Monaco Guglielmo, Scott Emeritus Lawrence T, Zanasi Riccardo
Dipartimento di Chimica e Biologia "A. Zambelli" Università degli studi di Salerno via Giovanni Paolo II 132 Fisciano 84084 SA Italy.
Department of Chemistry University of Nevada Reno NV 89557-0216 USA.
ChemistryOpen. 2020 May 27;9(5):616-622. doi: 10.1002/open.202000091. eCollection 2020 May.
The three-fold HOMO-LUMO gap oscillation, typical of finite length armchair carbon nanotubes (CNT), has a major effect on the magnetic response of ultrashort, single-end-capped [5,5] carbon nanotubes to a perturbing magnetic field parallel to the main symmetry axis. For the CNT's containing 40, 70, and 100 carbon atoms, for which 100 % of the C=C double bonds can be grouped into aromatic-sextets, i. e., fully or complete Clar networks, large paratropic (antiaromatic) global circulations around the cylindrical axis are predicted at the DFT level of calculation. Local and semi-global diatropic (aromatic) currents of strengths not larger than that of the benzene molecule are determined for a perpendicular perturbing magnetic field. CNTs of intermediate lengths do not display this enhanced antiaromatic response. The paratropic current flow clearly shows that these complete Clar networks can be viewed as stacked cycloparaphenylene belts, each providing a double annulene circuit as a consequence of the quinoidal resonance structure that results from their closure. Paradoxically, the fully aromatic Clar structure itself is responsible for the enhanced global antiaromaticity.
典型的有限长度扶手椅型碳纳米管（CNT）的三重HOMO-LUMO能隙振荡，对超短单端封端的[5,5]碳纳米管对平行于主对称轴的扰动磁场的磁响应有重大影响。对于含有40、70和100个碳原子的碳纳米管，其100%的C = C双键可归为芳香六隅体，即完全或完整的克拉尔网络，在密度泛函理论（DFT）计算水平下预测，围绕圆柱轴会有大的顺磁（反芳香）全局环流。对于垂直扰动磁场，确定了强度不大于苯分子强度的局部和半全局抗磁（芳香）电流。中等长度的碳纳米管不显示这种增强的反芳香响应。顺磁电流流动清楚地表明，这些完整的克拉尔网络可视为堆叠的环对亚苯基带，由于其闭合产生的醌型共振结构，每个环对亚苯基带都提供了一个双轮烯回路。矛盾的是，完全芳香的克拉尔结构本身导致了增强的全局反芳香性。
