AI 中文总结
本文提出统一键合熵模型，发现Kekulé石墨烯纳米片开壳层稳定的新机制，为筛选开壳层碳纳米结构及设计可调分子自旋提供了新途径。
AI 中文摘要
Kekulé石墨烯纳米片（GNFs）的开壳层特性通常通过电子未成对时Clar芳香π六重态的增益来解释。尽管该规则成功解释了许多醌型双自由基化合物，但它仅考虑了六重态的最大数量，而忽略了实现相同Clar数的共振构型的多重性和空间分布。本文中，我们确定了开壳层稳定化的第二条途径：最大Clar六重态数保持不变，而可及Clar共振器的数量显著增加，我们将该机制称为“Clar数不变的共振空间扩展”。通过枚举闭壳层和开壳层Clar共振器，并结合键合熵模型（BEM）分析，我们表明电子未成对可释放闭壳层配对约束、扩大共振流形并重新分布C-C键占据率，使其远离定域单键和双键的极限。BEM预测的未成对电子数量及空间分布，与广泛GNFs的密度泛函理论双自由基特性、局域磁矩、优化后的C-C键长及相对能量密切相关。该框架提供了一种基于图且物理上透明的途径，用于筛选开壳层碳纳米结构及设计可调分子自旋，无需增加最大Clar数。
英文摘要
The open-shell character of Kekulé graphene nanoflakes (GNFs) is conventionally rationalized by the gain of Clar aromatic $π$-sextets upon electron unpairing. While this rule successfully explains many quinoidal diradicaloids, it treats only the maximum number of sextets and neglects the multiplicity and spatial distribution of resonance configurations that realize the same Clar count. Here, we identify a second route to open-shell stabilization in which the maximum Clar-sextet number remains unchanged while the number of accessible Clar resonators increases substantially. We term this mechanism \emph{Clar-number-invariant resonance-space expansion}. By enumerating closed-shell and open-shell Clar resonators and combining this analysis with a bonding entropy model (BEM), we show that electron unpairing can release closed-shell pairing constraints, enlarge the resonance manifold, and redistribute C--C bond occupancies away from localized single- and double-bond limits. The BEM-predicted number and spatial distribution of unpaired electrons correlate strongly with density-functional-theory diradical character, local magnetic moments, optimized C--C bond lengths, and relative energies across a broad set of GNFs. The resulting framework offers a graph-based and physically transparent route for screening open-shell carbon nanostructures and for designing tunable molecular spins without requiring an increase in the maximum Clar number.
