Cyclic Conjugated n Systems

The electronic spectra of cyclic conjugated n systems depend inherently on the number of n electrons. Closed-ring systems with AN+l n electrons in the perimeter are aromatic compounds, of which benzene is the most important representative. Benzenoid hydrocarbons constitute a class of compounds whose UV spectra have been investigated most extensively both experimentally and theoretically. The fact that the spectra of aromatic compounds are so characteristic meant that formerly they were of considerable importance in the structure determination of organic compounds. However, these spectra cannot be explained in terms of the simple HMO model. If one seeks a theoretical basis for an understanding, one has the choice between the perimeter model and the Pariser-Parr-Pople or a more complicated numerical method. Before discussing these theoretical models, some empirical relations will be presented. Finally, cyclic systems derived from a perimeter of 4N Jt electrons will be considered. 

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Aromaticity was found to be a general property of many (but not all) cyclic, conjugated jt systems. Moreover, it was found diat aromaticity in molecules can be predicted by Huckel s rule. The structural requirements implicit in Huckel s rule are that there be An + 2 (n is an integer) n electrons in a cyclic, conjugated n system. Obviously benzene, which has six jt electrons (An + 2, n = 1) in a conjugated it system, is aromatic. However, Huckel s rule predicts that molecules such as cyclodecapentaene An + 2 = 10 (n = 2) and [18]-annulene An + 2 = 18 (n = A) should be aromatic, have equal bond lengths, and be planar—and they are. 

MCD measurements are useful not only in purely spectroscopic investigations, for example, in the detection of hidden absorption bands or for the identification of degenerate absorptions, but also in structural organic chemistry. There is, for instance, a rule that describes the influence of the molecular environment on the MCD effect of the n- r band of a ketone (Seamans et al., 1977 Linder et al., 1977). However, the method is clearly most useful for investigating cyclic conjugated n systems. 

Model 2 Cyclic Conjugated n Systems ( Racetrack Analogy)... 

The MO diagram of a cyclic conjugated n system takes the shape of the ring involved, with one vertex pointing down. For example... 

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