Topological resonance calculations

Topological resonance energy (TRE) values suggest that 346 is considerably less stable than 345. CNDO/2 calculations with inclusion of d-orbital participation for 346 show considerable 7i-bonding between carbon and sulfur of which over half is attributable to pTt-dn overlap the degree of charge separation indicates that canonical resonance forms of type b (carbonyl ylide type) are more important than those of type c (thiocarbonyl ylide type)." ... 

Results of MNDO calculation of lA-azonine (35 X=NH) are in agreement with experimental evidence that this is a planar, aromatic molecule. The calculated geometry of oxonin (35 X = 0), as a buckled, unsymmetrical polyenic heterocycle, is also in agreement with its known properties. The MNDO calculations on thionin (35 X = S) indicate that this molecule is planar, which should allow effective tt delocalization, and at least some aromatic character (86MI927-OI). The topological resonance energy model also predicts lA-azonine and thionin to be aromatic, and oxonin nonaromatic (84JHC273). 

The degree of 7t-electron stabilization exhibited within the A,B-diheteropentalenes is dependent on the relative orientation of the two heteroatoms [64,65], Topological resonance energy values [64] and heats of formation from MNDO calculations [65]... 

Gimarc et al. have calculated topological resonance energies for a series of sulfur-nitrogen rings and have suggested that the hypothetical species should be planar and aromatic <85rCA105>. 

The topological resonance energy model has also been used to study the aromatic stabilities of the heteronins. l//-Azonine (1) and thionin (14) were predicted to be aromatic, whereas oxonin (11) was predicted to be nonaromatic <84JHC273>. Hiickel energy levels have been calculated for the neutral S5N4 ring system <80PAC1443>. 

Attachment of substituents to the cage has been a blossoming branch from the onset of fullerene chemistry. " The concomitant removal of electrons from the n system (by localizing them in o bonds) is bound to affect any resonance mechanism that might be operative. For multiple adducts of Ceo, the aromatic character is indicated, by Fluckel-type calculations of the topological resonance energy, to depend notably on the addition pattern, and the same is to be expected for the magnetic and NMR proper-... 

Finally, if we abandon Hiickel s topological approach altogether and consider more elaborate quantum-mechanical approaches, the concept of aromaticity derived purely from a consideration of -electrons becomes blurred and tends to disappear completely. In fact, allelectron methods allow the calculation of aromatic properties (Section V,B) of a given substance without introducing explicitly the concept of aromaticity. Certain authors, notably Dewar,19 have published resonance energies derived from self-consistent field molecular-orbital (SCF-MO) calculations, and these could be used as a measure of aromaticity. 

HMO calculations, based on localized polyenes instead of isolated alkenes, can account for the heats of atomization of furan (41.64 observed, 41.69 eV) and of dibenzofuran (109.09 observed, 108.92 eV). For resonance energies (quoted as resonance energy per electron, REPE) they give furan, 0.007 oxepin, -0.006 benzo[6]furan, 0.036 benzo[c]furan, 0.002 and dibenzofuran, 0.047 /3 (72T3657). That furan emerges as hardly more aromatic than a diene while pyrrole (REPE 0.039/3) is clearly aromatic is in line with other results, including those from MINDO/3 and topological methods (see Section... 

Having reproduced our old proposal, the outcome is now briefly described. According to Aihara s (1988) topological theory of aromaticity, the resonance energy per 7t electron (repe) of 5 is calculated to be 0.0274 / , or about 60% of that of benzene (0.0454 / ). It is thus clear that there is no dramatic increase in the conjugative stability in 5. As far as aromatic stabilization in the football structure is concerned it should be regarded as an extension of two-dimensional aromaticity. Aihara Hosoya (1988) call it spherical aromaticity. 

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