Ayub Rabia, El Bakouri Ouissam, Smith Joshua R, Jorner Kjell, Ottosson Henrik
Department of Chemistry - Ångström Laboratory, Uppsala University, Box 523, SE-751 20, Uppsala, Sweden.
Department of Chemistry, Humboldt State University, One Harpst Street, Arcata, California 95521, United States.
J Phys Chem A. 2021 Jan 21;125(2):570-584. doi: 10.1021/acs.jpca.0c08926. Epub 2021 Jan 11.
The aromaticity of cyclic 4π-electron molecules in their first ππ* triplet state (T), labeled Baird aromaticity, has gained growing attention in the past decade. Here we explore computationally the limitations of T state Baird aromaticity in macrocyclic compounds, 's, which are cyclic oligomers of four different monocycles (M = -phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 π-electrons in their main conjugation paths we find that for their T states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans ('s) retain Baird aromaticity until larger than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a with a specific is more strongly Baird-aromatic than the analogous while the magnetic indices tell the opposite. To construct large T state Baird-aromatic 's, the design should be such that the T state Baird aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel aromaticity of one or a few monocycles and semilocalized triplet diradical character. Monomers with lower Hückel aromaticity in S than benzene (e.g., furan) that do not impose steric congestion are preferred. Structural confinement imposed by, e.g., methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg-Haas description of T state aromaticity, we reveal the analogy to the Hückel aromaticity of the corresponding closed-shell dications yet observe stronger Hückel aromaticity in the macrocyclic dications than Baird aromaticity in the T states of the neutral macrocycles.
在过去十年中，处于其首个ππ*三重态（T）的环状4π电子分子的芳香性，即所谓的贝尔德芳香性，受到了越来越多的关注。在此，我们通过计算探索了大环化合物中T态贝尔德芳香性的局限性，这些大环化合物是由四种不同单环（M = -亚苯基（PP）、2,5-连接的呋喃（FU）、1,4-连接的环己-1,3-二烯（CHD）和1,4-连接的环戊二烯（CPD））组成的环状低聚物。尽管对于主共轭路径中π电子数高达20的大环化合物，我们发现基于UB3LYP几何结构而非UM06 - 2X和UCAM - B3LYP几何结构，其T态在规范UCCSD(T)和近似DLPNO - UCCSD(T)水平下的单点能量最低，但我们仍力求得出适用于各种DFT泛函的一般性结论。这一发现与之前在扩展卟啉基态中观察到的情况形成对比。然而，无论使用何种泛函，含有2,5-连接呋喃的大环化合物（）在比由其他三种单环组成的大环化合物更大时仍保持贝尔德芳香性。此外，从芳香性的几何、电子和能量方面来看，具有特定的的比类似的更具强烈的贝尔德芳香性，而磁学指标显示的情况则相反。为了构建大的T态贝尔德芳香性，设计应使大环周长的T态贝尔德芳香性主导一种情况，即一个或几个单环具有局部闭壳休克尔芳香性以及半局域化三重态双自由基特征。在S态中休克尔芳香性低于苯（例如呋喃）且不会造成空间拥挤的单体是优选的。例如亚甲基桥所施加的结构限制也是构建更大的贝尔德芳香性大环化合物的一种方法。最后，通过使用T态芳香性的齐尔伯格 - 哈斯描述，我们揭示了与相应闭壳双阳离子的休克尔芳香性的类比，但观察到大环双阳离子中的休克尔芳香性比中性大环化合物T态中的贝尔德芳香性更强。
