Aromaticity, aromatic character Mobius

Hehre66 69 and independently Jorgensen72,73 pointed out that the Mobius and Hiickel description of homoconjugated molecules (Figure 9) is consistent with the assumed homoaromtic and homoantiaromatic character of these compounds. However, it was also realized that in the general case such a classification might not be sufficient to describe subtle differences in orbital interactions, which determine the homo(anti)aromatic character of a molecule. 

Mobius aromaticity A monocyclic array of orbitals in which a single out-of-phase overlap (or, more generally, an odd number of out-of-phase overlaps) reveals the opposite pattern of aromatic character to Hiickel systems with 4n electrons it is stabilized (aromatic), whereas with 4n + 2 it is destabilized (antiaromatic). In the excited state 4n + 2, Mobius pi-electron systems are stabilized, and 4n systems are destabilized. No examples of ground-state Mobius pi systems are known, but the concept has been applied to transition states of PERI-CYCLIC REACTIONS (see AROMATIC [3]). 

MO calculations (MP2/6-31G ) on the TS confirmed that it is helical and conforms to the expected conrotatory mode. This is a Mobius type TS. The NIGS and magnetic properties attributed to the TS by MO calculation also indicate that it has aromatic character. ... 

The An cyclononatetraenyl cation was studied by Anastassiou and Yakali via the treatment of deuterium-labeled 9-chlorocyclononatetraene with liquid SO2 (an ionizing solvent), and they found that the deuterium became statistically distributed, presumably via the cation. They wondered why this An system could be so easily formed and proposed a helical geometry. Subsequent calculations by Schley-er et al. found that the lowest energy conformation of the ion did have a structure of this type, which leads to a Mobius 7r-electron system in which there is one inversion of phase. Compounds of this type were predicted by Heilbronner to have aromatic character, and the NICS value for this ion is in accord with this expectation. Calculations for other conformations of this ion have been reported. ... 

The first real Mobius systems ([16]annulenes) were only synthesized a few years ago [105], In these systems the authors achieved enough rigidity for the molecules so that they would not flip back to a Hiickel system. It was also determined that these Mobius-twisted annulenes are more aromatic in their character than the Hiickel-systems [106],... 

FIGURE 9. Hiickel and Mobius orbital systems for homoconjugated molecules. In each case, the number of participating electrons (e) is given and classification according to aromatic or antiaromatic character indicated... 

While all cyclic, conjugated six-membered phosphorus heterocycles with AV -phosphorus are clearly Hiickel-aromatic systems, the bonding situation in the cyclic conjugated six-membered systems containing AV -phosphorus has been discussed rather controversially. Schweig and coworkers, for example, discussed a mixture of Huckel and Mobius aromaticity and ylide character... 

An alternative analysis of sigmatropic reactions involves drawing the basis set atomic orbitals and classifying the resulting system as Htickel or Mobius in character. When this classification has been done, the electrons involved in the process are counted to determine if the TS is aromatic or antiaromatic. The conclusions reached are the same as for the frontier orbital approach. The suprafacial 1,3-shift of hydrogen is forbidden but the suprafacial 1,5-shift is allowed. Analysis of a 1,7-shift of hydrogen shows that the antarafacial shift is allowed. This analysis is illustrated in Figure 10.31. These conclusions based on orbital symmetry considerations are supported by HF/6-31G calculations, which conclude that 1,5-shifts should be suprafacial, whereas... 

An alternative analysis involves drawing the basis set atomic orbitals and, classifying the resulting system as Hiickel or Mobius in character. Once this classification is complete and the electrons involved in the process are counted, the transition state can be recognized as aromatic or antiaromatic. This analysis is illustrated in Fig. 10.7. 

An ab initio computational study has predicted that 87r-homologs (129 and 130) of planar aromatic carbenes and silylenes will form twisted 6 2-geometries, providing palladium complexes with Mobius-aromatic chiral character, and present an opportunity to design potentially chiral monodentate metal coordination. ... 

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