Charge distribution, allylic

Use Learning By Modeling to view the charge distribution in the allylic carbocation shown in the equation. 

Despite the cyclic character of these TSs, both the bond distances and charge distribution are characteristic of a high degree of charge separation, with the butenyl fragment assuming the character of an allylic carbocation. 

The ratio of isomeric ethers is strongly affected by polar substituents which induce an asymmetric distribution of charge in allylic cations. Photolysis of methyl 2-diazo-4-phenyl-3-butenoate (20) in methanol produced 24 in large excess over 25 as the positive charge of 22 resides mainly a to phenyl (Scheme 8).19 As would be expected, proton transfer to the electron-poor carbene 21 proceeds reluctantly intramolecular addition with formation of the cyclopropene... 

It is tempting to correlate this with charge distribution at. a and Y positions o.f the delocalized allylic active centers. In NMR... 

The positive charge of allyl cations essentially distributes itself among C-1 and C-3 of the allyl skeleton. Terminal alkylation concentrates the positive charge at the alkylated carbon, as shown for the 1-methylcyclopentenyl cation [494],... 

Information on electron delocalization in the bicyclo[3.1.0]hexenyl cations is available from their reported NMR spectra Data obtained with a variety of systems point to a completely different charge delocalization pattern to that found with the homotropenylium ions. For example, Olah and colleagues have obtained the NMR spectrum of the parent ion"", 61, and compared this with those of 42 and 11. As can be seen from the data summarized in Scheme 18, the chemical shifts of the five-membered ring carbons of 61 resemble those of the cyclopentenyl cation. There is a considerable difference in chemical shifts, and hence charge distribution, at C(2), C(4) and C(3) of 61. There is no evidence for the fairly even charge distribution as is found for the homotropenylium and homocyclopropenium ions (see previous Sections III. A and III. B). It was also noted by Olah that the chemical shift of C(6) is consistent with large delocalization to this position, i.e. to conjugation of the allyl system of 61 with the external cyclopropyl bonds. 

The three-dimensional shape of this fuzzy body of the electronic distribution has many important features not revealed by the simple, skeletal ball and stick model. One of the most important tasks of topological shape analysis of molecules is the precise analysis and concise description of the three-dimensional electronic charge distributions, such as that illustrated by the selected MIDCO s of allyl alcohol in Figure 1.2. Various methods and computational techniques of such topological shape analyses are discussed in detail in this book. 

Proceeding as we did for the allyl radical, it is easily seen that the electron charge distribution is uniform (one n electron onto each carbon atom, alternant hydrocarbon) and the spin density is zero, as expected for a state with S = Ms = 0 since the two bonding MOs are fully occupied by electrons with opposite spin. The delocalization (or conjugation) energy for linear butadiene is ... 

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