Stabilization, and aromaticity

The kinetic stability of 17 increases on deprotonation. The half-life times of 17 and its anion N 19 have been estimated [104] from the observed [105, 106] and computed free energy to be only 10 min and 2.2 days, respectively. The high kinetic stability of the anion 19 can be understood in terms of enhanced pentgon stability and aromaticity. The deprotonation raises the energy of lone pair orbitals and promotes cyclic delocalization of o- and rr-electrons. 

Bicyclic systems such as trithiapentalene 61 or thia-aza-pentalenes 62 (Scheme 30) show significant charge separation. Alternative representations for the above systems include nonclassical structures 61b,c and 62c with S(IV). Density functional calculations showed that the stability and aromaticity of bis-... 

Problem 10.12 Draw a conclusion about the stability and aromaticity of naphthalene from the fact that the experimentally determined heat of combustion is 255 kJ /mol smaller in absolute value than that calculated from the structural formula. 

These compounds are usually written in the uncharged form (788, 789 Z = NH, NR, O, S), but canonical forms of types (790) or (791) are of comparable importance, i.e. the compounds can also be considered as betaines derived from pyridinium, pyrylium and thiinium cations. They possess considerable stability and aromaticity in that in many of their reactions they revert to type . 

THE COMPARISON WITH C6H6 - THE VIBRONIC PERSPECTIVE ON RESONANCE STABILIZATION AND AROMATICITY... 

Stability and aromaticity of multiply N-confused porphyrins were systematically investigated using DFT calculations [236, 239], Ignoring tautomerism, there are... 

The potential of the m-benziporphyrin skeleton to produce aromatic macrocycles, foreseeable in the above discussion of its hypothetical tautomers, was actually revealed by introduction of hydroxy groups on the phenylene ring. These functionalities serve as additional tautomerization sites, which become effective aromaticity switches in appropriately designed systems. Theoretical insight into the switching capabilities of hydroxybenziporphyrins was gained from a theoretical survey of relative stabilities and aromatic character of various hydroxybenziporphyrin tautomers [287, 288], which was concurrently verified by experimental work (Table 2) [250],... 

The enol form of phenol benefits from the resonance stabilization and aromaticity of the benzene ring, but the keto form, with the single sp3 center in the ring, cannot. 

Although there is considerable interest in the potential aromaticity of the 1,3,2(AZ) diazasiloles and their benzo- or pyrido-fused analogues, such as 83, 85, and 89, there are no experimental studies tackling these problems. All information about stability and aromaticity is derived from computations. 

Again, these results are in agreement with the NIGS values for both rings of these molecules. With this MO analysis, the stability and aromaticity of these compounds are explained. The 7t-molecular delocalization agrees with the aromatic behavior of the two rings in these systems. It is possible to see the delocalization with two frontier MOs. However, caution is required for other systems where perhaps more occupied MOs are required. 

In an analogous manner, we succeeded finally in synthesizing the 5-carbonitrile of the kata-condensed tetracyclic azuleno(l, 2-f)azulene (776) in a 50% yield by condensation of (44) with azulene-2-acetonitrile (149) to the red-violet aminonitrile (174) and smooth thermolysis of it78. The hydrocarbon (176) is the first known member of the theoretical 12 isomeric azulenoazulenes of which considerable stability and aromatic properties are predicted79. Indeed, (7 76) can readily be formylated by dimethylformamide-phosphorylchloride in a typical aromatic-electrophilic substitution to the 1-carbaldehyde (7 77)78. ... 

On the other hand, there is the question of the close relationship between coke formation (catalyst stability) and aromatics cyclization (catalyst selectivity) over the acid sites present in the zeolites. On naphtha reforming, coke formation is a bifunctional reaction requiring the dehydrogenation capacity of the metallic function and the condensation capacity of the acidic function. Therefore, it is interesting to... 

These arguments form the core of Hiickel s rule for the relative stability (and aromatic character) of cyclic systems as a function of the electron count, a fortunate blend of elementary quantum chemistry and simple topology [55]. 

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