Hiickel theory isomers

The only closed-shell fullerenes predicted by simple Hiickel theory fall into two types the leapfrog fullerenes Ceo+en (n 1), and the cylinders 70+30 and 34+36 (m > 0) formed by stretching Dsh-Cio and Da-C 4 along then-respective 5 and 5 axes. The simplicity of the theory unfortunately prevents these rules having any general utility. Neither of the two IPR-isomers of 75 is predicted to be stable but Z>2- 76 has been isolated, whereas the expected fiillerene Ded-Cji has not, although its apparent low stability may be... 

The stability of the D2 over the Td isomer is easily understood, because the latter has an open electronic shell in elementary Hiickel theory. This degeneracy of the HOMO and LUMO is expected to greatly destabilize the candidate open-shell isomer. 

Within the limitations of simple Hiickel theory, two remarkably simple rules give a coherent account ofrt stability for all currently chemically interesting fullerene sizes. Qo and C70 are awarded special status by the rules, and both rules are compatible with the observed association of stability and pentagon isolation. However, it is also clear from experiment that a u-only theory is insufficient to explain the known isomer patterns of higher fullerenes. In addition to Ceo and C70, fullerenes such as C76, C78, and have been isolated and characterized by NMR the first of these has a nuclearity... 

It is of historical interest to note that the Baeyer strain theory with its planar rings predicts only one form of decalin with the ring-junction hydrogens on the same side of the molecule (Figure 12-23). The Sachse-Mohr concept of puckered strain-free rings allows for two isomers. In fact, Mohr predicted that the two isomers of decalin should exist before W. Hiickel (1925) succeeded in preparing them. Both isomers occur in petroleum. 

The TEMO pattern was detected for a large number of S, T pairs of isomers for MO energies computed at various levels of quantum theory, from Hiickel to ab initio. 

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