Department of Chemistry, University of Nebraska , Lincoln, Nebraska 68588-0304, United States.
J Am Chem Soc. 2013 Dec 4;135(48):18205-15. doi: 10.1021/ja409472f. Epub 2013 Nov 19.
Aminyl tetraradicals with planar tetraazanonacene backbones have quintet (S = 2) ground states and do not show any detectable thermal population of the low-spin excited states up to the highest temperature investigated (100 K) in the 2-methyltetrahydrofuran (2-MeTHF) matrix. This indicates that the nearest electronic excited state (triplet) is at least ~0.3 kcal mol(-1) higher in energy, that is, the triplet-quintet energy gap, ΔE(TQ) > 0.3 kcal mol(-1), which is consistent with the broken-symmetry-DFT-computed ΔE(TQ) of about 5 kcal mol(-1). In concentrated (ca. 1-10 mM) solutions of tetraradical 4 in 2-MeTHF at 133 K, a fraction of tetraradicals form a dimer (association constant, K(assoc) ≈ 60 M(-1)), with a weak, antiferromagnetic exchange coupling, J/k ≈ -0.1 K ~ 0.2 cal mol(-1), between the S = 2 tetraradicals. This weak intradimer exchange coupling is expected for two tetraradicals at the distance of about 6 Å. The most sterically shielded tetraradical 5 in 2-MeTHF has a half-life of 1 h at room temperature; the product of its decay is the corresponding tetraamine, suggesting that the hydrogen atom abstraction from the solvent is primarily responsible for the decomposition of the tetraradical.
具有平面四氮杂萘骨架的氨基四自由基具有 quintet（S = 2）基态，在最高研究温度（100 K）下，在 2-甲基四氢呋喃（2-MeTHF）基质中未检测到低自旋激发态的任何可检测热分布。这表明，最近的电子激发态（三重态）至少高出约 0.3 kcal mol(-1)的能量，即三重态-五重态能隙，ΔE(TQ) > 0.3 kcal mol(-1)，这与broken-symmetry-DFT 计算的 ΔE(TQ)约 5 kcal mol(-1)一致。在 2-MeTHF 中浓度约为 1-10 mM 的四自由基 4 的溶液中，一部分四自由基形成二聚体（缔合常数，K(assoc)≈60 M(-1)），S = 2 的四自由基之间存在较弱的反铁磁交换耦合，J/k≈-0.1 K~0.2 cal mol(-1)。这种较弱的二聚体交换耦合预计在距离约 6 Å 的两个四自由基之间存在。在 2-MeTHF 中具有最大空间位阻的四自由基 5 在室温下的半衰期为 1 小时；其衰变的产物是相应的四胺，表明溶剂中的氢原子提取主要负责四自由基的分解。
