AROMATIC AND ANTIAROMATIC COMPOUNDS

Aromatic compounds containing silicon have attracted considerable interest, both experimental and theoretical, and efforts in this area have intensified since the first synthesis of stable silenes and disilenes19. Despite considerable effort no stable silaaro-matic compounds have yet been isolated, pure or even in a solution. All direct observations to date have been in the gas phase or in matrix isolation. Experimental knowledge about these compounds is therefore quite limited19 (see also below), leaving theory as one of the main sources of reliable information. 

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The scheme for treating these subjects is as follows. We compare typical aromatic and antiaromatic compounds. Similar to the comparison between benzene and cyclobutadiene, we concern ourselves here with pyridine (34) and azete (57). Such an approach provides, apart from other advantages,... 

It is clear, as Katritzky et al. [7, 8] and ourselves [9] have pointed out, that aromaticity cannot be described with a single parameter. It is possible to select a parameter and classify aromatic compounds according to it and this approach is correct if one bears in mind that the aromaticity scale thus obtained is valid only for the chosen parameter. One of the most successful is Schleyer s NICS (nuclear independent chemical shifts) [10-12], a criterion we have used to separate aromatic and antiaromatic compounds [13], Cyranski et al. [14] as well as Sadlej-Sosnowska [15] have tried, with moderate success, to find an agreement between these different points of view. 

Oligounsaturated Five-Membered Carbocycles -Aromatic and Antiaromatic Compounds in the Same Family... 

I 2 Oiigounsaturated Five-Membered Carbocycies - Aromatic and Antiaromatic Compounds... 

Many aromatic and antiaromatic compounds were included in this study, and it provides the best justification for the use of NMR chemical shifts in studying... 

Since C < this is numerically less than the corresponding value for equal bond lengths (when C = ) Alternation of bond lengths makes aromatic compounds less aromatic and antiaromatic compounds less antiaromatic. Thus we arrive at the general rule ... 

Yet, compared to the other x-systems, the acyclic linear polyenes, and the cyclic oligoenes (i.e., the annulenes), cross-conjugated molecules have lived in the shadows so far. Acyclic linear polyenes are encountered widely in fatty acid, polyketide, and terpenoid natural product domains and the ultimate polyene, polyacetylene is a conductive polymer that was the focus of the 2000 Chemistry Nobel Prize. The annulenes played crucial roles in the development of practical and theoretical organic chemistry (aromatic and antiaromatic compounds), and serve as pivotal bulk chemicals in industry (benzene, toluene, the xylenes, styrene, etc.). 

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