Y vs. cyclic delocalization in small ring dications and dianions: The dominance of charge repulsion over Huckel aromaticity
✍ Scribed by Timothy Clark; Dieter Wilhelm; Paul von Ragué Schleyer
- Book ID
- 104221108
- Publisher
- Elsevier Science
- Year
- 1982
- Tongue
- French
- Weight
- 194 KB
- Volume
- 23
- Category
- Article
- ISSN
- 0040-4039
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✦ Synopsis
Coulombic repulsion in four membered ring dications and dianions is more important than HUckel aromaticity and leads to a preference for Y-delocalized isomers with more favorable n charge distributions. The most significant advances in the history of organic chemistry include the concept of cyclic aromaticity' and the development of Hflckel TI molecular orbital theory. 2 This simple oneelectron treatment is remarkably successful in predicting the relative stabilities of different types of n-systems,3 and is still used, despite the availability of more sophisticated MO methods, e.g., to evaluate the stabilisation energies of conjugated polyanions.4 One prediction of HUckel theory which long defied experimental verification is the aromatic nature of the 6 n cyclobutadiene dianion.5 The difficulties in obtaining this species are remarkable in view of the ease of formation of the C4 6 n trimethylenemethane 637 and butadiene' dianions. The situation is reversed for the corresponding dications. While substituted cyclobutadiene dications are known' in solution (but may be non-planar and have relatively low resonance energies) 10 , no trimethylenemethane dication derivatives appear to have been reported. These anomalies led us to investigate small dications and dianions with MNDO SCF-molecular orbital theory. 11 All geometries were fully optimised within symmetry constraints given. Variations in n-effects are emphasised by comparing the energies of isomers with common sigma skeletons but with different delocalized n-systems: linear, Y or cyclic. The calculated heats of formation (MNDO) indicate that the HUckel stability predictions (based on R-values) do not apply to highly charged small ring systems. Y-delocalization, the advantages of which have been pointed out by Gund,8 often is the most favorable of the three alternatives.