The State Key Laboratory of Refractories and Metallurgy, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China.
Aix Marseille Univ, CNRS, ICR, 13007 Marseille, France.
Molecules. 2020 Feb 11;25(4):776. doi: 10.3390/molecules25040776.
Stable organic free radicals are increasingly studied compounds due to the multiple and unusual properties imparted by the single electron(s). However, being paramagnetic, classical methods such as NMR spectroscopy can hardly be used due to relaxation and line broadening effects. EPR spectroscopy is thus better suited to get information about the immediate surroundings of the single electrons. EPR has enabled obtaining useful data in the context of host•guest chemistry, and a classical example is reported here for the stable (2,2,6,6-tetramethyl-4-oxo-piperidin-1-yl)oxyl or 4-oxo-TEMPO nitroxide () inside the macrocycle host cucurbit[7]uril (CB[7]). Generally and also observed here, a contraction of the spectrum is observed as a result of the reduced nitrogen coupling constant due to inclusion complexation in the hydrophobic cavity of the host. Simulations of EPR spectra allowed determining the corresponding binding constant pointing to a weaker affinity for CB[7], compared to TEMPO with CB[7]. We complement this work by the results of EPR spectroscopy of a biradical: bis-TEMPO-bis-ketal () with cucurbit[8]uril (CB[8]). Initial investigations pointed to very weak effects on the spectrum of the guest and incorrectly led us to conclude an absence of binding. However, simulations of EPR spectra combined with NMR data of reduced allowed showing inclusion complexation. EPR titrations were performed, and the corresponding binding constant was determined. H NMR spectra with reduced suggested a shuttle mechanism, at nearly one equivalent of CB[8], for which the host moves rapidly between two stations.
稳定的有机自由基因其单电子赋予的多种不寻常性质而越来越受到研究。然而，由于弛豫和谱线展宽效应，顺磁性使其很难用诸如 NMR 光谱等经典方法进行研究。因此，EPR 光谱更适合用于获取有关单电子周围环境的信息。EPR 已经能够在主体-客体化学背景下获取有用的数据，这里报道了一个稳定的（2,2,6,6-四甲基-4-氧代哌啶-1-基）氧自由基或 4-氧代-TEMPO 氮氧自由基（）在大环主体葫芦脲（CB[7]）内部的经典实例。通常，正如这里观察到的，由于包含络合作用导致氮偶合常数减小，从而导致光谱收缩。EPR 光谱的模拟允许确定相应的结合常数，表明与 TEMPO 相比，其与 CB[7]的亲和力较弱。我们通过葫芦[8]脲（CB[8]）的双 TEMPO-双缩酮（）的 EPR 光谱结果对此项工作进行了补充。初步研究表明，客体的光谱几乎没有受到很弱的影响，这使我们错误地得出了不存在结合的结论。然而，EPR 光谱的模拟与还原后的 NMR 数据相结合，表明存在包含络合作用。进行了 EPR 滴定，并确定了相应的结合常数。还原后的 H NMR 光谱表明，对于几乎相当于一个 CB[8]当量的情况，主体在两个位置之间快速移动，存在穿梭机制。
