A method for the synthesis of rigid nitroxide biradicals with various spatial orientations between the radical centers is reported. Diketones were employed as substrates for tin amine protocol (SnAP) reagents to provide the parent spirocyclic diamines. Oxidation by peroxyacids provided the corresponding nitroxide biradicals. A set of four different biradicals with various interelectron distances and torsion angles between the radical planes was synthesized using this method. The exact geometries were determined by X‐ray crystallography and the biradicals were investigated by EPR spectroscopy and evaluated for their dynamic nuclear polarization (DNP) performance. ¹H‐DNP enhancements in the range of 1.2–2.1 at 14.1 Tesla (600 MHz spectrometer) were achieved. This synthetic methodology opens a promising alternative to access nitroxide biradicals with various torsional angles and inter radical distances.

中文翻译：

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螺环一氧化氮双自由基：合成和评估作为动态核极化剂。

报道了一种在自由基中心之间具有不同空间取向的刚性氮氧化物双自由基的合成方法。将二酮用作锡胺规程（SnAP）试剂的底物，以提供母体螺环二胺。过氧酸氧化可提供相应的氮氧化物双自由基。使用该方法合成了一组四个不同的双基，这些基具有不同的电子间距和基面之间的扭转角。通过X射线晶体学确定确切的几何形状，并通过EPR光谱研究双自由基，并评估其动态核极化（DNP）性能。^1个在14.1特斯拉（600 MHz光谱仪）下，H-DNP增强了1.2-2.1。这种合成方法为使用具有各种扭转角和自由基间距的氮氧化物双自由基开辟了一个有希望的替代方法。