Delocalized structures

Aimulene offers a particularly significant test of the Hiickel rule. The internal cavity in [18]annulene is large enough to minimize steric interactions between the internal hydrogens in a geometry that is free of angle strain. Most MO calculations find the delocalized structure to be more stable than the polyene. ... 

The crystal structure of the potassium salt of 1,3,5,7-tetramethylcyclootatetraene dianion has been determined by X-ray dififaction. ° The eight-membered ring is planar, with aromatic C—C bond lengths of about 1.41 A without significant alternation. The spectroscopic and structural studies lead to the conclusion that the cyclooctatetraene dianion is a stabilized delocalized structure. 

SN)x are free to move under the influence of an applied potential difference and thus conduction occurs along the polymer chain. The S-N distances in the chain are essentially equal, consistent with a delocalized structure. The increase in conductivity with decreasing temperature is characteristic of a metallic conductor. The predicted Peierls distortion is apparently inhibited by weak interactions between the polymer chains (S—S = 3.47-3.70 A S—N = 3.26-3.38 A.). ... 

Compare the geometries of the cyclohexanone enolate and the cyclohexanone lithium enolate. Do both molecules show delocalized structures, or is the bonding in one of them more localized For comparison, examine the geometries of 1-hydroxycyclohexene md cyclohexanone. 

According to its IR spectrum, phospharinane 141 exists in the solid state exclusively as the tautomer 141b possessing the localized electronic structure (Scheme 43) (83UK1761). However, by means of NMR and IR spectroscopy, it was shown that in CDCI3 solution the delocalized structure 141a is also present, although at a low relative concentration. 

In the 1,4-dihelerocine scries 1 with different heteroatoms only the 1,4-oxazocines 2 are known. As in the case of the 1,4-diazocines, depending upon the substituent on the nitrogen, these systems exhibit either nonplanarity with a localized -electron system for acceptor substituents or planar delocalized structures for donor-substituted or the unsubstituted systems.10... 

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

The crystal structure of the phosphonium salt (118) confirms that the compound is a diene and not a delocalized structure. 

Formazans can be characterized as tautomeric structures 178 and 179. Schiele proposed that, in some cases at least, formazans are better represented bv the delocalized structure 180 implying coplanarity among substituents. 302... 

While 1,2,5-thiadiazole 1,1-dioxide has not yet been prepared, extrapolation of data on the known 3,4-dimethyl-l,2,5-thiadiazole 1,1-dioxide 23 <1998JP091> indicated that the nonaromatic or antiaromatic 1,2,5-thiadiazole 1,1-dioxide has a more delocalized structure than its isomeric thiadiazole 1,1-dioxide anologues <1997JMT119, 2001JMT285>. 

Optimizations at various levels either gave pronounced bicyclic structures or a completely delocalized structure with no transannular bonding. 

The hydroxycarbene isomer (H)Co(CO)3(CHOH) was also examined. It yielded a complex with molecular electronic energy more than 60 kcal/mole higher on the energy scale. The hydroxycarbene complex is not likely to play a significant role in the catalytic cycle. It is of some interest to inquire why the 18e hydroxycarbene complex (H)(CO) Co(=CH0H) is less stable than the 16e isomer (H)(CO)3C0(CH2O). The results suggest that the formation of the carbonyl double bond makes the critical difference. The electronically delocalized structure (H)(CO)3Co+5-CH2 0" may provide some extra stabilization for the formally unbonded formaldehyde moiety. The resonance form is dipolar and could be further stabilized by polar solvents. 

Conclusions about mesomeric forms arose from X-ray analysis of mesoionic A -[2-(5-methyl-l,3,4-thiadiazolo[3,2- ]pyri-dinio)]acetamidate 28 (Scheme 6). Among the six possible delocalized structures, a large contribution of the 1,3,4-thiadiazolium structures 28a and 28b was observed, rather than pyridinium structures. These two mesomeric forms are in agreement with the high double-bonded character of the S-C6 bond <1996BCJ1769>. 

Also the case of polyaniline is somewhat different from that of heterocyclic polymers. It has been proposed (MacDiarmid and Maxfield, 1987) that the doping process does not induce changes in the number of electrons associated with the polymer chain but that the high conductivity of the emeraldine salt polymers is related to a highly symmetrical 7r-delocalized structure. 

