the complexes and their electronic structure, multi-resonance EPR techniques at various microwave frequencies (X-, Q-, W-band, 244 GHz) have been employed in conjunction with DFT calculations. The planar five-membered ring system of the pyrroline-type nitroxide radical was found to form exclusively well-defined in-plane σ-type hydrogen-bonded complexes with one 2-propanol molecule in the first solvation shell in frozen solution. The measured hyperfine parameters of the hydrogen-bridge proton and the internal magnetic parameters describing the electron Zeeman and the electron-nuclear hyperfine and nuclear quadrupole interactions are in good agreement with values predicted by state-of-the-art DFT calculations. In contrast, multi-resonance EPR on the non-planar six-membered ring system of the piperidine-type nitroxide radical (TEMPOL) reveals a more complex situation, i.e., a mixture of a σ-type with, presumably, an out-of-plane π-type complex, both present in comparable fraction in frozen solution. For TEMPOL, the DFT calculations failed to predict magnetic interaction parameters that are in good agreement with experiment, apparently due to the considerable flexibility of the nitroxide and hydrogen-bonded complex. The detailed information about nitroxide/solvent complexes is of particular importance for Dynamic Nuclear Polarization (DNP) and site-directed spin-labeling EPR studies that employ nitroxides as polarizing agents or spin labels, respectively.

氢键在弱定向分子间相互作用中起着关键作用。它可用于确定分子构象和聚集，并控制许多化学系统的功能，从无机、有机到生物分子。尽管已经收集了大量关于具有闭壳电子构型的分子之间形成氢键的光谱信息，但开壳自由基与闭壳分子之间这种相互作用的细节仍然很少见。在这里，我们报告了对吡咯啉型和哌啶型中性氮氧自由基与醇，即 2-丙醇之间的氢键配合物的调查。这些氮氧自由基通常用作 EPR 自旋标记和探针。为了获得复合物的几何形状及其电子结构的信息，已将各种微波频率（X-、Q-、W-波段、244 GHz）的多共振 EPR 技术与 DFT 计算结合使用。发现吡咯啉型氮氧自由基的平面五元环系统与冷冻溶液中第一个溶剂化壳中的一个 2-丙醇分子形成完全明确的面内 σ 型氢键配合物。氢桥质子的测量超精细参数和描述电子塞曼和电子-核超精细和核四极相互作用的内部磁参数与最先进的 DFT 计算预测的值非常一致。相比之下，哌啶型氮氧自由基 (TEMPOL) 的非平面六元环系统上的多共振 EPR 揭示了一种更复杂的情况，即 σ 型与可能是面外 π 的混合物型复合物，两者在冷冻溶液中都以相当的比例存在。对于 TEMPOL，DFT 计算未能预测与实验非常吻合的磁相互作用参数，这显然是由于氮氧化合物和氢键复合物的相当大的灵活性。关于氮氧化物/溶剂复合物的详细信息对于分别采用氮氧化物作为极化剂或自旋标记的动态核极化 (DNP) 和定点自旋标记 EPR 研究特别重要。平面外 π 型复合物，两者在冷冻溶液中都以相当的比例存在。对于 TEMPOL，DFT 计算未能预测与实验非常吻合的磁相互作用参数，这显然是由于氮氧化合物和氢键复合物的相当大的灵活性。关于氮氧化物/溶剂复合物的详细信息对于分别采用氮氧化物作为极化剂或自旋标记的动态核极化 (DNP) 和定点自旋标记 EPR 研究特别重要。平面外 π 型复合物，两者在冷冻溶液中都以相当的比例存在。对于 TEMPOL，DFT 计算未能预测与实验非常吻合的磁相互作用参数，这显然是由于氮氧化合物和氢键复合物的相当大的灵活性。关于氮氧化物/溶剂复合物的详细信息对于分别采用氮氧化物作为极化剂或自旋标记的动态核极化 (DNP) 和定点自旋标记 EPR 研究特别重要。