Karadakov Peter B
Department of Chemistry, University of York, Heslington, York YO10 5DD, UK.
J Phys Chem A. 2008 Dec 11;112(49):12707-13. doi: 10.1021/jp8067365.
The levels of aromaticity of the most important geometries on the ground-state (S(0)), lowest triplet-state (T(1)), and first singlet excited-state (S(1)) potential energy surfaces (PESs) for cycloocta-1,3,5,7-tetraene (COT) are assessed using a wide range of magnetic criteria including nucleus-independent chemical shifts (NICSs), proton shieldings, and magnetic susceptibilities calculated using complete-active-space self-consistent-field (CASSCF) wave functions constructed from gauge-including atomic orbitals (GIAOs). It is shown that the ground state of D(8h) COT (transition state for the pi-bond-shift process on the S(0) PES) is markedly antiaromatic, even more so than the classical example of an antiaromatic system, the ground state of square cyclobutadiene. The CASSCF-GIAO magnetic properties of the ground state of D(4h) COT (transition state for the ring-inversion process on the S(0) PES) strongly suggest that it is much less antiaromatic than the ground state of D(8h) COT, whereas those of the ground state of D(2d) COT (local minimum on the S(0) PES) indicate that it is decidedly nonaromatic. The lowest triplet state and the first singlet excited state of D(8h) COT (local minima on the T(1) PES and the S(1) PES, respectively) exhibit surprisingly similar magnetic properties. These, in turn, are very close to the magnetic properties of benzene, which is a strong indication of a high degree of aromaticity.
使用多种磁性标准，包括核独立化学位移（NICSs）、质子屏蔽以及利用由含规范原子轨道（GIAOs）构建的完全活性空间自洽场（CASSCF）波函数计算得到的磁化率，来评估环辛 - 1,3,5,7 - 四烯（COT）在基态（S(0)）、最低三重态（T(1)）和第一单重激发态（S(1)）势能面（PESs）上最重要几何结构的芳香性水平。结果表明，D(8h) COT的基态（S(0) PES上π键迁移过程的过渡态）具有明显的反芳香性，甚至比反芳香体系的经典例子——方形环丁二烯的基态还要强。D(4h) COT基态（S(0) PES上环反转过程的过渡态）的CASSCF - GIAO磁性性质强烈表明，它的反芳香性比D(8h) COT的基态要小得多，而D(2d) COT基态（S(0) PES上的局部极小值）的磁性性质表明它绝对是非芳香性的。D(8h) COT的最低三重态和第一单重激发态（分别为T(1) PES和S(1) PES上的局部极小值）表现出惊人相似的磁性性质。反过来，这些性质与苯的磁性性质非常接近，这强烈表明其具有高度的芳香性。
