Resonance delocalisation

In the living cell these compounds exist in more complex bound forms, interacting with other molecules, for example flavones, and the actual observed colour depends on these interactions. However, it is interesting that even in vitro, simple pH changes bring about extreme changes in the electronic absorption of these molecules. For example cyanidin is red in acidic solution, violet at intermediate pH and blue in weakly alkaline solution, the deep colours being the result of extensive resonance delocalisation in each of the structures. 

Ketones are more reactive than esters due to the stabilising resonance delocalisation in the ester, and the nitrogen of hydroxylamine is more nucleophilic than oxygen, since nitrogen is less electronegative. 

The amine is the most nucleophilic site, although it is deactivated somewhat by the ester substituent at the p-position by a resonance delocalisation of the nitrogen lone pair. 

On the whole, resonance delocalises the n bonds, thus the C - C bonds in benzene is neither purely single nor purely double at any time. This was supported by X-ray diffraction and spectroscopic studies which revealed that benzene molecule is... 

Resonance states in the excited state carotenoid allowing delocalisation and stabilisation of the excited state. 

Apart from type 62, which is only slowly convergent to the optimised geometry, the other centres are well described by the ROHF method. Polyhedral views of the three type a structures are shown in Fig. 6. These all illustrate the change of hybridisation at the point of muonium attachment and at the adjacent carbon atom where the unpaired electron is effectively localised as expected from addition to an alkene. The bi and c defects (Fig. 7) are quite different. The expected hybridisation change to sp is clearly present for the atom bonded to muonium, but other significant distortions are not obvious. This is consistent with the prediction from resonance theory (Fig. 8) that the unpaired electron for these structures is delocalised over a large number of centres. 

Figure 8 Measure of delocalisation of each defect type predicted by resonance theory. The loops enclose centres which have numbers of classical structures larger than. 74 times the greatest number in the type. The cut-off point for type bi (or type 63) centres is particularly arbitrary since the delocalisation is spread around the equator. The small circles are the point of muonium attachment. The dotted circle is coincident with the equator of Cra-...

The valence-bond (resonance) description of the triphenylmethine dye Malachite Green (125) is illustrated in Figure 6.5. Comparison with Figure 6.4 reveals their structural similarity compared with cyanine dyes. Formally, the dye contains a carbonium ion centre, as a result of a contribution from resonance form II. The molecule is stabilised by resonance that involves delocalisation of the positive charge on to the p-amino... 

Related to these diphosphine dichalcogenides are the triphosphine trisulfides [R2P(S)]3CH (12) which can be prepared from lithiated methylene diphosphine disulfides upon treatment with thiophosphinic chlorides (Equation 9). Deprotonation of 12 with tBuLi gives a resonance-stabilised anion 13 containing a planar central carbanion whose charge is delocalised onto the three neighbouring phosphorus and sulfur atoms.32... 

The r-delocalisation in the parent phospholide anion I (Fig. 3, R =R =H) can be expressed in the valence bond picture by resonance between the canonical structures lA-IC (and their mirror images). Phosphonio-sub-stituents (R =R =PH3 ) increase the weight of the 1,2-dipolaric canonical structure IB and induce thus, in essence, a partial r-bond localisation and a shift of r-electron density from the phosphorus to the adjacent carbon atoms [16]. Consequences of this effect are the decrease in delocalisation energy for reaction (1) depicted in Fig. 4, and lower C2-C3/C4-C5 and higher C3-C4 bond orders which are reproduced in concomitant variations of computed bond distances [16]. 

Partial covalency in essentially ionic bonds changes somewhat the distribution of electrons, detectable as electron delocalisation by the modem methods of nuclear magnetic and electron spin resonance (NMR and ESR). Although the interpretations of these measurements widely differ (see 292, 293, 320) they doubtless prove the existence of partial covalency (in the order of magnitude of 10%) even in the most ionic fluorides AMeFg. Little work seems to have been done one fluorides of the heavier transition elements (96), but there is an abundant literature on first transition series fluorides, of which an arbitrary selection is given below for further information. ... 

Figure 9.13—Effect of resonance on carbonyl-containing compounds. Representation of the delocalisation of valence electrons in mesomeric forms of organic compounds. In1 C NMR, the signal corresponding to a carbonyl in an ester is at 165 ppm, whereas it is at 205 ppm for a ketone.

---