Some Comments on Topological Approaches to the 蟺-Electron Currents in Conjugated Systems
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- 作者：Timothy K. Dickens ; Jos茅 A. N. F. Gomes ; Roger B. Mallion
- 刊名：Journal of Chemical Theory and Computation
- 出版年：2011
- 出版时间：November 8, 2011
- 年：2011
- 卷：7
- 期：11
- 页码：3661-3674
- 全文大小：1179K
- 年卷期：v.7,no.11(November 8, 2011)
- ISSN：1549-9626
文摘
Within the past two years, three sets of independent authors (Mandado, Ciesielski et al., and Randi膰) have proposed methods in which 蟺-electron currents in conjugated systems are estimated by invoking the concept of circuits of conjugation. These methods are here compared with ostensibly similar approaches published more than 30 years ago by two of the present authors (Gomes and Mallion) and (likewise independently) by Gayoso. Patterns of bond currents and ring currents computed by these methods for the nonalternant isomer of coronene that was studied by Randi膰 are also systematically compared with those calculated by the H眉ckel鈥揕ondon鈥揚ople鈥揗cWeeny (HLPM) 鈥渢opological鈥?approach and with the ab initio, 鈥渋pso-centric鈥?current-density maps of Balaban et al. These all agree that a substantial diamagnetic 蟺-electron current flows around the periphery of the selected structure (which could be thought of as a 鈥減erturbed鈥?[18]-annulene), and consideration is given to the differing trends predicted by these several methods for the 蟺-electron currents around its central six-membered ring and in its internal bonds. It is observed that, for any method in which calculated 蟺-electron currents respect Kirchhoff鈥檚 Laws of current conservation at a junction, consideration of bond currents鈥攁s an alternative to the more-traditional ring currents鈥攃an give a different insight into the magnetic properties of conjugated systems. However, provided that charge/current conservation is guaranteed鈥攐r Kirchhoff鈥檚 First Law holds for bond currents instead of the more-general current-densities鈥攖hen ring currents represent a more efficient way of describing the molecular reaction to the external magnetic field: ring currents are independent quantities, while bond currents are not.