Gas-Phase Substituent Effects in Stabilized Benzylic Carbocations. Basicities of Benzaldehydes, Acetophenones, and Methyl Benzoates in the Gas Phase

The gas-phase basicities of a series of aromatic carbonyl compounds (ArCOR; R = OMe, Me, H) were measured based on the proton-transfer equilibrium method using a pulsed ICR mass spectrometer. The effects of ring-substituents on the gas-phase basicities were analyzed in terms of the Yukawa–Tsuno equation. It was shown that the resonance demand parameter (r+) decreases significantly from 1.06 for the benzaldehyde to 0.50 for the methyl benzoate in increasing order of the electron-donating ability of the R group directly linked to the carbonyl carbon. This indicates that the π-charge formed in the conjugate acid ion is competitively delocalized into an aryl group and an R group. In contrast, the ρ value is nearly constant at −11 (in kcal mol−1 per unit) in all series. Most importantly, it was found that the r+ value in the gas phase is in agreement with that for the basicities in aqueous solution, whereas the ρ value is significantly reduced in aqueous solution. This fact led us to the conclusion that the solvation of a cation reduces the central charge so as to lower the response to substituent perturbation, essentially without changing the nature of the intramolecular charge-delocalization.