Geometry resonance stabilization

Aeeording to your Lewis structure(s) and to the actual geometry of the molecule, is the bonding in planar corannulene fully delocalized (as in benzene), or are some CC bonds long and some short Do your results support the notion that planar corrannulene is resonance stabilized Explain. 

An acetyl group in the 2-position favors the monocyclic structure presumably because of the resonance stabilization.12 The same observation was made with oxepin-2,7-dicarbaldehyde, oxepin-2,7-dicarboxylic acid, and oxepin-2,7-dicarbonitrile.23 Substituents in the 4- and 5-positions of the oxepin such as methyl or methoxycarbonyl groups shift the equilibrium towards the epoxide.12 24 Low temperature 1H NMR studies on 7-ethyloxepin-2-carbonitrile and ethyl 7-ethyloxepin-2-carboxylate established a nonplanar boat geometry with a ring-inversion harrier of 6.5 kcal mol-1.25... 

It has been contended that here too, as with the benzene ring (Ref 6), the geometry is forced upon allylic systems by the a framework, and not the 7t system Shaik, S.S. Hiberty, P.C. Ohanessian, G. Lefour, J. Nouv. J. Chim., 1985, 9, 385. It has also been suggested, on the basis of ab initio calculations, that while the allyl cation has significant resonance stabilization, the allyl anion has little stabilization Wiberg, K.B. Breneman, C.M. LePage, T.J. J. Am. Chem. Soc., 1990, 112, 61. 

Because the resonance stabilization of the cu bond is always maximized in linear geometry (for which np—ctmf overlap is highest), each cu-addition characteristically results in an additional linear Fi-M-iF linkage. 

Hindered rotation can separate the NMR signals of apparently equivalent protons. This causes changes in the geometry of the molecule. This fact is illustrated by C - N bond if N, N dimethyl - formamide which has partial double bond character due to resonance stabilization of the molecule ... 

The integrals of the separated resonance absorptions provide a direct measure of the enantiomeric ratio from which the cc can be calculated. The chemical shift anisochrony of enantiomers (dR S<>) in the presence of a nonracemic chiral auxiliary compound is attributed to at least two contributions which are related to the geometry and stability of the resulting diastereomeric association complexes79 80 ... 

Much of the chemical reactivity of the /8-lactam antibiotics is associated with die /i-lactam moiety. The geometry and the accompanying increased ring strain results hr very little, if any. amide-resonance stabilization leading to a marked increase in chemical reactivity when compared to a normal amide. In fact, in many instances the reactivity of the lactam carbonyl is... 

Removal of a proton from Lj gives an anion whose lone pair is orthogonal to the it bond of the carbonyl group by virtue of the rigid geometry of the bicyclic system. Consequently the lone pair cannot overlap with the carbonyl n bond and delocalization via resonance is not possible—it is effectively a localized anion. Removal of a proton from L2 gives rise to a lone pair in a p orbital which can overlap with the carbonyl it bond and thus resonance delocalization is possible. Thus the anion from L2 is resonance stabilized and is thus formed more easily. 

In the meantime, other Tt-delocalized systems were discovered among open-chain systems, notably the three allylic systems, Scheme 4. All of these systems showed a behavior akin to the aromatic systems they are delocalized, have uniform geometries, and are more stable than saturated analogues.90-93 Furthermore, they all possess rotational barriers with heights related to the resonance stabilization of the species. 

The generality of Scheme 1 is apparent by now 7r-distortivity and resonance stabilization are two coexisting properties of delocalized species which are constrained to relatively small distortions around a symmetric geometry. This situation that typifies conjugated jr-systems of carbon allows one to measure the two properties independently of each other. 

If a stereogenic double bond is established by this Elcb elimination, one usually observes a trans- or an / -sclcctivity. This experimental finding could have two origins (1) product development control (Section 4.1.3), if the stereoselectivity occurs under kinetic control, or (2) thermodynamic control. Thermodynamic control comes into play if the cis, trans- or E,Z-isomeric condensation products can be interconverted via a reversible 1,4-addition of NaOH or KOH. In the trans- or isomer of an ce,/l-un saturated carbonyl compound the formyl or acyl group may lie unimpeded in the plane of the C=C double bond. This geometry allows one to take full advantage of the resonance stabilization C=C—0=0 <-> C—C=C—0 . ... 

On the other hand, in the as- or Z-isomer of an a,/J-unsaturated carbonyl compound the formyl or acyl group interferes to such an extent with the substituent at the other end of the C=C double bond that a planar geometry is no longer possible and the resonance stabilization consequently is reduced. 

---