Aromaticity of Fullerenes

Whether or not fullerenes have to be considered aromatic has been debated since their discovery [48]. This is because the definition of aromaticity [116-118] is controversial and has changed many times over the last 175 years. The structure of fullerenes, especially that of the icosahedral representatives such as Cjq, implies their consideration as three-dimensional analogues of benzene and other planar aromatics. In contrast to such classical systems, however, the sp -networks of 

---

For the spherical aromaticity of fullerenes, see M. Buhl, A. Hirsch, Chem. Rev. 

For polycyclic Jt-systems, there is not always a correlation between aromatic character and the total number of tt-electrons, as is the case for monocyclic annulenes91. In fullerenes, which are not only polycyclic but also three-dimensional, such a correlation is even less apparent. These carbon alio tropes embody completely conjugated spheroidal it-systems, so die carbon skeletons are boundary-less, and large numbers of Kekule structures can be drawn92. The aromaticity of fullerenes has been investigated theoretically and substantiated experimentally by using NMR studies93. 

Spherical aromaticity of fullerenes, polyhedral boranes, and related structures including heterofullerenes 05CRV3613. 

Nonplanar situations arise in fullerenes where a description of ji electrons in three dimensions is necessary. Haddon argued that orbital orthogonality is the key to the o—n separability, and he presents a recipe on how to conserve orbital orthogonality in three dimensions. Hirsch et al. showed that a new electron counting rule, different from the Hiickel An + 2 rule, can be used to describe the spherical aromaticity of fullerenes of symmetry. This interesting work may provide a stimulus to take a closer look into the a—n separation of fullerenes. 

Chapter 16 A section has been added that discusses the aromaticity of Fullerenes and their relationship to other allotropes of carbon. 

Biihl, M., Hirsch, A. (2001). Spherical aromaticity of fullerenes. Chemical Review, 101,1153. 

The aromaticity of fullerenes has been investigated theoretically and substantiated experimentally by NMR [154]. The best experimental method for studying the aromaticity of fullerenes was devised by Saunders and Cross, who encapsulated helium atoms inside fullerenes and measured their NMR spectra [155]. He NMR measurements show that C o exhibits the least aromatic character whereas C70 has the highest, and that the aromaticity of higher fullerenes lies between these two extremes. This is in contradiction to the initial suggestion in 1985 that C o is covered by a sea of n electrons that make it aromatic [14a, 156], and to the assumption of high aromaticity due to the large number of possible Kekule structures [157]. The reduction of fullerenes is not expected to modify their shape or symme-... 

These NMR experiments confirmed the conclusions of several calculations regarding fullerene anions. Intensive computational research was done into the spherical aromaticity of fullerene and fiiUerene anions and is summarized in the 2001 review by Bilhl and Hirsch [154]. The London calculations on Qo predicted only diatropic ring currents, a large diamagnetic susceptibility and a huge endohe-... 

The chemistry of fullerenes appears to continue to grow steadily. The aromaticity of fullerenes has been reviewed in the recent issue of Chemical Reviews on aromaticity in an article by M. Buhl and A. Hirsch, entitled Spherical Aromaticity of Fullerenes . The title immediately sends a message of unsuspected novelty that has opened with the recognition of buckminsterfullerene s spherical structure, which as Kroto suggested could be the first example of a spherical aromatic molecule. We will only supplement this review by reporting on some aspects of fullerene aromaticity as described by the conjugated circuits model, as reflected in the works of Klein, Schmalz, and co-workers. They performed conjugated circuits calculations on all isomers of buck-minsterfullerene Geo, a total of 1812 isomers. The... 

Buhl, M. Hirsch, A. Spherical Aromaticity of Fullerenes. Chem. (728)... 

---