Steiner Erich, Fowler Patrick W
Department of Chemistry, School of Biological and Chemical Sciences, University of Exeter, Stocker Road, Exeter, UKEX4 4QD.
Org Biomol Chem. 2004 Jan 7;2(1):34-7. doi: 10.1039/b309426j. Epub 2003 Nov 6.
Ipsocentric current density maps are computed at the coupled Hartree-Fock level in the 6-31G** basis set for the planar C(2v) B3LYP geometries of the expanded porphyrins, sapphyrin and orangarin. Both give clearly dominant global macrocyclic ring currents, but with opposite senses of circulation: in 22[small pi] sapphyrin, a diatropic current runs, with some bifurcation, around the conventional 22-centre delocalisation pathway; in 20[small pi] orangarin, a paratropic current runs around the inner 17-atom pathway. In agreement with the annulene analogy for these macrocycles, analysis of orbital contributions shows that in each case topology, energy and symmetry of the frontier orbitals together determine the macrocyclic ring current. In sapphryrin, 4-electron diamagnetism (aromaticity) arises from translationally allowed HOMO-LUMO excitations as in benzene itself; in orangarin, 2-electron paramagnetism (antiaromaticity) arises from rotationally allowed HOMO-LUMO excitations as in planarised cyclooctatetraene. The active orbitals invoked in the explanation of ring currents are those involved in the longstanding four-orbital model of porphyrin electronic spectra.
在6-31G**基组中，于耦合哈特里-福克水平下，针对扩展卟啉、蓝宝石卟啉和橙黄卟啉的平面C(2v) B3LYP几何结构计算了以自身为中心的电流密度图。两者均呈现出明显占主导的全局大环环流，但环流方向相反：在22π蓝宝石卟啉中，一个抗磁电流沿着传统的22中心离域路径运行，伴有一些分支；在20π橙黄卟啉中，一个顺磁电流沿着内部的17原子路径运行。与这些大环的轮烯类比一致，轨道贡献分析表明，在每种情况下，前沿轨道的拓扑结构、能量和对称性共同决定了大环环流。在蓝宝石卟啉中，4电子抗磁性（芳香性）源于如苯本身那样的平移允许的最高占据分子轨道-最低未占分子轨道激发；在橙黄卟啉中，2电子顺磁性（反芳香性）源于如平面化环辛四烯那样的旋转允许的最高占据分子轨道-最低未占分子轨道激发。在解释环流时所涉及的活性轨道是那些参与卟啉电子光谱长期存在的四轨道模型的轨道。
