Carbonium ions cyclohexadienyl

The first attempt to correlate proton chemical shifts of methyl groups in carbonium ions with charge on the adjacent carbon atom seems to have been made by Mac Lean and Mack or (1961). They examined the pnmr spectra of a number of cyclohexadienyl cations (benzenium ions) [6] where R was CH3, and found that chemical shifts for protons directly attached to positive carbons were linearly... 

Consideration of a cyclohexadienyl cation of the type shown in eq 2 where R and E are not readily lost as positively charged species should make clear the point that the reaction manifold so characteristic of carbonium ions is to be expected. 

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