Intrinsic barriers to the formation and reaction of carbocations

The Marcus equation relates the activation barriers (ΔG^‡) for organic reactions to the overall thermodynamic driving force (ΔG°) and the intrinsic barrier Λ, which is the value of ΔG^‡ when ΔG° = 0. Most treatments of substituent effects on the rate and equilibrium constants for the reactions of structurally homologous organic substrates make the assumption that the intrinsic barrier Λ remains constant with changes in ΔG°. However, there is good evidence for the nucleophilic addition of solvent to carbocations XArCH(R)⁺ that changes in the aromatic ring substituent X and the benzyl substituent R result in large changes in the intrinsic barrier. Evidence is presented that: (1) There is no significant work required for formation of the reactive complex for proton transfer from carboxylic acids to a homologous series of α-methoxystyrenes XArC(OMe)=CH₂. (2) There is a difference in the curvature of the energy surfaces for the reactant and product states that results in their intersection at a transition state in which proton transfer has proceeded ca. two-thirds of the way from RCO₂H to XArC(OMe)=CH₂.