Monaco Guglielmo, Zanasi Riccardo
Department of Chemistry and Biology "A. Zambelli", University of Salerno, via Giovanni Paolo II 132, 84084 Fisciano, Italy.
Phys Chem Chem Phys. 2016 Apr 28;18(17):11800-12. doi: 10.1039/c5cp07250f.
Benzene, planar cyclooctatetraene, and borazine have been taken into account as archetypal aromatic, anti-aromatic, and non-aromatic systems. Then, the making of the π-electron diatropic ring current of benzene, huge paratropic ring current of planar cyclooctatetraene, and weak diatropic ring current of borazine, has been monitored by means of DFT calculations of current density maps and bond current strengths along a concerted, highly symmetric reaction pathway for the trimerization and tetramerization of acetylene to benzene and planar cyclooctatetraene and the trimerization of the simplest iminoborane (BH2N) to borazine. Besides, a simple model is presented that permits to infer the presence of a ring current only on account of the sum of homotropic local vortices. The model works satisfactorily for borazine and surely as well for benzene with a substantial difference. On the one hand, for borazine, the evolution of the current density along the formation reaction can be recast summing three virtually unchanged diatropic current loops with respect to parent iminoborane molecules. On the other hand, the benzene ring current is an emerging property that can be re-elaborated as the sum of three diatropic current loops of increased diatropicity with respect to parent acetylene molecules, i.e., the radius of the maximum current increases form 0.76 to 0.97 Å and the current strength increases from 3.6 to 6.7 nA T(-1). In these terms, the difference between the aromatic benzene and non-aromatic borazine can be understood as the attitude of the acetylene molecules to form always wider and stronger current loops as they get closer, a behavior not shared by the iminoborane molecules. For planar cyclooctatetraene, the paratropic circulation arising from the HOMO-LUMO transition makes the model inapplicable, since the initial hypothesis of homotropic circulations over the reaction coordinate is violated. In a sense, the fact that the model works only for a bit of the planar cyclooctatetraene formation reaction is itself distinctive of the anti-aromatic magnetic response.
苯、平面环辛四烯和硼氮苯已被视为典型的芳香、反芳香和非芳香体系。然后，通过对电流密度图和键电流强度进行DFT计算，监测了苯的π电子抗磁环电流、平面环辛四烯的巨大顺磁环电流以及硼氮苯的微弱抗磁环电流的形成过程，该计算沿着乙炔三聚和四聚生成苯和平面环辛四烯以及最简单的亚氨基硼烷（BH2N）三聚生成硼氮苯的协同、高度对称反应路径进行。此外，还提出了一个简单模型，该模型仅根据同向性局部涡旋的总和就能推断出环电流的存在。该模型对硼氮苯的效果令人满意，对苯也肯定适用，但存在显著差异。一方面，对于硼氮苯，沿着形成反应的电流密度演变可以通过将相对于母体亚氨基硼烷分子的三个几乎不变的抗磁电流环相加来重现。另一方面，苯环电流是一种新兴性质，可以重新解释为相对于母体乙炔分子具有更高抗磁度的三个抗磁电流环的总和，即最大电流的半径从0.76 Å增加到0.97 Å，电流强度从3.6 nA T(-1)增加到6.7 nA T(-1)。从这些方面来看，芳香性的苯和非芳香性的硼氮苯之间的差异可以理解为乙炔分子在靠近时形成越来越宽且越来越强的电流环的倾向，而亚氨基硼烷分子则不具备这种行为。对于平面环辛四烯，由HOMO-LUMO跃迁产生的顺磁环流使得该模型不适用，因为违反了反应坐标上同向性环流的初始假设。从某种意义上说，该模型仅适用于平面环辛四烯形成反应的一部分这一事实本身就是反芳香磁响应的独特之处。
