Resonance formula

Ambident reactivity occurs for any substance displaying protomeric behavior. In basic medium anions in which negative charge is delocalized are formed this may be represented by the resonance formulas in Scheme 2. Each charged atom may react with an electrophilic center. The... 

It is interesting that the amount of ir-bond character in these oxygen complexes varies from about Vi to about2/3, and that for the sulfate ion it corresponds closely to the resonance formula... 

A similar explanation can be given for the larger Si-O-Si bond angles as compared to C-O-C. Electron density is given over from the oxygen atom into the valence shells of the silicon atoms, but not of the carbon atoms, in the sense of the resonance formulas ... 

Simple resonance theory suggests that for 2-substituted selenophenes the selenium atom can be considered to be situated in an ortho position and the influence of mesomeric and inductive effects or chemical shifts should be parallel for the 3-carbon (cf. resonance formula 5-7 for a-I-M substituted derivative). 

On the other hand, the situation is more complex for the 3-substituted derivatives. In such compounds the substituent can be considered electronically to be located either meta or para to the selenium ring atom (cf. resonance formulas 8 and 9 for a -I -M substituted derivative). However, the substituent-caused shifts in the 3-substituted derivatives indicate that the heteroatom and the substituents are para and not meta related. 77Se chemical shifts are more sensitive than the 13C shifts to changes in electron... 

Fig. 2.1-21. Bonding description of pentaborane(9) by localized bonds two resonance formula with closed 3c2e bonds and one with an open BBB 3c2e bond are shown.

For example, in benzene, all H- and C-atoms are constitutionally equivalent. In chlorobenzene, however, only the o- and m-atoms are pairwise equivalent. Note here that the individual resonance formulas of delocalized bond systems are not distinguishable. 

The particular substitution pattern of lithium carbenoids, the fact that both an electropositive metal and an electronegative substituent X are bound to the same carbon atom, causes the ambiphilic character of this species. The chameleon-like reactivity becomes evident from the resonance formulas of the carbenoid lb (equation 1) Whereas the carbanionic character is expressed by the resonance formula la, the electrophilic character is represented by Ic. In an analogous way, the reactivity of vinylidene carbenoids 2b is expressed by the mesomeric structures 2a and 2c. 

A considerable fraction of charge in adduct 26 seems to be delocalized into the benzenoid ring, as indicated by the shielding of the 6-position to which the charge can be effectively relayed from the reaction center (resonance formula 28). 

Relatively balanced carbon shifts of some fulvenes [241] indicate that polar resonance formulae contribute to the ground state of these crossed conjugated systems only slightly. 

Due to the partial n character of the CN bonds which are, in fact, vinylogous forma-mide or amidine connections in keeping with the resonance formulae, different methyl shift values are observed for some N,N-dimethylamine groups. Moreover, one-bond carbon-proton coupling constants decrease from the end to the center of the polymethine chain. This is explained by CNDO/2 calculations, in which corrected bond orders and bond angles alternating between 125° (a, y, e) and 117.5° (/5, 5) are taken into account [343],... 

Diazoalkane carbon shifts behave similiarly to those of the isoelectronic ketenimines [357, 358], The contribution of resonance formulae with carbanionic carbons predominates, as indicated by a considerable shielding of the carbon nucleus a to the diazonium residue. 

Isocyanate and isothiocyanate carbon nuclei resonate between 120 and 130 ppm [77a]. Isothiocyanate carbons are slightly deshielded relative to comparable isocyanates. Typical nitrile shift values (110-115 ppm) are characteristic of thiocyanates (rhodanides), while the carbon nuclei of rhodanide and cyanate anions shift to lower field due to significant contributions of heterocumulene-type resonance formulae. 

The NN connection of N-nitrosamines is a partial double bond described by two resonance formulae. 

Carbon-fluorine coupling constants of fluorobenzene and selected substituted derivatives are collected in Table 4.60 [402], Benzenoid JCF couplings are about 245+15 Hz. They depend on both type and position of the substituents Electron withdrawing groups increase while electron releasing ones decrease one-bond carbon-fluorine coupling in fluorobenzene, particularly when they are ortho and para to fluorine. These observations can be explained by cannonical resonance formulae which take ( + )- and (-)-M effects into account. The data of fluoroanilines (( + )-M) and fluorobenzaldehydes (( — )-M) provide typical examples (Table 4.60). 

Acyl cations [496] are stabilized by electron release of the alkyl groups and by interaction with adjacent n orbitals, as shown for benzoyl, acroyl and propiolyl cations. Kete-noid resonance formulae contribute to the states of these ions. 

When 2-benzopyrylium cations have a substituent with a fairly mobile hydrogen atom (a-alkyl group or heteroatom group in any other position of the cation), deprotonation of such a substituent occurs even under mild conditions by an acid-base interaction as the primary step (Section III,A). Although deprotonation in both cases leads to compounds whose structures can be depicted by two resonance formulas, either with charge separation (betaines) or without (anhydrobases), on discussing products of C-deprotonation, the term (and the corresponding formula ) anhydro-base is more often used, whereas products of O-deprotonation are called betaines. ... 

As described in Section III, practically all known conversions of 2-benzopyrylium salts are the result of their reactions with nucleophiles, and 2-benzopyrylium, similar to its monocyclic analog [82AHC(Suppl)], behaves chemically more like a carbenium ion, and not like an oxonium salt. This fact can be easily understood if one takes into account resonance formulas a-f, where an oxonium form f is added for the presence of an alkoxy (hydroxy) substituent in position 6. 

The asymptotic wavefunctions obtained from multichannel scattering calculations provide the S matrix, the eigenphases and eigenphase sum, and the cross sections. In principle, any of these quantities may be chosen for fitting the resonance formula to determine accurate values of the resonance parameters E, and T. 

Figure 25 Resonance formulas describing the gradual increase in polarization along the photoisomerization MEP of the cis-CsH6NH2 isomer. (From Ref. 37.)...

All of these dyes can be represented by various resonance formulas. They are characterized by high color strength. 

Much more stable are the 2- or 4-oxides, i.e., a-pyrones (unsaturated lactones) 20 and y-pyrones, which could, in principle, have aromatic character owing to their zwitterionic resonance structures (20A-20C). However, although protonated pyrones are definitely aromatic, the neutral compounds appear to have very little aromaticity. Bird s aromaticity index I6 for pyrylium is only 65.8% in comparison with benzene, whereas for 4-pyrone it is 37.2% and for 2-pyrone it is only 32.9%, as seen in Table 4 [22], In agreement with Table 2, the ring 0-(C = O) bond in 20A is a type X-Z bond, whereas the ring O = (C-O ) bond in the two other resonance formulas is a Y-Y bond. 

Other resonance formulas can he written with the positive charge on the nitrogen atoms, giving some double bond character to the affected C-N bonds. Notwithstanding the close O-S contacts and the fact that the four O-S-S-O atoms are in a linear arrangement, the S-S bond has a length practically normal for 1,2-dithiolium cations. 

The nitrite ion, NOz is bent with C2v symmetry, and its structure can be represented by two simple resonance formulas ... 

---