Benzyl cation rotational barrier

In a cation such as the (2,4-di-ferf-butyl-6-methyl)benzyl cation 147, a high rotational barrier around the v/r-hybridized atom is observed. The methylene protons are found magnetically nonequivalent in the1H NMR spectrum.356 Recent combined experimental and theoretical studies for the related cation 143 suggest357 that structure 143b is an important resonance contributor. 

Radicals and radical ions provide fruitful subjects of research. Room temperature fluorescence from the arylmethyl radicals Ph3C, Ph2CH- and PhCH2 and theoretical studies of rotational barriers in the benzyl cation, radical and anion as well as the singlet and triplet states of diphenylcarbene are typical examples of such contemporary studies. A very detailed paper considers the problems of the state assignment and reactivity of excited states of p-substituted benzyl radicals.Ketyl radicals containing the enthrone moiety and the 4-(methyl sulphonate) benzophenone ketyl radical anion are related studies in this field. 

The number of electrons changes stability in a more complex way in three-center systems, i.e. the allyl and related species. In this case, delocalization of charge is much more important than delocalization of spin. For example, rotation around the C-C bond becomes much more difBcult in the allyl cation (-38 kcal/mol) compared to the allyl radical (-13 (calculated), 15.7 (experimental)kcal/mol). Allylic anions have a lower rotation barrier relative to the cation (-23 vs. -38kcal/mol). In the case of anions, additional stabilization to the twisted form (-8-14 kcal/mol) is provided by rehybridization, which partially offsets the lower efficiency of hyperconjugation in the twisted anion than in the twisted cation. The calculated barriers for the allyl system depend strongly on the methods employed, but the trend of cation > anion > radical remains. The same trend is observed for the rotation barriers in the benzyl radical and cation (Figure 3.10). ... 

Use HMO theory to estimate the electronic energy barrier for rotation of an allyl radical about C2-C3 as shown in Figure 4.79. How does your result compare with an experimental activation energy How does the calculated value compare with the electronic energy barriers calculated for the allyl cation and the allyl anion How do the calculated values for the allyl systems compare with those of the corresponding benzyl systems ... 

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