Density functional theory and MC-SCF calculations have been applied to a number of pericyclic reactions including cycloadditions and electrocyclizations. It has been established that the transition states of thermally allowed electrocyclic reactions are aromatic. Apparently they not only have highly delocalized structures and large resonance stabilizations, but also strongly enhanced magnetic susceptibilities and show appreciable nucleus-independent chemical-shift values. 

It is well known that crystal and electronic structures are interdependent and define the reactivity of chemical substances. In Section 1.4.2, it was noted that copper-porphyrin complex gives cation-radicals with significant reactivity at the molecular periphery. This reactivity appears to be that of nucleophilic attack on this cation-radical, which belongs to n-type. The literature sources note, however, some differences in the reactivity of individual positions. A frequently observed feature in these n-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of porphyrin consistent with a localized structure rather than the delocalized structure usually ascribed to cation-radical. A pseudo Jahn-Teller distortion has been named as a possible cause of this alternation, and it was revealed by X-ray diffraction method (Scheidt 2001). 

It has been suggested that the effect of selective deuteration on C-13 spectra of symmetrical carbenium ions can be used to distinguish between rapidly equilibrating structures (ie. involving hydride and methide shifts) and delocalized structures (l3). 

When R is hydrogen, more tautomeric forms similar to those of monometalated acetonitriles can be written . These modes appear at too low wavenumbers to be acetylenic or nitrile absorptions and therefore they were attributed to ketenimine derivatives of form R(Li)C=C=NLi. For the ionic species, Gornowicz and West proposed a delocalized structure (Scheme 8) °. Klein and Brenner called it sesquiazacetylene in a previous study . 

Tetrathiatetrazaazulene (15) has long wavelength maxima at 322 (log = 3.35) and 492 nm (log e = 3.31). This pattern together with simple IR and C NMR spectra is indicative of a highly symmetrical and delocalized structure <85CC1654>. 5-Phenyl-1,3,2-dithiazole-4-thione exhibits absorbtion at 550 nm <89JCS(P1)2489>. 

In contrast with previous studies on He2Cl2 cluster, in the present work localized structures are determined for the lower He2Br2 vdW states. Traditional models based on a He2Cl2 tetrahedron frozen stucture have failed to reproduce the experimental absorption spectrum, suggesting a quite delocalized structure for its vibrationally ground state. Here, based on ab initio calculations we propose different structural models, like linear or police-nightstick , in order to fit the rotationally resolved excitation spectrum of He2Cl2 or similar species. 

Internal complexation has to be considered additionally as chelating substituents may alter the situation completely. For example, in the lithiated allyl A,A-diisopropylcarbamate 3 possessing a delocalized structure, it is clearly a 1-lithiated hetero-substituted allyl compound despite the fact that oxygen is a donor and the silyl group can be regarded more as an acceptor13. 

Problem 11.8 (a) Give the delocalized structure (Problem 11.1) for the 3 benzenonium ions resulting from the common ground state for electrophilic substitution, C H,G + E. (6) Give resonance structures for the par -benzenonium ion when G is OH. (c) Which ions have G attached to a positively charged C (d) If the products from this reaction are usually determined by rate control (Section 8.5), how can the Hamnrand principle be used to predict the relative yields of op (i.e., the mixture of ortho and para) as against m meta) products (e) In terms of electronic effects, what kind of G is a (i) op-director, (ii) m-director (/) Classify G in terms of its structure and its electronic effect. M... 

Several molecules displaying antitumour properties are efficient electron acceptors. Besides their ability to intercalate between DNA base pairs they can be reduced by enzymes. 5 /Y- Pyridophenoxazin-5-one (21) is a new anticancer iminoquinone which exhibits such properties. It can be readily reduced to its radical anion (Em = -0.496 V vs. NHE) with a delocalized structure it is able to form Cu(II) complexes and acts as a mediator for the formation of superoxide radical anions243. 

Structures A and B, for example, represent the molecular ion of cyclohexadiene. Compound A is a delocalized structure with one less electron than the original uncharged diene both the electron and the positive... 

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