Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan.
J Phys Chem B. 2012 Jan 26;116(3):1053-9. doi: 10.1021/jp211391x. Epub 2012 Jan 11.
An ESR hyperfine splitting pattern of a biradical in solution depends on the magnitude of the intramolecular exchange interaction J(intra) compared with the hyperfine coupling constant A. Some biradicals exhibit their hyperfine splitting patterns characteristic of a monoradical, even though their exchange interaction is strong enough, |J(intra)| ≫ |A|. The contradiction in ESR spectroscopy is known as "biradical paradox", puzzling scientists for a long time. In this study, it is shown from ESR spectral simulations underlain by a theoretical model of a series of spin Hamiltonians that noncovalent aggregation of biradical molecules in solution leads to the appearance of paradoxical ESR spectra. Most of the spins in an aggregate of one dimension lose their contribution to the ESR spectra owing to intermolecular antiferromagnetic interactions J(inter), leaving two outermost spins ESR-active in the aggregate of one dimension. Paradoxical ESR spectra appear only when J(intra) and J(inter) fall within a particular range of the magnitudes which depends on the number of molecules in the aggregate.
溶液中双自由基的 ESR 超精细分裂模式取决于分子内交换相互作用 J(intra)与超精细耦合常数 A 的大小。尽管某些双自由基的交换相互作用足够强，|J(intra)| ≫ |A|，但它们仍表现出单自由基的超精细分裂模式。这种在 ESR 光谱学中的矛盾被称为“双自由基悖论”，长期以来一直困扰着科学家。在这项研究中，通过一系列自旋哈密顿量理论模型的 ESR 光谱模拟表明，溶液中双自由基分子的非共价聚集导致了悖论 ESR 光谱的出现。由于分子间反铁磁相互作用 J(inter)，一维聚集物中的大多数自旋失去了对 ESR 光谱的贡献，一维聚集物中只剩下两个最外层的自旋具有 ESR 活性。只有当 J(intra)和 J(inter)落在特定的数值范围内时，才会出现悖论 ESR 光谱，该范围的大小取决于聚集物中分子的数量。
