New nitroxide biradicals 15NR6–C≡C–(p-C6H4)2–C≡C–14NR6 (B2) and 15NR6–C≡C–(p-C6H4)2–C≡C–15NR6 (B3), where R6 denotes 1-oxyl-2,2,6,6-tetramethyl–1,2,3,6-tetrahydropyridine ring, are synthesized and studied in liquid solutions by continuous-wave electron paramagnetic resonance (EPR) spectroscopy. Hyperfine interaction constants and exchange integrals are determined from the simulation and fitting of experimental spectra and calculated ones. The best fitting of the EPR spectra detected in experiments and their temperature dependence was achieved under the assumption that the biradicals perform transitions between conformations with different exchange integrals. The conformation of about 75% of biradicals has the exchange integral |$$J$$| = 4.4 G. Thus, the ground electronic state of these biradicals has the conformation with |$$J$$| = 4.4 G. About 20% of biradicals have the conformation with the zero exchange integral. A minor fraction of biradicals have conformations with |$$J$$| = 11.2 G and |$$J$$| = 57 G. The activation energy for the transition from the conformation with |$$J$$| = 4.4 G to the conformation with $$J$$ = 0 is 8.5 kcal/mol, while that for the reverse reaction is 1.7 kcal/mol. It is shown that the isotope substitution provides a valuable resource in the EPR studies of the exchange interaction in biradicals.

中文翻译：

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氮氧化物双自由基 EPR 研究中同位素替代的潜力

新的氮氧双自由基 15NR6–C≡C–(p-C6H4)2–C≡C–14NR6 (B2) 和 15NR6–C≡C–(p-C6H4)2–C≡C–15NR6 (B3)，其中 R6 表示1-氧基-2,2,6,6-四甲基-1,2,3,6-四氢吡啶环，在液体溶液中合成并通过连续波电子顺磁共振（EPR）光谱进行研究。超精细相互作用常数和交换积分由实验光谱和计算光谱的模拟和拟合确定。实验中检测到的 EPR 光谱及其温度依赖性的最佳拟合是在双自由基在具有不同交换积分的构象之间进行跃迁的假设下实现的。大约 75% 的双自由基的构象具有交换积分 |$$J$$| = 4.4 G。因此，这些双自由基的基电子态具有 |$$J$$| 的构象。= 4.4 克。大约 20% 的双自由基具有零交换积分的构象。一小部分双自由基具有 |$$J$$| 的构象 = 11.2 G 和 |$$J$$| = 57 G。从具有 |$$J$$| 的构象跃迁的活化能 = 4.4 G 到 $$J$$ = 0 的构象为 8.5 kcal/mol，而逆反应为 1.7 kcal/mol。结果表明，同位素取代为双自由基交换相互作用的 EPR 研究提供了宝贵的资源。