Delocalization of electron density

The second important characteristic of aromatic isocyanates is that they are much more reactive than aliphatics due to delocalization of electron density into the aromatic ring. Of the mesomer structures shown in Scheme 4.3, if the... 

Aromatic rings containing, s/j2-hybridized silicon atoms also constitute members of the silene family. Interest in these compounds largely focuses on the possible delocalization of electron density in the rings, the extent to which they display aromatic character, and their relative stabilities. Raabe and Michl6 have reported much data, which will not be repeated here other computational studies are reported in Section III.D. A few interesting additions to our knowledge of these systems have been reported in recent years. 

P)GeFc2 compounds, suggesting delocalization of electron density from the porphyrin to the ferrocenyl group. The conclusion is that the stability of Ge—C bonds is higher in the ferrocenyl complexes than in the alkyl complexes165. 

There is evidence for extensive delocalization of electron density between the Ru centers in [(NH3)5Ru(/i-L)Ru(NH3)5] where L = 4,4 -dithiodipyridine, and an overview of a range of Fe, Ru, and Os-containing complexes containing 4,4 -dithiodipyridine or bis(4-pyridine) sulfide provides a useful summary of the properties of these species. ... 

The intermediate a radical RCHOR is not stabilized by delocalization of electron density by the adjacent O through extended v bonding. One resonance structure would have 9 electrons on O. 

In the proposed mechanism, Craq002+ attacks the phenyl ring of ArO + and produces a peroxochromium intermediate I. This step is in agreement with literature precedents (133-137) and with the recent discussion of delocalization of electron density in phenoxyl radicals (138). The peroxochromium intermediate was not observed, implying that the dissociation of CraqOH2+ in the next step is fast. Finally, the loss of teri-butanol was written in analogy with the known reactions in organic solvents (139,140). 

Here the C—H bond is weakened and the frequencies lowered by delocalization of electron density toward the positively charged center (hyperconjugation see Section 10.2 for further discussion). A bending mode is probably again the most important one 58 A H/A D is greater than 1, values ranging up to about 1.4 for favorably situated hydrogens,59 but more typically on the order of 1.1. 

The presence of two electrophilic reaction centers in the molecule of o -unsaturated carbonyls is responsible for their ability to participate in the synthesis of heterocycles. Such compounds can react as ambident electrophiles owing to delocalization of electron density in a C=C-C=0 system. The addition of nucleophiles to these molecules can proceed in one of two main directions—via attack of the carbonyl group (1,2-addition) or involving the / -carbon (1,4-addition). 

A depending on the size of the lanthanide metals. Delocalization of electron density on four equivalent nitrogen atoms causes elongation of the Ln-N bonds at about 0.10-0.15 A compared to silylamides. The close proximity of the macrocyclic 7i-systems in sandwich complexes proved to be useful as structural and spectroscopic models for the bacteriochlorophyll [Mg(Bchl)]2, the special pair in the reaction center of bacterial photosynthesis [211,212]. The distance between the pyrrole rings in [Mg(Bchl)]2 is about 3 A. 

For many simple compounds having no more than one double bond, the modern picture may be quite adequately represented by the Lewis structures (although the Lewis rules are noncommittal about the shapes of molecules). For compounds such as butadiene, benzene, and nitrous oxide, where there is extensive delocalization of electron density, the Lewis structures are not as suitable as the x-electron structures or, better still, as the streamer structures. Both of the latter type, however, are more difficult to draw and, for more complex molecules, more difficult to visualize they become extremely unwieldy when one attempts to use them to represent the progress of a chemical reaction. 

The stabilization of the cation weakens the C-Si bond by the delocalization of electron density so that the bond is more easily broken. Attack of a nucleophile, particularly a halogen or oxygen nucleophile, on silicon removes it from the organic fragment and the net result is electrophilic substitution in which the silicon has been replaced by the electrophile. 

Pure Component Reference Spectra. An ESCA Cls spectrum of the pressed PEEK wafer is shown in Figure la. An ESCA Ols spectrum of the pressed PEEK wafer is shown in Figure lb. The Cls spectrum can be resolved into peak components indicative of hydrocarbon-type environments (285.0 eV), carbons singly bound to oxygen (at approximately 286.5 eV), and a distinctly resolved peak indicative of carbon in a ketone environment at 287.3 eV. Since this Q=0 peak is expected at 288.0 eV, this suggests delocalization of electron density along the backbone chain. The PEEK surface stoichiometry is as expected (see Table I). This further supports the idea that the Q=0 peak is... 

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