Alternant non-benzenoids

The EC values of alternant non-benzenoids have not been discussed till now. We shall limit the discussion to cata-fused non-benzenoids having two or three condensed rings with ring sizes equal to 4, 6, or 8, and to peri-condensed non-benzenoids with the above ring sizes having at most four rings. Figure 8.19 craitains structures of systems that are discussed in this section. 

Fig. 8.19 Alternant non-benzenoids their EC values. Note that for 13 and 14 the EC value for the eight-membered ring is 5.6. and not 7.2...

Resonance Energy (RE) Expressions in Terms of 4n Conjugated Circuits for Several Alternant and Non-Alternant Non-Benzenoid Hydrocarbons of Eigure 10.11... 

Figure 66. Selection of biphenylene derivatives representing alternant non-benzenoid hydrocarbons. (RE values are shown in Table 25.)...

In Table 26 we show the decomposition of Kekule valence structures for a selection of non-alternant non-benzenoid hydrocarbons having odd rings (illustrated in Figure 67). They include azulene (1/67), acepleiadylene (8/67), and corannulene (15/67). Observe that all of the non-benzenoid systems shown in Figure 67, despite having odd-member rings, have... 

Table 26. Expressions for Molecular RE for Non-alternant Non-benzenoid Hydrocarbons with Odd Rings Having 4n -I- 2 Conjugated Circuits (Shown in Figure 67)...

It is regrettable that there are so few calculations of higher quality on a sizable collection of non-benzenoid hydrocarbons and non-alternant non-benzenoid systems that would allow a better estimate of the contributions made by An conjugated circuits. Therefore, better and more precise parametrization of various Qn terms, which would be possible if there were reliable calculations of the resonance energies of these molecules, is lacking. The challenge here for theoretical chemistry is not so much in making quantum chemical calculations as such, but to be able to extract from such calculations a non-observable known as RE. 

Figure 79. Isoconjugate structures. Top, benzenoids middle, alternant non-benzenoids and bottom, non-alternant non-benzenoid structures.

Rule A-56. Radicals for Non-benzenoid Ring Systems (Alternative in part to Rule A-55.1)... 

Because of structural variety that allows one to modify the mode of deformation, distorted condensed benzenoid (and non-benzenoid as well) aromatics will continue to be a source of exotic reactions. These molecules will continue to accept challenge of synthetic chemists because new alternative methods should always be developed. Moreover, these investigations of special 71 bond systems will certainly deepen our understanding of chemical bonding, but also would provide opportunities of finding useful reactions and functions of organic molecules. 

FIGURE 10.8 Two dozen non-benzenoid non-alternant polycyclic hydrocarbons having only 4 + 2 conjugated circuits whose RE expressions are listed in Table 10.3. 

Kekule valence structures continue to offer an alternative basis for description of benzenoid and non-benzenoid conjugated hydrocarbons, which complements characterization of molecules by quantum chemistry, which tends almost exclusively to consider molecular orbital approaches as the frame for discussion of molecular properties. We have already seen applications of conjugated circuits (which are extracted from individual Kekule valence structures) for clarification of the notion of aromaticity in polycyclic conjugated hydrocarbons and characterization of the relative magnitudes of molecular resonance energies and will also later consider the use of conjugated... 

Quite evidently, all states except leuco-emeraldine can be protonated. The states with no protonation are denoted as the "base" form, e.g. emeraldine base (which would be non-conductive). The reader may note tlie presence and alternation of "benzenoid" and "quinonoid" segments, as defined in an earlier Chapter, in the structures of Fig. 13-9. Upon protonation, the polymer is denoted as the salt, for example protonation of the emeraldine base form (Fig. 13-9c with HCl would yield emeraldine hydrochloride-. 

With some exceptions, i.e. non-planar systems, variances in geometry are small within the class of benzenoid (alternant) hydrocarbons and thus can be neglected. Hence the individual representatives of this class of compounds differ only with regard to their molecular topologies. Provided the influence of the non-topological structural characteristics (kind of atoms, geometry, additional electronic interactions that are not referred to in the constitutional formulae) on the physical and chemical properties... 

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