School of Chemistry, University of Hyderabad, Prof. C. R. Rao Road, Gachi Bowli, Hyderabad-500 046, India.
Org Biomol Chem. 2012 Mar 28;10(12):2439-46. doi: 10.1039/c2ob06875c. Epub 2012 Feb 15.
This paper presents the synthesis of a series of nitronyl-nitroxide (NN), oxoverdazyl (OVZ) based mono-, and bi-radicals attached to 4-phenyl-2,6-bispyrazolylpyridine coupling unit, their optical, electron spin resonance (ESR) spectroscopic studies and computational analysis. The ESR studies revealed that the axial zero-field splitting (zfs) parameter of the NN biradical (|D/hc| = 0.00719 cm(-1)) is larger than the OVZ biradical (|D/hc| = 0.00601 cm(-1)). Additionally both biradicals displayed forbidden half-field transitions (ΔM(s) = ±2; g(av) ~ 4.01) at 170 K demonstrating their triplet nature. The cryogenic ESR measurements of the two biradicals showed a Curie magnetic behaviour of the ΔM(s) = ±2 signal intensities (χ(EPR)) down to 4.2 K. A detailed comparative analysis of the strength of hyperfine coupling, spin density distribution, zfs and the spin-spin exchange coupling (J) of both NN and OVZ based biradicals showed that the ground state spin multiplicity of both biradicals is probably triplet (S = 1) or it is nearly degenerate singlet-triplet states with J(NN)≫J(OVZ).
本文介绍了一系列连接到 4-苯基-2,6-双吡唑基吡啶偶联单元的硝酰基-氮氧自由基（NN）、氧代重氮基（OVZ）单自由基和双自由基的合成、其光学和电子自旋共振（ESR）光谱研究和计算分析。ESR 研究表明，NN 双自由基的轴向零场分裂（zfs）参数（|D/hc| = 0.00719 cm(-1)) 大于 OVZ 双自由基的（|D/hc| = 0.00601 cm(-1))。此外，两个双自由基都在 170 K 下显示了禁戒的半磁场跃迁（ΔM(s) = ±2; g(av) ~ 4.01），证明了它们的三重态性质。两个双自由基的低温 ESR 测量显示出 ΔM(s) = ±2 信号强度（χ(EPR)) 的居里磁行为，直至 4.2 K。对 NN 和 OVZ 双自由基的超精细耦合强度、自旋密度分布、zfs 和自旋-自旋交换耦合（J）的详细比较分析表明，两个双自由基的基态自旋多重性可能是三重态（S = 1），或者它是近简并的单重态-三重态，其中 J(NN)≫J(OVZ)。
