Theoretical Studies of Substituent and Solvent Effects on Protonation Equilibria of Benzaldehydes

Abstract

The effect of substituent on the protonation equilibria of benzaldehydes ( A + H + ⇌ AH +, where A and AH + represent benzaldehyde and its protonated form) in the gas phase and in solution has been investigated theoretically at the MP2/6-31G*//MP2/6-31G* level applying two solvation (IPCM and SCIPCM) models. The absolute as well as relative Δ G ° (or log K ) values of the gas-phase MP2 results are in good agreement with the experimental values. All the solvation models are inadequate to reproduce experimental ρ + (slope of log K versus σ + plot) value in aqueous solution. The SCIPCM model gives the best correlation but with a much larger magnitude of ρ + (=−5.29 relative to the experimental value of ρ + = −1.88) mainly due to neglect of specific solvation (i.e., hydrogen bonding) effect. It was found that the neglect of hydrogen-bonding effect in the solvation of aldehydes ( A ) results in unduly higher equilibrium constants ( K ) for electron donors (σ + < 0); in contrast, the neglect of hydrogen bonding in the solvation of the protonated forms ( AH + ) leads to unduly lower K values for electron acceptors (σ > 0).